A Multipurpose Listening Device
An electric microphone is used to pick up sounds. The advantages of using it are its excellent frequency range and high sensitivity. Four transistors and a few resistors and capacitors are required for fabricating the circuit.
The circuit operates off two 1.5V AA-size cells. Transistors T1 and T2 amplify sounds picked up by the microphone. Transistor T3 with resistor R2 (1-mega-ohm) in feedback path ensures a constant level. This is necessary because faint sounds should be boosted by the device and strong sounds should be attenuated to a safe level.
Earphones of the types used with Walkman and cellphones (32-ohm impedance) are used for listening.
Wire the stereo headphone socket such that both the right and left earphones are connected to the circuit. Multi-turn trimmer VR1 (10-kilo-ohm) is used to control the sensitivity of the microphone.
The article was first published in June 2006 and has recently been updated.