The circuit diagram below shows the two distinct parts of the circuit. They are based upon standard amplifier designs. The top section is the low signal pre-amplifier for the microphone and the lower section is the power amplifier for the headset. Due to its simplicity, an intercom complete costs less £10 to build.
Note there are many components not mounted on the PCB. Connections to the PCB are denoted by SIL or TB.
This amplifier is a standard inverting amplifier based around a 741 op-amp with decoupling capacitors. The gain is set by
-R5/R4 = 1M/10K = 100.
This provides an output around 500-800mV which is fed to the level preset on the PCB to adjust the level of signal placed on the Link. Connection to the Link is via SW2 to enable muting, but audio audio receptions will still be heard from other intercoms.
To enable the amplifier to work on a single voltage supply, a psuedo 0v was created by R2,R3,C1 which gives the amplifier 9 - 4.5 - 0v to operate from.
This is based around the LM386 IC which provides about 500mW with just 4 components. The volume is set by VR2 which is mounted on the lid.
This uses a 9v/7.2v PP3 which powers the intercom via a switched 2.1mm DC power socket. The switched contacts inside allow for the battery to be charged through R9 when a supply of 10v or more is pluged in. If you are using a power supply, which is advised, then remove the battery unless it is rechargeable.