Telephone Intercom

  
The Link circuitry is simple and efficient, employing just two ICs, half a dozen transistors, and a handful of garden variety components. It all runs on 12 volts and is easily assembled. You can have your own home intercom between the kitchen, the garage, the rumpus room and at your poolside ?barby? and all for less than $100! The ?Link? intercom has been designed in such a way that you can buy parts for it ?off the shelf? at just about any decent electronics retail chain. 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?).
Telephone Intercom - schematic

It is suitable for radio field days and sporting events (providing you can scrounge enough 4 wire cable) and may find a place in pre-schools, old folk?s homes, boy scout/girl guide halls, churches, kids? tree houses/fortresses, or maybe even more serious uses such as small offices, factories, workshops and many other applications. 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 is designed around two ICs. The first, IC1, is an NE 556 dual timer chip, which is wired up to provide dial tone, ring tone (busy tone too, which will be explained along with a few add-ons to be mentioned later on) and ring pulses for the ringer circuit attached to each line circuit. The other chip, IC 2, is a CD 4017B decade counter, which is wired to count each train of dial pulses as they are received and buffered by the two opto-couplers, OC1 and OC 2 and their associated R/C networks. Each...



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