This circuit can detect very faint, remote sounds with a good clarity. It is useful in large conference halls, auditoria, cinema halls, lecture rooms in colleges, etc. The circuit can be housed in a small plastic box and kept in a shirt pocket. It is especially useful for watching TV programmes at a low volume so as not to disturb other family members in the house.
An electret 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. Multiturn trimmer VR1 (10-kilo-ohm) is used to control the sensitivity of the microphone.