The Use of a Photometer to Detect Luminance for the Visually

Challenged

BE 310: Independent Laboratory Project

Group M8John Ho

A. Laurance MichaelsAnthony NapoliKarl Orishimo

Introduction:

For this laboratory project, an optoelectric device was constructed to judge an

obstacle’s distance from the user. This device used the infrared spectrum as the light

source from which the distance could be determined. The theory behind this device is

based on the fact that as the person gets closer to an obstacle, the intensity of infrared

being detected by an infrared sensitive detector will increase. Therefore, by using an

infrared sensitive phototransistor, the voltage will increase as the device detects a higher

intensity of infrared light. This device, coupled with a voltage-regulated timer chip,

would produce a discernible sound change when the person gets closer to an object. The

preliminary testing yielded reproducible results of 1.403 ± 0.93 feet. This means that

when a person approaches an obstacle, at 1.403 feet, the device would produce a

significant frequency change from the speaker. This device was developed successfully,

however, there was not ample time to fine tune the device for optimal performance.

Future testing would include incorporating different, perhaps improved filters in the

black box and also increasing the volume outputted so that the frequency change would

not be so faint.

2

Background:

Until now, the only aid used by blind people was the walking stick. By swinging

the stick back and forth and tapping the ground, the person can get a sense of the

obstacles near them. Using this device for long periods of time may become an

inconvenience and even a source of embarrassment to the user. The aim was to construct

a device which essentially serves the same purpose as the walking stick while being a bit

more inconspicuous. This device will allow a blind person to freely move about and

avoid injury without feeling self-conscious.

This device will also benefit non-blind people by helping to dispel stereotypes

and generalizations about blind people. Because the blind person using the device is less

conspicuous, they will integrate more with society. Seeing how independent these people

are, the non-blind members of society will more readily welcome them into normal,

functioning society. With the device, both the blind and the non-blind will share the

benefits of increased interaction and integration.

Although no device like this is currently out on the market, similar devices do

exist. For example, Realtors use an IR-based device to measure the dimensions of rooms.

The device emits an IR beam which strikes a wall in the room and is then reflected back

into the device. A chip in the device measures the time for the IR beam to reach the wall

and be reflected back. Using the speed of light and the time taken for the beam to return

to the device, the distance to the wall can be calculated. This distance is then displayed

via a LED or LCD display.

When dealing with light and reflection, one important issue must be realized and

addressed. Because different surfaces reflect light in different ways and also because the

reflected beam does not always have the same intensity level as the original, one must

attempt to maximize the original signal and minimize interference from other forms of

light. Because the transmitter sends out the beam in almost all directions a lens is needed

3

to focus the beam in a particular direction. Similarly, the receiver, most of the time, is

generalized and functions when any and all light is shined on it. It is of great importance

that the receiver be specialized to receive only the reflected beam and not any ambient

light from the environment. The most common method of doing this is by using a filter

that only passes light which is the same as the originally transmitted beam. Even with the

filter and lens, some difficulty may be encountered in transmitting and receiving the

signal because the reflection off of some surfaces is not always exact. Instead of

completely reflecting the original beam, some surfaces may scatter the beam, hence

losing some of the original intensity.

There are multiple types of measurements for the determination of light, however

relevant photometric quantities are radiance and luminance. Radiance is the density of

light per unit surface area while luminance is the intensity of light per unit surface area.

There is an equation that defines the relationship between luminance and radiance:

L K V Lm e

where KmV represents the luminous efficiency and Le is the radiance. (Le Grand, p.75)

Since this device measures the intensity of light, luminance is a more relevant

measurement

The study of optoelectrics combines both the technologies of optics and

electronics. Optoelectric devices are mainly concerned with the interface between

electromagnetic radiation in the form of visible and infrared light and electronic circuits.

Most optoelectrical devices operate over a light spectrum with approximate ranges of 300

nm to 1100 nm. Included in this range are visible light (400 nm to 600 nm) and infrared

light (700 nm to 1000 nm).

Optoelectric circuits are primarily used for two purposes, emission and detection.

Emission involves converting an electrical signal into a light source while detection

converts light into an electrical signal. A photon is emitted when excited electrons fall to

lower energy levels. This transition from one energy state to another is perceived in the

4

form of light energy. Light emission usually occurs in the semiconducting material of the

optoelectrical component. The materials that are most often used for this purpose, and are

most responsive to light, are Germanium and Silicon. The simplest form of an

optoelectric emitter is a light emitting diode (LED). Like a regular diode, an LED only

passes current in one direction. When current is directed in the forward direction (anode

positive, cathode negative), light energy is given off. There is little current flow or light

emission until the forward-biased voltage is equal to or greater than the forward voltage

drop of the LED. Because little or no current flows in the reverse direction, no light is

produced in the reverse-biased condition. By alternating the input signal to the LED, a

signal emitter or a signal indicator can be produced. One must be careful not to raise the

voltage above the LED’s threshold because when the LED’s threshold is surpassed, an

short circuit is created. A short circuit is when the voltage goes to zero and the current

goes to infinity.

The second type of optoelectric device is an optical detector. These devices

transform light energy into an electrical signal. One of the most common light sensors is

the phototransistor. A phototransistor is the same as a normal transistor with the

exception of a small glass window which allows light to strike the base of the transistor.

In a dark environment or when no light shines on the phototransistor, there is very little

base current flow. Under these conditions the circuit is basically open. When light strikes

the phototransistor, a large current flows through the emitter and the circuit is closed.

The intensity of the light striking the phototransistor controls the amount of current

flowing through the circuit. Using emitter and collector resistors, both positive and

negative outputs can be produced provided that light is striking the phototransistor.

Another important aspect of the device design is the safety precautions that must

be observed during the course of constructing and utilizing the device. The first safety

precaution deals with the power source. Care has to be taken to find two batteries of the

same potential. If a new and a slightly used battery were to be connected, the resulting

5

ground would not necessarily be zero volts. Instead, it would have some voltage equal to

the differences of the volts coming into the point to be used as ground. Therefore, there

should be a warning on the device, such as:

“When using this device, both batteries must be replaced simultaneously.”

In this experiment the voltage was initially about 8.5 volts. (most nine volt batteries are

less then nine volts even when new). However, it is important to note that when the

battery drops to half of its original voltage, not enough voltage will be supplied to the

oscillator because device limitations specify that the 555 timer needs a minimum of 4.5

volts. This limited voltage range, which would result in a limited duration of use, should

also be made clear to the user. Another danger that comes from batteries with different

voltages is the fact that batteries have an affinity to achieve the same voltages, when

connected together. Therefore, if one battery were to have more charge than another,

then the higher charged battery would create a flow of current to balance the lower

voltage battery. This type of charging can cause severe problems, such as leakage of the

battery contents.

