Multipurpose Listening Device

Ashok K. Doctor

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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.

Fig. 1: Circuit of multipurpose listening device
Fig. 1: Circuit of multipurpose listening device

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.

Fig. 2: Pinconfiguration ofBC548 and BC337
Fig. 2: Pinconfiguration ofBC548 and BC337

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.


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