Normally, hearing aid circuitsconsume battery powercontinuously once they areswitched on. The circuit given heresaves battery power by switchingon the sound amplifier section only when sound is detected. The sensitivityof the detection section and the‘on’ time duration of the sound amplifiercircuit can be set by the user.Also the circuit uses only a singlecondenser mic for sound detectionand amplification.As is clear from the above, thishearing aid consists of a condensermicrophone, earphone, and sounddetection and amplification sections.The sound detection section employsa quad op-amp IC LM324 (IC1(A)) anda timer NE555 (IC2). The sound signalreceived at the mic is pre-amplifiedby transistor BC549 (T1). The voltageat its collector is fed to the invertingterminal (pin 2) of op-amp IC1(A) which is used as a comparator. Thereference voltage (Vref) at the noninvertingterminal (pin 3) of IC1(A)is set using preset VR1. The preset isalso used to control the sensitivity ofthe sound signals received by the circuit.The output from pin 1 of IC1(A)is fed to the trigger input (pin 2) oftimer NE555, which is configured inmonostable mode.When sufficient sound signalstrength is detected at the base of transistorT1, the pulsating voltage at itscollector exceeds the reference voltageat pin 3. As a result, output pin 1 ofIC1(A) goes low. The low output fromIC1(A) triggers the NE555 timer and itsoutput goes high for a preset duration.R4 and C2 are the timing componentsfor setting the time duration. The highoutput of the timer is directly used asthe power source for the sound amplifiersection.The sound amplifier section is builtaround transistors T2 through T5. Thelast amplifier stage T5 (pnp transistorBC558) drives the earphone. The soundsignal received from the mic is fed tothe non-inverting pin of the secondop-amp of IC1(B) which is wired inunity follower configuration. The unityfollower mode resolves the problemof impedance mismatch which wouldhave occured if the output of the micis fed directly to amplifier stage. Theoutput from pin 7 of IC1(B) is fed to thebase of transistor T2. The weak signalreceived at transistor stage T2 is furtheramplified by transistors T3, T4 and T5.An earphone to listen to the sound isconnected between the collector of T5and ground. It is recommended to usea mono earphone with volume controlattached.With 9V DC supply, when sound isdetected through the mic, the amplifiersection is automatically triggered andthe current consumption of the circuitis about 96 mA. When the amplifier circuitis ‘off,’ the circuit draws a currentof about 6 mA only, thus saving considerableamount of battery power

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