Another consideration is that the circuit is placed in a black box. Since black

absorbs electromagnetic radiation and warmth, it is best that this device be kept out of the

sun so that the circuit resistance does not increase and so specific components such as the

555 timer will not be heated any more then necessary. The soldered components in the

circuit need to have their exposed wires taped with electrical tape, so that the circuit does

not have any shorts. As well, this will protect the user of the device in the event that the

batteries need to be replaced.

Since this device is using infrared light, it is necessary to become familiar with

this sort of wave. Infrared radiation is an electromagnetic radiation that has a shorter

frequency than humans can visibly discern. This frequency range falls below the visible

6

spectrum, but above radio waves in the electromagnetic spectrum. Infrared radiation can

be related to electromagnetic radiation using the equations:

fc

E hf

where f = frequency,c = velocity of light = WavelengthE = radiation quantum energyh = Planck’s constant

Kirchhoff derived laws of radiation to describe the nature of infrared light. These

laws are that 1) a good absorber is a good radiator, 2) a good reflector or a transparent

body is a poor radiator, 3) emissivity e can be determined by measuring the absorption a,

4) the absorption cannot exceed unity, since it is impossible for a body to absorb more

energy than the total amount radiating onto it, and 5) all the energy radiating onto an

opaque body is either absorbed or reflected away, and can be determined by 1 = a + R =

e + R (where R = reflectance) (Vanzetti, 16).

The advantages of using infrared radiation over other conventional methods of

detecting obstacles are that it is a non-contact technique and is passive (very little energy

is imposed on the target). Choosing infrared light as the device’s emission was a major

factor in the design process, and is discussed later in the potential problems section of the

report.

Finally, the project utilizes the principles of sound and human hearing. Because

the main output is a sound generated by the device, human hearing had to be taken into

consideration. There are two main aspects of sound which are central to human hearing:

frequency and intensity. The frequency of a sound wave is the distance between the

7

crests of the wave and is measured in cycles per seconds or hertz. The frequency of a

sound wave is directly related to the pitch. The pitch of the sound increases as the

frequency increases. The second aspect of sound that is important to human hearing is

intensity. Intensity is the loudness of the sound and is directly related to the amplitude of

the sound wave. Intensity is measured in decibels where an increase of one dB causes a

tenfold increase in the intensity. The human ear can perceive sound over a frequency

range of 20 to 20,000 Hz. It can also distinguish between pitches that differ by 0.3% in

frequency. The normal range of intensities that can be perceived by the human ear spans

a range of twelve orders of magnitude.

8

System Overview:

There were three basic components that had to be integrated on to a small

breadboard ( JE21 ), 3.3” long by 2.1” wide. These components were the transmitter, the

receiver, and the voltage-regulated oscillator.

The transmitter consisted of two infrared Light Emitting Diodes (LED). Two

LEDs, instead of the original one, were used to enhance the intensity of the infrared light

being returned to the phototransistor. These infrared LEDs were available in the

laboratory. A push-button switch was used to open and close the circuit. A 1KW variable

resistor with a value of 284 W was used to limit the amount of voltage going through the

LEDs. The construction and testing procedure will elaborate on the method for testing

for the correct resistance of the variable resistor. If the correct voltage was not obtained

either the LED would not light, (If the voltage was too low, the LED would not light.

Also, if the voltage was too high, the voltage would go to zero, causing a short circuit,

thereby not allowing the voltage to run through the LED) or the voltage would not be at

its maximum. It was determined that the best results would be obtained with the

maximum voltage through the LED. There were two additional pieces of equipment that

were used to enhance the transmitting and receiving of the IR LED. A Fresnel Lens ( 1”x

1”, A431794 ) was used to focus the infrared light emitted. This allowed for a more

intense emission. The lens would focus the infrared light and make the light easier to

detect. Another piece used was an infrared filter ( 1” diameter, A43948 ). This filter

only allowed infrared light to pass through. This was necessary to ensure that the

phototransistor was picking up the infrared light that was emitted from the device and not

light from an outside source (i.e. ambient light, room light). The final change made

9

between the initial proposal and the final project was the use of the 9-volt battery.

Initially, one battery was used. Yet, as the project began, it became apparent that a

positive, a negative, and a ground terminal were necessary. To accomplish this, two

batteries were used. The negative terminal of one battery was connected to the positive

terminal of the other battery. This “new” terminal was ground. This successfully allowed

all three necessary terminals to be used in conjunction.

The next portion of the device that must be understood is the receiver. A 741

operational amplifier was used to amplify the voltage through the circuit. A particular

voltage was necessary to activate the 555 timer chip correctly. This voltage was

calculated and the appropriate gain was applied. The gain was created using a non-

inverting amplifier. Calculations for the gain can be found in the testing procedure. A

non-inverting amplifier was used instead of the original proposal of a comparator circuit

for a number of reasons. First, a comparator would only provide the circuit with positive

or negative nine volts. Since it was necessary to allow for a change in voltage to vary the

oscillation in the timer chip, the comparator circuit would render the phototransistor

useless because as long as the voltage input to the v+ was larger then zero the same

voltage would be input into the oscillator. Therefore, it was determined that a non-

inverting amplifier was necessary. A non-inverting amplifier was chosen over an

inverting amplifier so that a negative voltage would not be supplied to the 555 timer

chip. The most important piece of the receiver circuit was the phototransistor. The

phototransistor was the same as the one used in Laboratory #8 of the BE 310 curriculum.

The specifications and further information on the phototransistor and the IR LED are

included in the appendix.

10

The last piece of the circuit was the voltage-regulated timer chip and the speaker.

A speaker with a resistance of 8W was used as an output device. Depending on the

voltage input into the 555 timer chip, a certain frequency was emitted from the speaker.

This allowed the user to determine his or her orientation with respect to an object. The

capacitor in the 555 timer chip serves as a voltage threshold for the timer chip to

discharge. The charge on the capacitor will range from 1/3 Vcc to 2/3 Vcc. When the

voltage reaches 2/3 of Vcc, the capacitor discharges. The frequency for this event can be

described by the equation:

frequencyR R C

144

1 2 2 1.

( )

However, it is important to note that the oscillation frequency is independent of

Vcc. The frequency of the chip is dependent, though, on the input voltage (pin 5). As

the input voltage increases, the oscillation frequency decreases. In order to produce a

frequency that the human ear can detect, the frequency must be calculated to fall between

the human hearing threshold. To increase the volume, the resistor in series and prior to

the speaker was taken out and replaced by a 4.7 mF capacitor after the speaker. This was

to increase the volume, as well as to dc-couple out the DC component of the voltage, so

the speaker would not burn out.

