The hearing aid circuit available in the market are fully transistorized and generally use a single pen cell. The circuit hearing aid uses an IC LM358 (IC1) for the same purpose which give better sensitive and performance.
Operational amplifiers are used here in their simplest from non-inverting AC amplifier with a gain of 100. Op-amp A1 forms the first-stage amplifier. Second-stage amplification is given through op-amp A2. Hence the total gain is 10,000.
Resistor R1 and R2 provide biasing to the condenser microphone, and capacitor C1 is used for coupling. Finally, transistor T1 drives the earphones.
The circuit hearing aid is small enough to fit into a match-box as it uses only one IC and very few discrete components. It is very sensitive and can pick even single weak signals from a circular area of over ten meters diameter. The circuit hearing aid can also work as a baby-phone by using a speaker instead of an ear-phone.
To reduce the hissing noise the feedback resistor R8 and R9 value may be reduced from 1 MΩ to 680 KΩ.
Note: In case a microphone other than a condenser type is being used, the reader may have to experiment with change of the coupling capacitor as well, whose value may not exceed 10 µF, 10V in any case.
PARTS LIST OF HEARING AID
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 560 Ω
R2 – R5 = 10 KΩ
R6, R7 = 100 KΩ
R8, R9 = 1 MΩ
C1, c2 = 0.1 µF
C3 = 0.01 µF
IC1 = LM358
T1 = BC547
SW1 = On-Off Switch
B1 = 3V Battery
MIC1 = 34K8P Condenser Microphone