Hearing Aid Circuit Based 4 Transistor

The hearing aids that are available have to be purchased at a high cost. But if you’re creative, you can make your own inexpensively and only use four transistors. The results obtained from this series are very good for helping to hear.

Circuit Diagram

hearing aid circuit


  • 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 = 2×1.5V Cells
  • SW1 = On/Off-Switch

Circuit Operation

when switching S1 to the ON position, the condenser microphone will detect sound signals from the surroundings (omnidirectional). The signal from this microphone is with Q1 and Q2. Now the amplified signal passes through the coupling capacitor C3 to the base of Q3.

The signal amplification is then carried out by Q4 to be directly connected to low-impedance earphones. Capacitors C4 and C5 work as power supply decoupling. The circuit is so simple that it can fit on a small PCB.

This series of hearing aids can operate properly at a voltage of 3 Volts. If the circuit is not used for a long period of time, then switch S1 must be in the OFF state, otherwise the battery will not run out by itself.