After all of the circuitry was complete, the final product was placed inside a black

carrying box ( H2853 ). The black box was 4.9” long by 2.5” wide by 1.5” tall. This

allowed sufficient room for the circuit and the batteries. First, two holes a ½” wide were

drilled in the front to allow openings for the infrared LED and the phototransistor. The

infrared filter was placed in front of the phototransistor. This filtered out all of the visible

light that would be received and might interfere with the phototransistor. There were two

11

theories behind the placement of the Fresnel lens. One theory was that the lens should be

placed in front of the LED. In this case, the infrared light would be focused from the

LED and a compact beam would leave the box. This would allow for a strong output.

The second theory believes that the lens should have been placed in front of the

phototransistor. In this case, the lens would focus the reflected IR that was emitted from

the LED, and focus it. It would then be received by the phototransistor. For this

experiment, it was decided that the lens should be placed in front of the emitter. This

would allow the beam to be focused initially rather then dispersed over a wider angle, as

is common of a LED. Ideally, a second lens would be added before the phototransistor

as well. After the filter and lens were installed, a hole was made for the push button

switch. This switch was placed on top of the box, to simulate a remote control device.

Care was taken to place the switch in such a way as to be ergonomically correct. The

final piece of construction on the black box dealt with the speaker. Because the sound

emitted from the speaker was faint, holes were drilled into the box in the back where the

speaker would be housed. This allowed for a larger volume to be emitted from the box.

After this was complete, testing was begun.

12

Construction / Testing Procedure:

After checking to ensure that all parts for the circuit were available and

undamaged, a testing procedure was defined and carried out. The following outlines the

procedures. Each week, a procedure was laid out beforehand for what was to be

performed in lab. These procedures will be defined along with the points of refinement

in the procedure due to adjustments determined to be necessary as a result of

experimentation.

Week 1:

The first week consisted of organizing the newly acquired materials, completing

preparations for construction of the circuit, and actual construction of the circuit. It was

decided that two members of the group would build the receiver portion of the circuit on

the breadboard to be used in the device, while another two members of the group used a

different breadboard to construct the voltage-controlled oscillator. The LED transmitter

was left out because it would be relatively trivial to build (although room was left for it

on the front end of the breadboard) and could easily be substituted later.

The receiver portion of the circuit is shown below:

13

0.1 uf

100K

741

9V

100K

10K

This circuit primarily consists of three significant areas. First, the phototransistor portion

of the circuit consists of a phototransistor powered by a nine volt battery in series with a

resistor. The resistor lowers the amount of current going into the collector region of the

phototransistor to prevent saturation. The second part of the circuit is the operational

amplifier. It is obvious that the operational amplifier is designed to act as a comparator.

When the voltage going into the inverting voltage input (v- ) is larger then the non-

inverting voltage input ( v+ ), the output at pin 6 is intended to be negative nine volts.

However, when the voltage is larger at v+ than at v- the voltage output is intended to be

positive nine volts. This output is then sent to the third major portion of the circuit, the

10kW variable resistor. This is considered a major portion of the circuit because aids in

varying the voltage going into the oscillator circuit, hence varies the frequency of the

resulting sound.

14

Construction of this circuit was completed, and it was quickly tested using a

previously constructed emitter circuit to see if the circuit was effective. When the circuit

did not work, a testing procedure was begun that consisted of checking various nodes

throughout the circuit to try and determine the error. It was determined that there must

have been a short somewhere in the circuit because the op-amp became extremely hot.

The circuit was slowly deconstructed to determine where the problem was and it was

found to be the operational amplifier. The circuit was rebuilt, with a new operational

amplifier, and tested again. This time the circuit worked, but not well. It would return

very small voltage changes at the emitter end of the diode as a result of the light from an

LED. A multimeter was connected below the diode and the diode was continuously

covered and uncovered to see if the voltage would vary. A varying voltage would show

that there was a varying degree of intensity of light entering the photodiode via the LED.

Even though there was very little sensitivity to the incoming light, the receiver portion of

the circuit was put aside for experimentation on the voltage-controlled oscillator to

determine if this portion of the circuit was effective.

The original design of the voltage-controlled oscillator is shown below. This

design is, for the most part, the same as the voltage-controlled oscillator published by

Radio Shack. (Mims, p. 15)

15

100K

555

220

100K

1K

0.01uf

Output toSpeaker

5V

R1

R2

C1R3

The voltage-controlled oscillator, as well, contains three major portions that need to be

considered. First, the 100 kW variable resistor is an important part of this circuit because

it is what determines the voltage going into pin 5. This is especially important because it

is this voltage which controls the 555 timer and changes the properties of its output,

hence the properties of the sound (how exactly it does this will be explored later).

Second, the right side of the circuit that consists of the two resistors (R1 and R2) and the

capacitor (C1) is what controls the oscillations. Without the speaker and the input into

pin 5, this circuit is almost exactly the same as an astable oscillator. Hence, the frequency

of the output can be expressed by the following equation.

FrequencyR R C

14421 2 1

.( )

16

The variable resistor is very important because it can help vary the frequency of

oscillation, hence the frequency of the sound that is produced by the speaker. By varying

this resistor, one can set the base frequency at which the speaker will produce a sound

when none of the infrared light is being returned to the phototransistor. The third portion

of the circuit is the speaker connected in series with a small resistor to the 555 timer.

The small resistor functions to limit the amount of current going through the speaker.

Since the speaker has a very small resistance, too much current flowing through the

speaker could burn it out. Therefore, a small resistor (big relative to the speaker though,

almost 30 times the size) is connected in series to limit the current and have much of the

voltage drop across it.

The oscillator was checked and then transferred to the breadboard containing the

receiver. Testing was then carried out on the oscillator because it did not seem to

oscillate. Again, certain points in the circuit were tested for their expected voltage drop.

One of the problems was that the variable resistor produced significantly different

resistances when it was in the circuit as compared to what it was set at outside of the

circuit. This had a significant impact on the oscillation frequency, especially considering

the other resistance is one hundredth its size. When the non-inverting op-amp was built,

the gain was calculated to be 22. When tested experimentally, the gain was confirmed to

be 22. However, once the non-inverting op-amp was placed in the receiver portion of

circuit, the gain dropped significantly. This could be attributed to the fact that when

placed in the circuit, the non-inverting op-amp components (mainly the resistors) were in

parallel with resistors elsewhere in the circuit (or even the phototransistor which may

17

have acted as a resistor), thereby changing the actual resistance. When the actual

resistances of the non-inverting op-amps are changed, the gain will be significantly

changed. Therefore, it was removed and replaced by a single resistor. This was simply an

arbitrary resistance, even though it was recognized that this would have an impact on the

frequency of the speaker output. However, the oscillation frequency was still within the

audible range of human hearing, so this was not a problem At this point, there was no

more time left for experimentation, so it was discontinued until week 2.

