A Simple Hearing Aid
Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components. Hearing Aid Circuit Diagram 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 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 wh...