The basic operation of the circuit is based on impedance matching and then amplifying the signal to give it to the output. Two normal speakers can be used both as the input and the output devices. The first stage which is at BC109C receives the signal in the common base configuration. It gives a good voltage gain and low input impedance which is used for impedance matching with the 8ohms speaker.
Self DC bias is used allowing variations in the transistor’s current gain.
The signal is then filtered for any DC part that may be present, through the capacitor. Thus a pure AC audio signal is sent to the 2nd part of the circuit which is the audio amplifier booster.
A constant Vcc is provided to this circuit, the two diodes check the backward flow of current. The signal is amplified on the 2nd stage through 2 transistors in cascade and later the signal is purified through a capacitor and sent to the output speaker. The basic function of the amplifier is to boost the voltage gain of the circuit and than provide it to the output speaker.
The double pole double throw switch, reverses the loudspeaker positions, so that one is used to talk while the other to listen.
The circuit i used instead of LM386 is:-

The output analysis of the circuit tells that its providing 2 watts gain to the speaker so that the voice is audible. If the gain of the amplifier exceed s above the limit than the output signal is clipped off.

2 Responses so far.

  1. Webmaster says:

    The loudspeaker attached in door phone is really of great use.

  2. pankaj says:

    Thank you for sharing very useful information.

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