Week 2:

Experimentation began where it left off in the previous week. Since the receiver

circuit appeared to be working, experimentation continued on the oscillator portion of the

device. Since the frequency of sound output by the speaker decreases as the voltage

increases, adjusting the voltage entering pin 5 was the initial test to see if this might

result in a recognizable oscillation. Experimentation then focused on trying to ensure

that the capacitor was reaching threshold and actually discharging as an oscillator circuit

should. However, before a significant amount of experimentation was done on this

another possibility arose. The speaker may not have been receiving enough voltage in

order to produce an audible signal. In order to increase the volume of the speaker, the

220W resistor was removed and a 1mF capacitor was connected after the speaker at

pin 3, and then connected to ground. This served to increase the volume of the output

from the speaker, and the speaker immediately began to produce an audible output.

Moreover, variation of the potentiometer connected to pin 5 varied the frequency of the

signal output by the speaker. This is necessary because increasing the resistance of the

18

potentiometer would decrease the voltage going into pin 5. This would then increase the

frequency output of the speaker and allow greater control over the output. The new

oscillator circuit looks this like:

555

4.7u

Output toSpeaker

+9V

1u

270

220

R1

R2

C1C2

The circuits were then connected together with the assumption that the receiver

circuit still functioned; however, this was not the case. The system was then debugged in

the following way: Starting with the bottom of the circuit (since it was known that the

oscillator worked because it was producing a noise), the voltages at different nodes were

tested and compared to what might be expected. This meant that the resistance of the

variable resistors associated with varying the voltage going into pin 5 of the timer had to

be tested as well. Experimentation led back to the operational amplifier, when it was

determined that using the operational amplifier did not serve its intended purpose. This

was because the operational amplifier was acting as a comparator, resulting in only a

positive supply voltage or a negative supply voltage. As a result, no matter what the

19

intensity of incoming light to the phototransistor might have been, it would not have

mattered because the voltage leading to the 555 timer would always be the same. Hence,

a non-inverting operational amplifier with a chosen gain replaced the comparator. The

non-inverting amplifier that replaced the comparator is shown below.

200

741

+9V

20022K

New receiver portion

to speaker

R2R1

R3

As well, the capacitor that originally connected to v- was removed. Putting a known

voltage in and measuring the voltage out through use of an oscilloscope then tested the

non-inverting amplifier. Both R2 and the variable resistor, R3, (From the old circuit on

page 14) were removed because they were no longer necessary now that the comparator

was converted to a non-inverting amplifier. The non-inverting amplifier was then

connected to the circuit and it was found that the circuit was still not functioning

properly. Since, at this point, every portion of the circuit had been tested except the

phototransistor, experimentation focused there. Again, the original circuit design

contained an error. By having the resistor above the phototransistor as seen in the circuit

20

diagram, the phototransistor was always connected to ground at the emitter end as well as

connected to the v+. Because of this, when the phototransistor was in saturation or

cutoff it would result in zero volts at v+ either way. It was shown that the

phototransistor was in cutoff because the voltage drop was 7.7 volts across it. If it had

been in saturation, almost all of the voltage (except for 0.2 volts) would have dropped

across the resistor and there would be minimal voltage difference between the collector

and the emitter. Again, the voltage would be zero at v+ but would not coincide with the

result of a 7.7 volt drop across it. It was then determined that the circuit had to be

redesigned to have the resistor after the phototransistor. This way, if the transistor was

ever in cutoff, the voltage would be zero at v+ still, but if it was in saturation it would

approximately be equal to the potential of the battery. At this stage, it was then

determined that testing needed to be done on the phototransistor to determine its active

range. By doing so, the range over which the phototransistor would show an increased

voltage due to increased intensity of light could be determined. This was a very

important step now that the operational amplifier was changed from a comparator

to a non-inverting amplifier. The signal that was produced at the emitter as a result of

the intensity of the light could be amplified and sent into pin 5 to change the frequency

of oscillation of the speaker.

Week 3:

The procedure for this week consisted of four tasks. Since it had been determined

in the previous week that the only things not functioning correctly were the non-inverting

amplifier and the phototransistor, this was the focus of the testing. However, it was first

21

necessary to examine the voltage-controlled oscillator and test it to select a base

frequency. (The frequency at which no light is returned to the photodiode and sound is

based purely on the initially chosen oscillation frequency) The base frequency is one at

which it would oscillate independent of pin 5.

Simply putting a variable resistor in at R1 and varying it tested the oscillator. The

output was recorded via an oscilloscope in place of the speaker. The frequency at which

the circuit was oscillating was simply determined by changing the time/division knob on

the oscilloscope to the most accurate setting and recording the distance between identical

voltages. This was because the same curve was repeated at every pulse. This was done at

frequencies ranging from 16Hz. to 2631Hz. The large frequency range was necessary

because it was yet to be determined what frequencies of sound would be used. The most

important quality was which speaker frequency would be most useful in terms of both the

quality of sound emitted from the speaker and the ability of the listener to discern

changes in frequency due to changes in input voltage at pin 5.

The voltage at pin 5 was then tested to examine how the input voltage would

effect the output of the speaker. Simply using a variable voltage source at pin 5 and

again recording the output frequency from the oscilloscope accomplished this. When this

was completed, the speaker was placed in the circuit and the procedure was carried out

once again to determine the range of frequencies that the speaker could support and the

range of voltages that would produce a sound that was clear and distinct. Varying the

voltage and qualitatively assessing when the voltage corresponded to a discernible sound

accomplished this. This range of voltages would then be used to determine the gain of

the non-inverting op-amp portion of the circuit. An appropriate gain would take the

22

maximum voltage at v+ (when the user is standing the minimum distance away from an

obstruction) and multiply it to obtain the maximum input that can be received at pin 5

that could produce an audible output from the speaker. The same would ideally be the

case with the minimum input voltage to v- so that the minimum input could be received

at pin 5 when there is no obstruction at all.

The next part was the connection of the non-inverting amplifier to the circuit and

testing to ensure that it worked within the circuit. This was to be done by applying a

voltage to the v+ of the op-amp and obtaining a representative gain that would vary the

frequency of the oscillator. However, there was some difficulty in this portion of the

testing because it was found that the circuit was not constructed in such a way as to allow

for both a ground and a negative bus. The ground and negative were assumed to be the

same. This problem was corrected by using two batteries connected in series. This

created a theoretical power source of ±9 volts, or a net potential difference of 18 volt.

