Normally, the hearing aid circuits consume battery power continuously once they are turned on. This project is designed to save battery power by turning on the sound amplifier section only when the sound is detected. The sensitivity of the detection section and the duration of the activation time of the sound amplifier circuit can be adjusted by the user. Also the circuit uses only a single condenser microphone for the detection and amplification of the sound. This headset design consists of a condenser microphone, headphones and sections for detection and sound amplification. The sound detection section employs a quadruple operating amplifier and a timer. The sound signal received in the microphone is pre-amplified by the transistor BC549. The voltage in its collector is fed to the op-amp inverter terminal, which is used as a comparator. The reference voltage (Vref) on the non-inverting terminal is set via a preset. The preset is also used to control the sensitivity of the sound signals received by the circuit. The output is fed to the trigger input of the timer, which is set to monostable mode. When sufficient sound signal strength is detected at the base of the transistor, the Opamp is triggered.
The sound amplifier is designed with NPN transistors. The sound signal received from the microphone is fed to the non-inverting pin of the second operational amplifier that is wired in drive follower configuration. The drive follower mode solves the impedance mismatch problem that would have occurred if the microphone output is fed directly to the amplifier stage. The output is fed to the base of the transistor. The weak signal received in the transistor is further amplified by another amplifier stage. A headset to listen to the sound is connected to the output. With 9V DC power, when sound is detected through the microphone, the amplifier section is automatically activated and the circuit current consumption is approximately 96 mA. When the amplifier circuit is 'off', the circuit draws a current of approximately 6 mA only, thus saving a considerable amount of battery power.