The circuit describes here is used to amplify the weak signals so that it can be heard properly. Many people lose there their hearing power because of old age or due to hearing loud music or because of any another reason. So to amplify the weak signals they can use this circuit which is used to enhance the weak signals as similar to common audio amplifier. Advantage of this circuit is high sensitivity, it consumes low current and uses commonly available components and light weight.
This circuit is based on single IC that is LM386 with few more components. LM386 is a very popular and widely use 8 pin audio amplifier used to amplify sound signals. To understand the working of circuit first we should understand its pin description-
Pin 1 and pin 8 is used for gain control of the amplifier.
Pin 2 and pin 3 is used to feed the input signal which is required to be amplified. Pin 2 is used for negative input and pin 3 is for positive signal.
Pin 4 is used as ground.
Pin 6 is used to connect DC power supply.
Pin 5 is used as output.
Pin 7 is used as bypass terminal with a capacitor to prevent oscillation.
Working of circuit is very simple. When you speak in front of microphone small signals receive by microphone is amplified by the transistor T1. Now this pre amplified signals are feed to the pin 3 of IC1 through capacitor C1. Variable log resistor VR1 is used to adjust the volume of the input signals. In this circuit we are using resistor R5 and capacitor C5 as a decoupling component which will provide constant power supply to preamplifier section.
Gain factor of IC1 that is LM386 can be adjusted through pin 1 and 8. If it is left open the gain will be 20 as it is internally set in IC but it can be vary by adding a resistor and a capacitor up to 200. Now these amplified signals are feed to pin 5 of IC1 through which capacitor and amplified signals can be heard through earphone. We have used Capacitor C6 and C5 to remove the any noise due to inductive headphone. And we have used capacitor C8 between pin 7 and ground to nil the effect of ripple rejection. If you are not able to hear the voice clearly from head phone than vary the variable resistor VR1.
Note- In the video I have used both speaker and headphone to demonstrate that signals are amplified