This obviously resulted in a positive and a negative terminal but it also allowed for a

ground because the positive of one battery was connected to the negative of another,

effectively canceling the two. When this problem was corrected, the circuit was again

tested and it worked. However, it did not work as it was expected. When the amplifier

was placed in the circuit, the gain that was calculated based on simple use of the op-amp

laws was not obtained. Gain for a non-inverting op-amp is:

GainRR

1 2

1

This problem was to be fixed later because the objective was primarily to ensure that the

non-inverting amplifier could produce a high enough voltage so that a varying sound

could be produced based on the input to pin 5. Theoretical gain was secondary to the

23

actual output of the sound. It did not matter whether the experimental gain correlated

with the theoretical gain, as long the audible sound from the speaker was heard. This

was achieved so the next step was to integrate the transmitter and the phototransistor

portions of the circuit onto the protoboard.

Before the phototransistor and transmitter could be integrated, they had to be

tested separately to determine their optimal operating region. The transmitter had to be

tested to determine the voltage at which the largest intensity of light could be produced.

The phototransistor had to be tested to find its active region, the region at which the

voltage varies with the intensity of light received.

Finding what maximum voltage the LED would allow tested the transistor. It was

assumed that the highest intensity of IR was emitted at the highest voltage. This was

assumed because a red LED was tested first. Until approximately 1.90 ± 0.02 volts the

LED would not light. Afterwards, the LED emitted light at higher intensities with the

increase of voltage. At approximately 5.90 ± 0.04 volts, the voltage became too high.

Since the device used a 9V battery, the correct resistor had to be inserted into the circuit

to allow for the appropriate voltage drop. It was determined that a 284 W resistor was

needed. Since the lab did not have 284 W resistors, a 1kW variable resistor was used,

and set at 284 W. This allowed for the maximum intensity to leave the IR LED.

The next piece of the receiver circuit was the phototransistor. A phototransistor is

similar to a normal transistor in that there are three terminals: the base, collector, and

emitter. However, the phototransistor does not have a separate pin for the base terminal.

Instead, the infrared sensitive base varies the voltage based on the intensity of the

infrared light absorbed. The collector voltage is the input power of the transistor, which

24

in the circuit, was approximately nine Volts. The emitter voltage was connected to the

non-inverting op-amp and amplified to fall within a pre-determined range for the timer

circuit so that the optimum frequency output could be produced.

Week 4:

Since week 3 ended with obtaining a functional circuit, week 4 was primarily

based on constructing the circuit casing, finishing up other extraneous testing, and

making further improvements on the circuit to try and make the circuit work as a

function of distance.

One of the more important parts of the laboratory was designing the black box so

that it could hold the circuit and function properly. This involved drilling two holes in

the front of the box for the emitter and the phototransistor. The infrared filter was then

placed in the hole for the phototransistor and the IR lens was placed in the hole for the

emitter. Each one of these was tested previously to ensure that the circuit would work

when these filters were used. Simply putting the filters in between the emitter and

phototransistor and checking to ensure that a signal was still produced did this. Further

drilling then produced a hole on the top of the box so that a push button could be placed

in the circuit. Finally, a series of holes were drilled in the back of the box where the

speaker was to be placed so that the sound would not be dulled by being a complete

enclosure.

The next step was to check the voltages of certain portions of the circuit that had

not been checked in the previous week. These included such things as the gain of the op-

amp compared to the theoretical prediction, and the voltage input to pin 5.

25

Once this further testing was completed, it was necessary to try and use all of the

accumulated data to make the circuit work as originally designed. However, one

immediately came across the problem that most of the values obtained separately (when

each component was tested on its own) were different than the values that were being

found when the circuit was completely connected. For example, the initial testing of the

voltage-controlled oscillator and the phototransistor showed that a gain of 22 would be

necessary to change the maximum voltage input to the non-inverting op-amp to the

optimum frequency range. However, when the whole circuit was connected, it turned

out that this gain was too high for the 555 timer and it had to be lowered. As well,

because of this new addition of voltage to pin 5, the base frequency had to be recalibrated

to a frequency of higher quality output from the speaker. At this stage, simple guess and

check methods were employed to vary the R2 in the oscillation portion and the gain was

varied until the appropriate sound was received. The sound that was selected was one

that corresponded to high quality output from the speaker, and was most aesthetically

pleasing to the listener. The device was then tested to see that it worked and changed

frequency based on distance. Testing showed that this was the case, but the range at

which the device worked was very small; the reflecting object had to be within a foot for

the speaker to produce any output. The gain and the base frequency were changed some

more to see if this would make a difference and it was found that it did have a slight

effect of increasing the object reflection distance recognition to approximately a foot. At

this point, the only other alternative reason for this lack of signal was that the infrared

emitter was not putting out a strong enough signal. To counter this, another emitter

was placed in parallel with the first one to see if this might have an effect on the

26

object reflection distance by enabling the phototransistor to take in more light.

This procedure only increased the object reflection distance a few more feet, and at this

point further testing could not be completed because of time constraints.

Another obstacle in the working of the circuit as a whole was that when the

circuit began to produce a sound when it was within the object reflection distance, this

sound did not vary much at all. In fact, the sound initially produced seemed to have

three frequency stages. First, the base frequency would appear very faintly prior to the

object distance. Then, when the object distance was reached, the frequency would make

a distinct change. The third and last stage of frequency change seemed to be when the

device approached a few inches of the reflecting surface where it quickly changed once

again. The continuously varying frequency that was expected in theory was not obtained

in practice. The technique of altering some of the circuit components to see if this would

have an effect on the frequency change was tried, and this did not seem to matter except

the three frequencies would all be similarly affected. For example, if the circuit were

altered so that the frequency would be slightly lower, all of the three stages would be

slightly lower in terms of voltage, and therefore the net change in voltage would not

change with respect to distance.

Overall, it was found that the circuit worked as a voltage-controlled oscillator, but

it worked as a very weak one. The voltage would vary but only in discrete stages. One

last attempt was made by putting the circuit in the box because it was believed that other

factors such as ambient light could have affected the output. However, when the circuit

was put in the box, which now included the filters, the signal was even more faint, yet

the three stage outputs seemed to become more continuous. It was later found that the

27

phototransistor only had a viewing angle of 16 degrees. Since the phototransistor was

inside of the hole and not sticking slightly outside of it, this could have explained the

much more faint signal. However, because of time constraints, modifications of the

black box and the filters to allow for the phototransistor to receive as much of the 16

degrees as possible could not be made so this theory could not be tested. However, the

device was shown to be operational and most of the original problems were resolved.

