The hearing aid circuit available in the market are fully transistorized and generally use a single pen cell. The hearing aid circuit uses an IC LM358 (IC1) for the same purpose which give better sensitive and performance. You can also check DIY Hearing Aid Circuit using 555.
Circuit Description of Hearing Aid Circuit
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.
Resistors 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 CIRCUIT
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Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise) |
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R1 = 560 Ω R2 – R5 = 10 KΩ R6, R7 = 100 KΩ R8, R9 = 1 MΩ |
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Capacitors |
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C1, c2  = 0.1 µF (Ceramic Disc) C3 = 0.01 µF (Ceramic Disc) |
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Semiconductors |
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IC1 = LM358 (Dual Operational Amplifiers) T1 = BC547 (General Purpose NPN Bipolar Junction Transistor) |
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Miscellaneous |
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SW1 = On-Off Switch B1 = 3V Battery MIC1 = 34K8P Condenser Microphone Ear phones |
excellent, well chosen schematics
How can i remove the hum sound in the hearing aid circuit? I’m only using an electret mic.
How can i improve the range of sound, heard by the mic? I use electret mic.
I am trying to simulate this circuit using pspice, what can I put in place of the microphone? And also, regarding A1 and A2, is +Vcc connected to 3v and -Vcc to ground? Just making sure, thanks.