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An Electric Stethoscope

Need to do a bit of self-diagnosis? Make sure your heart is still beating or check other body sounds? Maybe you would like to sort out some unusual rattles or other noises in your car's engine or other machinery? This electronic stethoscope will do the job ? and you can listen via headphones or a loudspeaker. It has switchable frequency shaping in four bands so you can hone in on sounds which might otherwise be masked out.

By John Clarke

Features

Portable
Battery supply
Volume control
Boost or cut control
Selectable tone frequency
Reverse supply protection
Power and battery condition indicator

SPECIFICATIONS

Supply voltage:........9V @ 12mA quiescent current

Battery life:............Typically 30 hours
         (with alkaline battery)

Selectable bands:........63Hz, 250Hz, 1kHz and 4kHz

Boost or cut range:........±15dB

So why have an electronic stethoscope when a traditional cheap and cheerful stethoscope might be all you need?

Well, a conventional stethoscope is OK if you have keen hearing and you are listening in a quiet environment but its sound levels are quite low, particularly at low frequencies.

Secondly, on a cheap stethoscope there is no way of tailoring the frequency response of sound heard at the earpiece (apart from choosing the diaphragm or bell on the chestpiece of a medical stethoscope).

The SILICON CHIP Electronic Stethoscope has plenty of gain – you can adjust the volume level to suit and you can use switchable filtering to cut or boost a particular band of frequencies. As well, it can be connected to headphones or a loudspeaker, in which case more than one person can hear the sounds.

If you wanted to, you could record the monitored sounds and display the waveforms on computer screen.

Our Electronic Stethoscope comprises a chestpiece (sound pickup) that connects to a small amplifier box via a shielded cable. It has a headphone socket, knobs for volume and equaliser (EQ) and switches for power and for frequency band.

The equaliser provides boost or cut in the frequency band selected by the 4-position slide switch.

These bands are centred on 63Hz, 250Hz, 1kHz and 4kHz and are labelled Low, Mid1, Mid2 and High respectively.

To simulate the bell sensor (of a medical stethoscope) where low frequency sounds are more prominent, the Mid2 band can be selected and the equaliser control set for an amount of signal cut. Or the low band could be selected with the equaliser control set in the boost position. To simulate the chestpiece diaphragm, the high band can be selected and the “EQ” pot set to the cut position.

Alternatively, any one of the bands can be selected by the switch and the equaliser pot can be set to boost or cut. Boosting the frequency band selected will make more prominent any sounds of interest within that band.

Conversely, the cut position will remove prominent sounds in that band that may otherwise mask out the sounds of interest.

The chestpiece is adapted from a low cost stethoscope but with a piezo transducer fitted inside.

For use with car engines or other machinery, the chestpiece is further modified to provide a more direct contact with the piezo transducer element.

Click for larger image
Fig. 1: the circuit is based on two low-cost ICs – an amplifier (IC1a), buffer (IC1b), frequency band selection (IC1c and d) and finally, a power amplifier capable of driving a set of headphones or ear buds (IC2). It’s all powered by a 9V battery.

Circuit details

The Electronic Stethoscope is based on two low-cost ICs: a TL074 quad op amp IC and an LM386 power amplifier IC. The op amps are used for amplification and filtering of the signal from the piezo element in the chestpiece. The power amplifier drives the headphones or a loudspeaker. The full circuit is shown in Fig.1.

Signal from the piezo element is applied via CON1, 3.5mm socket and a 100nF capacitor to op amp IC1c. The associated 33pF capacitor and 10Ωresistor attenuate high frequencies and thereby reduce the possibility of picking up radio signals. IC1c is biased at +4.3V via the 1MΩresistor connected to the 10kΩvoltage divider resistors across the 8.6V supply. The 1MΩresistor also sets the input impedance of the amplifier.

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