28

Addressing original potential problems:

There were certain potential problems that needed to be addressed when

constructing the transmitter and receiver portion of the circuit. First of all, the issue of

how to calibrate the circuit to the correct distance needed to be resolved. This was done

by choosing values for the resistors that would cause the sound to shut off when the

device was too close to an obstacle. Then this distance would be measured, and this was

the value for the resistors at a certain distance from the obstacle.

Another issue that needed to be resolved was whether or not the light was

rebounding back to the device. This was not a major issue, since infrared light is

invisible light and can be detected with the appropriate detector. Even at night, when it

is extremely dark, infrared light can be reliably utilized (as in night vision goggles),

which makes infrared light a very attractive waveform to use in this circuit design.

The final issue that needed to be resolved was how the infrared light would fare

on black objects. In theory, the color black absorbs all visible light, and reflects none of

the light back. However, the black only absorbs the visible light spectrum, which

infrared light does not fall under. Therefore, infrared can be used without diminished

effects.

Improvements:

Although the device was functional, there were many improvements that could

have been made if time and money allowed. Some of the major obstacles encountered in

designing, building and testing the device included focusing and filtering the IR signal

and supplying sufficient power to the components of the circuits. When the IR signal

29

strikes the surface of an obstacle, some of the light is scattered and the reflected signal is

not as strong . While nothing can be done to improve the IR emitter, a Fresnel lens can

be placed in front of the emitter in order to focus the beam. Also, an IR filter can be

placed in front of the phototransistor to decrease any interference by other types of light.

With the lens focusing the outgoing beam and the filter improving the reception of the

reflected beam, the signal into the non-inverting op-amp will be improved. This can also

be accomplished by using a more sensitive phototransistor. A longer lasting power

supply that weighed less would also be a great improvement. One could also try another

speaker to get a larger possible voltage range. Aesthetically, decreasing the size of the

device so that it is less noticeable and easier to carry would make the device

ergonomically superior.

30

Operational Specifications:

For a device as complicated as this, hours of testing, reconstructing, and re-testing

each component part are necessary. As a result, specific experimental values for each

part independently must be acquired so that they may be applied to the circuit as a whole.

In examining these components parts, the first portion to be tested was the

oscillator. The following values for the error in the resistors is based on the given 5%

error and is calculated for each resistor individually. The error in taking the frequency

readings off of the oscilloscope is half of the smallest division. The change in frequency

that this produced was the combined with the error in the frequency purely due to

inherent errors in the components. This procedure was carried out to determine the

optimum base frequency as well as examine the inherent errors in the oscillator.

Table 1: Theoretical vs. Experimental Frequency of oscillation for the Voltage-Regulated Oscillator

Resistor 1 Resistor 2 Theoretical Frequency

Experimental Frequency

% Error

100 kW ± 5000 1 kW ±50 14.1 Hz. 16.1 Hz. ± 4.7%

12.4

47 kW ±2350 1 kW ±50 29.4 33.3 ± 3.5 13.233 kW ±1650 1 kW ±50 41.1 50 ± 3.5 21.622 kW ±1100 1 kW ±50 60 83.3 ± 4.6 38.815 kW ±450 1 kW ±50 84.7 100 ± 5.5 18.010 kW ±500 1 kW ±50 120 153.8 ± 2.9 28.25.1 kW ±255 1 kW ±50 202 294.1 ± 3.6 45.61 kW ±50 1 kW ±50 480 555.6 ± 3.9 15.8470 W ±23.5 1 kW ±50 583 666.6 ± 4.2 14.3220 W ±11 1 kW ±50 648.7 714.3 ± 4.5 10.11 kW ±50 220 W ±11 1000 1333.3 ± 2.4 33.3470 W ±23.5 220 W ±11 1582.4 2083.3 ± 2.4 31.6220 W ±11 220 W ±11 2181.8 2631.6 ± 2.7 20.6

31

Assuming that the error in the resistor is 5% and the error in the capacitor is 10%,

the error for the experimental output can be calculated via the following differential

equation:

fR

R R CR

R R CC

R R C

144

22 88

2144

21

1 22

1

2

1 22

1

1

1 22

12

. *( )

. *( )

. *( )

This result can then be put into the following equation that takes into account both the

errors found above and the error from reading the frequency off of the oscilloscope. It is

calculated via the following equation:

totalerrorff

dOO

2 2

100*

where dO is the error of the reading from the oscilloscope and O is the actual value. The

R2 value measures how the data corresponds to the best fit line. A R2 equal to one is an

exact fit. The blue diamond shaped points represent theoretical data while the pink boxes

represent actual data. According to the following relationship between the variable

resistor and frequency,

frequencyR R C

14421 2 1

.( )

the data should behave logarithmically. The results show this is the case, with a high

degree of precision (The graphical program used for the two figures included in this

laboratory will not plot individual error bars. It was therefore necessary to average the

percent error and apply this as a representative for each. This average was 3.7%.

32

Unfortunately, we could not activate the Excel functions in Word. Therefore, we were

unable to add error bars to this graph.)

Figure 1: Theoretical vs. Experimental frequency variation due tovarying resistance at R2

y = -131.26Ln(x) + 1428.2R2 = 0.9656

-100

0

100

200

300

400

500

600

700

800

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

Resistance (ohms)

Frequency(Hz.)

The following data demonstrates the relationship between the voltage input to pin

5 and the output frequency as a result of this input. This was necessary in order to

examine the maximum voltages that would correspond to an audible sound output as well

as an aesthetically pleasing output. The error for the voltage measured via the

multimeter is ± 0.3% plus the last digit.

Table 2: Effects of input voltage on output frequency

33

Voltage Input to pin 5 Frequency output0 volts 0 Hz. 0.50 ± 0.025 01.00 ± 0.04 01.50 ± 0.055 02.00 ± 0.07 1041.7 ± 21.3 Hz.2.50 ± 0.085 1000 ± 19.63.00 ± 0.10 961.5 ± 18.13.50 ± 0.115 862.1 ± 14.64.00 ± 0.13 833.3 ± 13.64.50 ± 0.145 793.7 ± 12.55.00 ± 0.16 769.2 ± 11.65.50 ± 0.175 740.7 ± 10.86.00 ± 0.19 714.3 ± 10.16.50 ± 0.205 689.7 ± 9.47.00 ± 0.22 666.7 ± 8.87.50 ± 0.235 645.2 ± 8.38.00 ± 0.25 625 ± 7.78.50 ± 0.265 606.1 ± 7.39.00 ± 0.28 0

Figure 2: Frequency as a function of input voltage

y = -65.877x + 1127.9R2 = 0.9532

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7 8 9

Voltage input (V)

Frequency(Hz)

One can see that the graph of voltage vs. frequency is linear. This is helpful to

the experimentation not only because of the high degree of accuracy with which the

oscillator follows this linear relationship, but also because it becomes easier to determine

the frequencies that are desired by simply applying the correct gain to the non-inverting

34

op-amp. It should be noted that this graph was utilized primarily in the higher frequency

range because it was found that the higher frequencies produced a better quality sound

and would produce a more discernible frequency change. The table above shows that

there were a number of frequencies at which a frequency output is no longer produced.

