Telephone dialer circuit.

In this portion of telephone dialer circuit we use the telephone dialer circuit. For

this we use one dtmf generator circuit to generate a dtmf tones from the circuit. For

this purpose we use one DTMF generator circuit to generate a dtmf tones from the

circuit. We connect one telephone keypad or push to on switches with this circuit.

We use total 14 push to on switches with this circuit to dial the dtmf tones. Once

we dial the number in the dialer circuit. Now when micrcontroller press the redial

switch then circuit is on and redial the pre-dial number .

All the switches are connected to the pin no no 12,13,14,15,16,17,18. All the

switches are connected in row and matrix combination including redial and flash

switch

DTMF LOGIC

In DTMF there are 16 distinct tones. Each tone is the sum of two frequencies: one from a low and one from a high frequency group. There are four different frequencies in each group.

Your phone only uses 12 of the possible 16 tones. If you look at your phone, there are only 4 rows (R1, R2, R3 and R4) and 3 columns (C1, C2 and C3). The rows and columns select frequencies from the low and high frequency group respectively. The exact value of the frequencies are listed in Table 3 below:

TABLE 3: DTMF Row/Column Frequencies

LOW-FREQUENCIES

ROW # FREQUENCY (HZ)

R1: ROW 0 697

R2: ROW 1 770

R3: ROW 2 852

R4: ROW 3 941

HIGH-FREQUENCIES

COL # FREQUENCY (HZ)

C1: COL 0 1209

C2: COL 1 1336

C3: COL 2 1477

C4: COL 3 1633

C4 not used in phones

Thus to decipher what tone frequency is associated with a particular key, look at your phone again. Each key is specified by its row and column locations. For example the "2" key is row 0 (R1) and column 1 (C2). Thus using the above table, "2" has a frequency of 770 + 1336 = 2106 Hz The "9" is row 2 (R3) and column 2 (C3) and has a frequency of 852 + 1477 = 2329 Hz.

The following graph is a captured screen from an oscilloscope. It is a plot of the tone frequency for the "1" key:

You can see that the DTMF generated signal is very distinct and clear. The horizontal axis is in samples. The frequency of the tone is about 1900 Hz - close to the 1906 Hz predicted by Table 3 (697+1209).

CIRCUIT DIAGRAM USE IN THIS PROJECT FOR TELEPHONE DIALLER IS

GIVEN BELOW. In this circuit we use ic 91215 as adtmf generator. Pin no

15,16,17,18 is a row and pin no 12,13,14 is a row. All the switches are connected

to these pins. Redial button and flash button is connected to the microcontroller

output pin s no 13 and 12 . Output from the 91215 is available on the pin no 7 of

the ic . Output from the 91215 is further amplify by the ic 741. Here pin no 7 of

the ic 741 is connected to the positive supply and pin no 4 is connected to the

negative supply. Pin no 6 is the output pin and amplify signal is available on this

pin and again further amplify by the two transistor circuit. Output from the

transistor circuit is connected to the one coil of the output transformer.

Now when we press a redial number then 91215 generate a pre-dial tones and this

signal is further connected to the coil and further pass on the telephone line.

Voice processor circuit.

ISD produces a series of very handy single-chip voice record/playback devices with record/playback durations from 16 to 120 seconds. In this workshop, we will use the ISD1420 chip, a 20 second record/playback device. Interesting facts from the ISD1420 datasheet include the chip's 100 year message retention and its 100,000 record cycles.

Connecting the ISD1420 is rather straightforward. Below you see a circuit diagram for a very basic application example. Vcc in all of the examples shown on this page is 5V. Please make sure to use your 7805 Voltage regulators!

Note that this circuit has three pushbuttons: one for recording, one for playback on edge detection and one for playback on level detection. Since we don't have three pushbuttons in our lab kit, we will use simple wire bridges to simulate the buttons (to simulate a pushed button we connect the wire bridge to ground). The recording button does exactly what it says: pushing it allows you to record sound. Releasing the button stops the recording. If the end of memory (EOM) of the chip is reached before the button is released, the chip will automatically terminate the recording and go to the power down mode. There is also a red recording indicator LED. This LED lights up when you record sounds. Sounds can be recorded in two different ways: using the microphone as seen in the above circuit example - or using a direct input from e.g. your stereo system's cd-player which is connected to ISD1420's

analog input pin. Unfortunately, this is quite tricky - the following application note shows you why. In most cases, if you want to record CD sound, holding the microphone close to your stereo's speaker will be a faster solution with only a little quality payoff.

There are two versions of play buttons: an edge-activated (PLAYE) and a level-activated (PLAYL) button. PLAYE starts the playback upon edge detection. Edge detection in this case means a change in Voltage from HIGH to LOW or from a LOW to a HIGH state. This allows you to use it like a toggle switch. The level-activated play function triggers playback when a change from a HIGH state to a LOW state is detected. Accordingly, it stops playback once the pin is pulled HIGH from a LOW state.

The above circuit only show the most basic functionality of the chip. In addition, much more features can be used. Using the address inputs (A0-A7), i.e. pins 1-6 and 9/10 or the operational modes (using the same pins but with differnt status bytes - pin 9 and 10 pulled HIGH), multiple sounds can be recorded and played back. Please take a look at the ISD1400 datasheet to learn more about the use of the address inputs and operational modes (see pages 11/12). The following application note illustrates how to use the ISD1420 to record and play back multiple messages. If you consider using this feature of the chip, replace the lo-tech wire bridges for S1-S3 with real push button switches.

In the following we will look at one more advanced example that includes the ISD1420 operational modes. This example demonstrates a very useful feature of the ISD1420 chip: looping. Looping is a feature that can be found in the ISD1420's operational modes. Looking at the datasheet for the ISD1420 we see that we have to change the two most significant bits (MSB) on the chip from LOW to HIGH in order to access the operational modes. The two MSB are pin 9 and 10, so instead of having them go to GND we pull them HIGH (to Vcc). In the second step, we locate the address pin that activates the looping feature in the operational mode. This is pin 4 (A3). We also change this pin's connection from GND to Vcc. The corresponding circuit diagram can be found below (note that the only change that makes this circuit

different from the one above is the connections from pin 4,9 and 10). If you want your circuit to go into the looping mode automatically after powering up, you should take a look at the following application note. More advanced circuit examples on how to use the ISD1420 can be found at http://www.isd.com/products/isd_products/chipcorder/.

A final word on sound output from the ISD1420. The chip itself has a weak on-chip amplifier. This is why the output seems not very loud. ISD provides an application note that describes how to connect the ISD1420 to an audio amlifier circuit. Also, the speaker output for the ISD1420 is designed for the use with a 16Ω speaker. However, our kit includes only an 8Ω speaker. Normally, you shouldn't replace a 16Ω speaker with a 8Ω speaker. Instead you should rather connect a 8Ω resistor in series with the 8Ω speaker to increase the total resistance of the load to 16Ω. For testing purposes we can temporarily use the 8Ω speaker with this circuit. If you want to use the circuit for a more permanent installation, consider using one of the above mentioned strategies to connect a speaker.

5