This electronic hearing aid circuit consists of a condenser microphone, earphone, and sound detection and amplification sections. The block of sound detection section used a quad op-amp IC LM324 (IC1(A)) and a timer NE555 (IC2). The sound signal received at the condenser mic is pre-amplified by transistor BC549 (T1). The voltage at its collector is fed to the inverting terminal (pin 2) of op-amp IC1(A), which is utilized as a comparator. The reference voltage (Vref) at the non-inverting terminal (pin 3) of IC1(A) is set with preset VR1. The preset is also used to control the sensitivity of the sound signals received by the circuit. The output from pin 1 of IC1(A) is fed to the trigger input (pin 2) of timer NE555, which is configured in monostable mode.
When sufficient sound signal strength is detected at the base of transistor T1, the pulsating voltage at its collector exceeds the reference voltage at pin 3. As a result, output pin 1 of IC1(A) goes low. The low output from IC1(A) triggers the NE555 timer and its output goes high for a preset duration. R4 and C2 are the timing components for setting the time duration. The high output of the timer is directly used as the power source for the sound amplifier section.
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