This range was from 1.8 - 8.85 volts, or 588.2 - 1041.7 Hz. As well, there was a sound

range at which the speaker produced a clearer sound. Because of the quality of the

speaker, there was a limited range at which the speaker produced a clear, consistent

sound. This sound range was from 2.6 - 5.1 volts, or 769.2 - 1000 Hz.. This is important

because it shows that even with the oscillating circuit isolated from the rest of the circuit,

there was a limited voltage and frequency range at which optimum output characteristics

desired were displayed.

The clear data shown above was different when the full circuit was fully

integrated with all the components. The voltage range at which the speaker produced a

clean, acceptable frequency output diminished. At this stage, the circuit as a whole was

quite variable. The same circuit could be induced to return a certain base frequency but

it was found that the voltage at pin 5 varied little when the infrared light was obstructed

at closer distances. In fact, even though the phototransistor produced a maximum of a

0.3 Volt variation when separated from the circuit and tested in conjunction with the

infrared LED, the range of audible frequency outputs was diminished. This was despite

the fact that a non-inverting op-amp was placed after the phototransistor for the sole

purpose of amplifying the signal to obtain a wider voltage range entering pin 5. For

example, with the initial estimation of a gain of 22 (experimental testing verified the gain

was 22.2) a voltage range from 1.8 to 6.6 volts could theoretically be obtained. Even

35

though the phototransistor could drop to zero volts when no infrared light was coming in,

it was found that the low voltage is 1.8 because of the fact that it will not produce a

frequency output below this voltage at pin 5 (see table 2). This data, in conjunction with

the data on the optimum voltage range at pin 5 for the oscillator discussed earlier, should

have produced a frequency change much like that found in the earlier testing of the

oscillator alone. However, this was not the case.

When qualitative testing showed a very minor frequency change with the distance

of the obstruction, the qualitative assessment was made that the base frequency was too

low. This was based primarily on the earlier testing which showed that the speaker

performed better at higher frequencies, and experimental testing supported it. The base

frequency was increased from 464 to 1000Hz. The gain of the op-amp was then varied

as well until a sound was obtained that was clearer, corresponded to the better end of the

speakers performance, and was more aesthetically pleasing. This was all done

qualitatively because quantitative experimentation could only do so much as to give the

range at which the individual component parts worked, but it was the subjective ear that

determined where in this range the optimum results were produced for the circuit as a

whole.

At this point it was found that there was a very limited range at which the speaker

produced the type of sound that was being sought. More importantly, the frequency

range corresponding to the object reflection distance was very small except at very small

distances, such as six inches. A considerable effort was made to improve the accuracy

and capacity of the circuit to vary the frequency at higher distances, but as the table

36

below demonstrates, a discernible change in the frequency could only increase to a

maximum of 1.403 ft. ± 0.093.

Table 3: Maximum distance of frequency variation discernabilityTrial Number Reflection distance (in.) Reflection distance (ft.)1 16.00 ± 0.0625 1.332 16.25 ± 0.0625 1.353 17.00 ± 0.0625 1.424 16.50 ± 0.0625 1.385 17.89 ± 0.0625 1.496 19.00 ± 0.0625 1.587 15.13 ± 0.0625 1.268 16.00 ± 0.0625 1.339 17.13 ± 0.0625 1.4310 17.50 ± 0.0625 1.46

The above data was tested by having one of the experimenters stand with the

circuit outside of its housing and slowly walking towards a blue obstruction. When the

first noticeable frequency change to the observer occurred, he stopped and the distance

was measured.

37

Device Specifications:

The following table is a summary of all of the important components of the final

device.

Table 4: Device SpecificationsParameter RangeSupply Voltage 4.5 - 15 VLED Voltage range 1.9 - 5.9 VOscillator Base frequency 1000 Hz. Minimum Reflection Distance 1.403 ftOptimum Operating Voltage 2.6 - 5.1 VOptimum Operating Frequency 769.2 - 1000 Hz.On/Off Oscillation Range 1.8-8.8 VOn/Off Oscillation Frequency 588.2 - 1041.7 Hz. Circuit Gain 22.2Applied LED Forward Current 30 mAOperating Temperature 0° to 70° CEstimated Battery Replacement Time 2.67 days

The supply voltage is based on the individual component parts. The limiting

piece seemed to be the 555 operational amplifier, which is why the supply voltage range

of the circuit corresponds to the supply voltage range of the operational amplifier.

Note that the Optimum Operating Frequency is begun at the upper limit. Since an

increase in voltage will decrease the frequency of the impending sound (as shown by

figure 1) the problem of exceeding the On/Off Oscillation Range is removed. Because of

this, the circuit will always be in its operating range. This is because the gain is set so as

to limit the voltage going into pin 5 so that there is never too much voltage going into the

oscillation portion of the circuit so as to exceed the operating range. Since the functional

active operating range of the phototransistor is known, one could theoretically set the

gain so that the maximum voltage out of the phototransistor would correspond to the

38

maximum voltage of the Optimum Operating Voltage range. The forward current for the

LED was determined by finding the resistance of the variable resistor and using a voltage

input of 8.5 volts (since a battery was never found that could exceed this). The 30mA is

much less then is necessary to ensure that the current is below the maximum forward

current (which is 100mA as detailed in the appendix).

The operating temperature of the device is adapted from the specification sheets

of the circuit’s components. It is important to note that the operating temperature could

be higher than the ambient temperature for the device because it is enclosed in a black

box that absorbs electromagnetic radiation of the whole visible light spectrum. However,

there are holes in the back of the device that not only act to increase the volume of the

sound that reaches the users ears, but it also acts to cool the device.

The estimated battery replacement time was determined in the following way.

During testing it was noted that the circuit was supplied with the two nine volt batteries

for approximately a half-hour (not continuously, but over the course of the lab). During

that time, the batteries went from approximately 17 volts to 14 volts. This is a loss of 1.5

volts per battery. If the batteries were to begin at 8.5 volts and drop to 4.5 volts (this

being the lower limit of the supply voltage range), the person would have an estimated

operation time of 80 minutes for the device. Using an upper limit estimate for a highly

active visually impaired person of about 30 minutes of device activity per day, this would

result in an estimated battery replacement time of 2.67 days. This is why it would be

recommended that rechargeable batteries be used with this device because the cost of

batteries alone could exceed the cost of the device in the long run.

