Here the cheap hearing aid to help people who have hearing loss. Commercially available assistive hearing devices are quite expensive. This is a cheap hearing aid device circuit which works by using just four transistors and several passive electronic components.
On shifting on / off switch S to “on” position, the condenser mic detects the actual sound signal, that is amplified by transistors T1 and also T2. The amplified signal passes through coupling capacitor C3 towards the base of transistor T3. The signal is additionally amplified by PNP transistor T4 to drive a low impedance earphone. Capacitors C4 and C5 are the power source decoupling capacitors.
The circuit can be simply constructed on a small, general-purpose Printed Circuit Board as well as a Vero board. It works off a 3V DC supply. For this purpose, you can utilize two small 1.5V battery. Always keep switch S to “off” condition when the circuit is not being used. To improve the level of sensitivity of the condenser mic, house it inside a small tube or pipe.