Mic Amplifier

Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components.

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Circuit Diagram:

Hearing Aid Circuit Diagram

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Parts:

R1 = 2.2K

R2 = 680K

R3 = 3.3k

R4 = 220K

R5 = 1.5K

R6 = 220R

R7 = 100K

R8 = 680K

C1 = 104pF

C2 = 104pF

C3 = 1uF/10V

C4 = 100uF/10V

C5 = 100uF/10V

Q1 = BC549

Q2 = BC548

Q3 = BC548

Q4 = BC558

J1 = Headphone jack

B1 = 2x1.5V Cells

SW1 = On/Off-Switch

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Circuit Operation:

On moving power switch SW1 to ‘on’ position, the condenser microphone detects the sound signal, which is amplified by Q1 and Q2. Now the amplified signal passes through coupling capacitor C3 to the base of Q3.

The signal is further amplified by Q4 to drive a low impedance earphone. Capacitors C4 and C5 are the power supply decoupling capacitors. The circuit can be easily assembled on a small, general-purpose PCB or a Vero board.

It operates off a 3V DC supply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the sensitivity of the condenser microphone, house it inside a small tube.