The Link Telephone Intercom

  
It uses old pulse dial handsets and replaces the AC bell set with a 9 volt DC buzzer. The whole circuit runs from a 12 volt regulated DC supply and is suitable for short term battery operation (eg: Gel Cell). The Link is designed to enable one call at a time within a small area (about 100 meters from the black box is about the max per handset) and is not suitable for connection to the PSTN (public network) as the voltages and currents used by the PSTN are higher, and will damage the simpler 12 volt circuitry, that employs CMOS ICs etc. The Link will run quite happily off a 12 volt regulated DC supply of only 200mA or so, and this can be a simple affair, such as a DC plug pack, wired to a 7812 regulator chip and appropriate filter caps on the output. Add some leds if you want!
The Link Telephone Intercom - schematic




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