Commercially available hearing aids are quite costly.Here is an inexpensie hearing aid circuit that
uses just four transistors and a few passive components.On moving power switch S to "ON" position,
The condenser microphone detects the sound signal,which is amplified by transistor T1 and T2.Now the
amplified signal passes thorough coupling capacitor C3 to the base of transistor T3.The signal is further
amplified by pnp transistor T4 tp drive a low impedence earphone.
Capacitor C4 and C5 are the power supply decoupling capacitor.The circuit can be easily assembled
on a small ,general purpose PCB or a vero board.It operates off a 3v DC supply.For this,you may
use two small 1.5V cells.Keep switch S to "OFF" state when the circuit is not in use.To increase the
sensitivity of the condensor microphone,house it inside a small tube.