In order to listen to your heartbeat, you would normally use a listening tube or stethoscope. This circuit uses a piezo sounder from a musical greetings card or melody generator, as a microphone. This transducer has an output signal in the order of 100 mV and its low-frequency response is governed by the input impedance of the amplifier. For this reason, we have chosen to use an emitter follower transistor amplifier. This has a high input impedance and ensures that the transducer will have a very low-frequency response. At the output, you just need to connect a set of low impedance headphones to be able to listen to your heartbeat. Replacing the emitter follower with a Darlington transistor configuration will further increase the input impedance of the amplifier.