Appendix:

39

The following specifications are provided as supplementary as well as additional

information to both demonstrate the parameters portions of the circuit are restricted to,

and for those who may desire further modifications of the circuit.

The cycle repeats independent of the supply

voltage and the frequency is given by the

following equation:

frequencyR R C

144

21 2 1

.( )

The 555 Operational amplifierFunction Pin

numberGround 1Trigger 2Output 3Reset 4Control V 5Threshold 6Discharge 7Vcc 8

40

Note, this frequency is the frequency output of a basic astable circuit. The voltage-

controlled oscillator is extremely similar to the astable oscillator. In fact, the only

tangible difference in the two circuits is the addition of an input across pin 5 and ground.

It is this input voltage that controls the frequency of the output signal. The equation

above simply describes the base frequency at which the 555 op-amp will operate at

without application of a voltage to pin 5. The following parameters for the 555 op-amp

were adhered to for this circuit,

although the table shows that there is

some variability in the range of device

limitations. Further research might

include varying such things as the

supply voltage to examine exactly what voltage resulted in the optimum volume output.

Many of the limitations described above apply to the 741 operational amplifier as

well. However, the 741 operational amplifier was not used as part of an oscillating

circuit in the photometric device, but rather it was used as part of a non-inverting

amplifier.

The 555 Device SpecificationsSpecification RangeSupply Voltage (Vcc) 4.5 to 15 VSupply Current (Vcc = 5V) 3 to 6 mASupply Current (Vcc = 15V) 10 to 15 mAOutput Current (maximum) 200 mAPower Dissipation 600 mWOperating Temperature 0 to 70°C

The 741 Operational AmplifierFunction Pin numberOffset Null 1Input ( - ) 2Input ( + ) 3Negative Voltage Supply 4Offset Null 5Output 6Positive Voltage Supply 7Unused 8

41

Note that the 741 op-amp was initially used as a comparator, in which case the important

inputs to pay attention to are the negative and positive power supply inputs (pins 4 and 7)

because these inputs determine the voltage that input into the oscillator. It is also

important to note that the input voltage should not exceed the supply voltage. However,

this was not a consideration in this circuit because the gain occurred across the op-amp

and not before it. Had the gain occurred before the op-amp (for example, if another op-

amp preceded this one) then the input voltage could have exceeded the supply voltage,

especially as the batteries began to lose their charge. This is another reason the batteries

would have to be changed quite often. It is therefore recommended that rechargeable

batteries be supplied with this device because of the frequent necessity of restoring

maximum voltage supply to the circuit for a number of reasons.

Few of the operational

characteristics played an

integral of role in the design of

the device. However, some of

these limitations, such as the

maximum supply voltage, input

voltage, and operating

temperature are important to

keep in mind when testing and

using the device.

Operational Limitations and characteristicsMaximum Ratings Supply Voltage ± 18 VPower Dissipation 500 mWDifferential Input Voltage ± 30 VInput Voltage ± 15 VOperating Temperature 0 to 70° CCharacteristics (typical)Input offset voltage 2 to 6 mVInput resistance 0.3 to 2 MWVoltage Gain 20,000 to 200,000Common-mode rejection ratio 70 to 90 dBBandwidth 0.5 to 1.5 MHzSlew Rate 0.5 V/msecSupply Current 1.7 to 2.8 mAPower Consumption 50 to 85 mW

42

The specifications below for the NPN silicon phototransistor and the Infrared

LED have been selectively taken from the specification sheet provided by the supplier.

If further specifications are desired one can consult the BE310 Laboratory Manual.

There are a couple of things to note about these characteristics below.

First, it is important to note that if any soldering is to be done on the phototransistor then

it should be applied for no longer then five seconds. The experimenter can also provide a

heat sink to redirect the heat from the phototransistor. Also, since the viewing angle of

the phototransistor is only 16 degrees, this is going to have a significant impact on the

phototransistor’s ability to receive the infrared light emitted by the LED. Because of

this, it is necessary to place the phototransistor so that it slightly emerges from the black

box in order to utilize as much of the receiving angle as possible. This was not done in

experimentation, and would be a further modification of the circuit. As well, since there

is such a small viewing angle, if the LED were to emit light and that light was to fall on a

surface that would diffract the light, then a small amount of it would return to the

phototransistor. This is why it is necessary to have the maximum amount of light

emitted from the LED, maybe even place two of them in the device.

If the infrared emitter were put in parallel with another they could emit the same

strength signal, resulting in a higher intensity of light received by the

NPN Silicon Phototransistor Parameter Maximum RatingsCollector-Emitter Voltage 30 VEmitter-Collector Voltage 5 VOperating Temperature -50°C to 100°C Soldering Temperature 260°C for 5 secondsPeak emission wavelength 940 nmViewing angle 16 °

43

phototransistor. Also, note that

the longer end of the emitter is

the cathode so the anode should

point in the direction of current

flow. This is important because

diodes are nonlinear devices and

only operate properly when there is a forward voltage drop.

TLN110 Infrared EmitterCharacteristic Maximum RatingForward Current 100 mAPulse Forward Current 1 AReverse Voltage 5 VDiode Power Dissipation 150 mWOperating Temperature range -20° to 75°CForward Voltage 1.5 VPeak Emission Wavelength 940 nm

44

Pictures:

45

References:

Hughes, Fredrick W. Illustrated Guidebook to Electronic Devices and Circuits. Prentice-Hall, Inc. Englewood Cliffs. 1983. p 101-104

Le Grand, Yves. Light, Colour and Vision. Chapman and Hall Ltd. London. 1968, pg. 75.

Mims, Forrest M. Engineer’s Mini-Notebook: 555 Timer IC Circuits. Siliconcepts, 1984. pp. 4-7, 15.

Mims, Forrest M. Engineer’s Mini-Notebook: Formulas, Tables and Basic Circuits. Siliconcepts, 1988, p. 37.

Mims, Forrest M. Engineer’s Mini-Notebook: Op-amp IC Circuits. Siliconcepts, 1985. pp. 8, 12-13.

Mims, Forrest M. Engineer’s Mini-Notebook: Optoelectronics Circuits. Siliconcepts, 1986. pp. 6-11, 19-21, 26-27, 38-39.

Tooley, Michael. Electronic Circuits Handbook. Butterworth-Heineman Ltd. Oxford.1993. p 185-195

Vanzetti, Riccardo. Practical Applications of Infrared Techniques. New York: John Wiley and Sons, 1972.

46