If you compare this DIY intercom circuit to modern FM and wireless designs, you will certainly sau it outoded. The intercom schematic fulfills its tasks – to provide a reliable communication line and is very simple to construct. The circuit is made up of an amplifier, two switches and two loudspeakers.
If additional stations (speakers) are wished, additional switches are just incorporated into the circuit. IC LM382 is used as the final amplifier. It delivers nearly 2 watts of audio power by 15 volts supply voltage. Amplification is preset to 50. C9 serves as supply bypass. Another IC is used to boost the input signal before it is passed into the LM384. IC2 is an op-amp with an amplification factor of 11. The frequency bandwith is 160 Hz to 10 kHz. These values were chosen since the device is to be used only as intercom and not as HIFI amplifier.
To achieve the best performance, use speakers that can also function as microphones. The intercom must be constructed inside a box so that the microphone function is optinum. Since the impedance of most speakers are quite low, an impedance converter is needed to maintain a good audio quality. This is done by the transformer TR1. TR1 is a normal 220/6V step-down transformer. The 6V winding of the transformer is connected to the speaker and raises its impedance to about 10 kohms.
To reduce signal loss in the transormer, a 4.5 Watt transformer is used.
In constructing the intercom circuit, house the power supply unit separately from the main circuit to avoid interference. C1 suppresses HF interference. The current consumption is about 210 mA with 1.8 watt output.