Audio Intercom by LM380 and BC109C

Figure 1 

This one is a high quality audio intercom circuit that consists of two identical intercom units and each unit consists of four parts which are a power supply, a microphone preamplifier based on direct coupled BC109C transistors, an audio amplifier based on LM380 2.5W audio power amplifier IC and “Push to Talk” relay circuitry.

The circuit is directly connected to the mains and 12VAC, 12VA transformer reduces the voltage. A 1A fuse is connected after the transformer to prevent the circuit against high voltage transients. Four 1N4001 diodes connected in bridge configuration rectify the input and 4700uF and 100nF capacitors filter and stabilize the supply voltage.

The microphone is a three terminal electret condenser type and connected to the microphone preamplifier stage formed by two direct coupled BC109C transistors. The gain is very high so use the 10 kΩ potentiometer to adjust it not to transmit all the room noise to the other side. On the other hand, you don’t have to speak too near to the mic, you can stay away since the sensitivity is high.

The amplifier stage is based on LM380. This audio power amplifier IC can deliver up to 2W into 8 ohm load with a supply current of only 50mA. Speaker choice is also important.

The relay circuitry is also simple.The relay contacts switch between the preamplifier output and the loudspeaker input.

More detailed information and PCB file is also provided in the project page.

Read More:zen22142.zen.co.uk/Circuits/Misc/intercom.htm