Stethoscopes are associated primarily with medical devices used to diagnose diseases based on the noise of the lungs and heart. However, in addition to this, they find great application in other areas, for example, they are used by auto mechanics to listen to extraneous sounds when the engine is running. And if the stethoscope has great sensitivity, then, by fixing its sensor on the wall, it will be easy to hear the conversation of people on the other side of the wall – however, it is worth remembering that eavesdropping on other people’s conversations is prohibited, and simply immoral. The presented stethoscope scheme has a high gain, which provides good sensitivity, especially with a good sensor.
Description of the circuit
In the first two stages of the stethoscope, a low-noise operational amplifier OPA350 is used, which is very important, because the extra noise in the audio path will make the useful signal less intelligible. The OPA350 indicated on the diagram is quite expensive, you can replace it with a cheap and affordable NE5532, which is also considered low-noise, albeit with a deterioration in parameters – that’s exactly what I did. The gain is determined by the value of the resistor R18 in the feedback circuit of the amplifier, the 10 MΩ rating may even be excessive; in the event of self-excitation or simply excessive sensitivity, it can be reduced.
At the output of the operational amplifier there is a variable resistor – a volume control. Practice has shown that the required volume is achieved even at a small angle of rotation of the control from the minimum. Next, the diagram is followed by the frequency response control unit and the corresponding regulator, with its help you can correct the frequency response of the signal in the high frequency region, which in some cases will help to make the received signal more intelligible. On the last transistor, a follower is assembled, which switches the headphones, shown in the diagram in the form of a dynamic head. The output of the circuit is mono, and the headphones have an input for the right and left channels, they are simply connected in parallel. It is pointless to use a circuit with a speaker connected at the output, since it will not be possible to get rid of acoustic feedback, given the very high gain of the circuit. Sometimes the acoustic connection even occurs with the headphones, if they are close to the sensor.
The supply voltage of the circuit is 9..12 volts, while it is important that the power source is as clean as possible and does not give pulsations at the output, since they can easily get into the sound signal of the stethoscope. Therefore, a good source, in this case, will be a 9 volt battery, given the low current consumption of the circuit.
You can download the board here:
In the diagram, the sensor is designated “Micro1” and is located in the leftmost part of it. A piezoelectric plate will be used as a sensor – the most common squeaker found in many toys, multimeters and other electronics. Its brass substrate is soldered to the ground of the circuit, and the piezoelectric crystal itself is soldered to the input, you should be careful when soldering piezoelectric plates, as they lose their properties from heating. Different piezoplates can give different results in sensitivity, good results were shown by large specimens (diameter 4 cm), also, according to some information, excellent sensitivity is given by domestic ZP-3 and others from this series. A piezo plate can be used naked, gluing it with double-sided tape to the object to be listened to, or make a full-fledged sensor out of it by mounting it in the body and making a spring-loaded pickup stylus – this will give a good increase in sensitivity. The piezoplate is soldered to the input of the circuit with a flexible shielded wire, I use MGTPE for this.
Assembling the structure
The stethoscope is assembled on a fairly spacious printed circuit board, which will be attached to the archive. The printed circuit board assumes the use of output elements, in addition to the operational amplifier – it is in an SMD package.
The printed circuit board was carried out using the usual LUT method, which has been repeatedly described on the Internet. Variable resistors are soldered directly to the board, next to them there are 3.5 mm jacks for connecting a headphone plug (output) and a piezo plate (input), power is supplied via two wires. The assembled board without a case will be good at catching interference, especially when approaching the network wires. To avoid this, the board can be placed in a metal case, which is connected to the ground of the circuit, only the shielded wire to the sensor can be brought out. The assembled stethoscope has really good characteristics, with its help you can clearly hear the conversation of people who are through a thin wall, even when using a primitive sensor.