2012Heart Rate MON Acoustic

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University of Massachusetts at Amherst

Department of Electrical and Computer Engineering

ECE 324 Electronics II Spring 2012

Design Project

Heart Rate Monitor

Goal: Design the electronics necessary to monitor heart rate. The heart rate will be

determined by processing the electrical output from a microphone connected to part of a

stethoscope. Your measurement must be updated every 15 seconds and must display the

number of beats per minute on a two digit LED display. This value should be displayed

for 15 seconds until the next measurement update is available. The monitor must becapable of measuring heart rates from 30 to 99 beats per minute. A speaker will beep at

each heart beat.

Overview

A block diagram of the monitor is shown in Figure 1. The stethoscope is placed

against the chest near the heart. The sound it picks up is transmitted up a plastic tube to a

miniature microphone pick up. The resulting signal is then amplified, filtered, and

transformed into a clean TTL pulse train. Counting electronics converts the pulse train

into a digital word which represents the number of heart beats per minute. The last stage

converts the word to a decimal number and drives the display.

The Microphone/Stethoscope Assembly: Everyone has had a doctor listen to their heart

using a stethoscope. The stethoscope conducts sound from the patient's chest through

a tube up to the doctor's hears. In this project, a small electronic condenser

microphone replaces the doctors ears and converts the sound in the tube to an

electrical signal. This microphone is the type often used in telephone answering

machines and has inside a small IC preamplifier which must be biased by an external

source.

Seven Segment LED Display:Each digit in the MAN74 display consists of seven light

emitting diodes, one for each segment of the digit. To light a segment you forward

bias the proper diode. To form each one of the numerals; 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 you

must light the proper collection of segments. The BCD-to-7 Segment Decoder Driver

IC will do the necessary coding for you.


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Design Discussion

A suggested design procedure is as follows.

1) Divide the project into parts. If, for example, three engineers are involved, the project

could be divided into three subsystems with each engineer responsible for one

subsystem. One subsystem might be the microphone amplifier/filter/waveshaper.

Another subsystem could be the counting electronics. A third could be the tone

generator electronics. Before starting, you must decide on the specifications necessary

for each part. It is likely that these specifications will change during the course of the

design project and so frequent communication will be necessary. As with most other

projects, each team member must fully understanding the overall concept or the end

result will not be as good, as cheap, or as quickly achieved.

2) You should review the spec. sheet for the microphone in this assembly and determine

how it should be biased. You will see that the manufacturer suggests a resistance(RL) of 2.2 Kohm connected to a voltage between 2.5 and 10 volts. There are two

wires connected , one to connect to RL and one to connect to ground. You must be

careful not to get them switched. After you have connected the bias, use an

oscilloscope set on a slow sweep speed to monitor the output voltage. If you blow or

tap on the end of the stethoscope, you should see a response on the oscilloscope. If

you place the stethoscope on your chest near your heart and turn up the sensitivity of

the oscilloscope, you should just be able to see pulses that correspond to your heart

beating. This is the signal that must be amplified, filtered, and converted to a digital

pulse train for the counting electronics. This can be tricky because the signal is quite

weak and may contain unwanted noise which may need to be filtered out. Usually

each heart beat will consist of a two pulses (lub-DUB) and you will need to insure

that only one beat is counted.

3) The counting electronics is another complicated part of the problem. You must count

the number of beats in some interval under 15 seconds and use this to determine the

number of beats that would take place one minute. Counters plus a precise timer or

oscillator may be useful for this section.

4) The tone generator would consist of an oscillator to generate the tone, a pulsegenerator to constrain the length of the beep, and circuitry to drive the speaker.

5) All members of the team should cooperate to minimize the overall part count. For

example, possibly a clock signal used in the timing circuitry could also be used to

generate the beep.


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6) Your final report should have; brief discussions of the design of each subsystem, a

neat drawing of the overall design, a parts list and itemized cost, and verification data

showing that the design does indeed meet the specifications.

Specialized Parts1 Stethoscope/microphone assembly

(Stethoscope made by Lumiscope Co. Edison N.J.; Electret

condenser Microphone Cartridge #WM54B)

Estimated cost: Materials $10.00

Assembly $20.00

Total $30.00

Documents

2012Heart Rate MON Acoustic