Link Telephone Intercom CircuitThe Link dent is simple and efficient, employing aloof two ICs, bisected a dozen transistors, and a scattering of garden array components. It all runs on 12 volts and is calmly assembled. You can accept your own home intercom amid the kitchen, the garage, the rumpus allowance and at your poolside ‘barby’ and all for beneath than $100!
The “Link” intercom has been advised in such a way that you can buy genitalia for it ‘off the shelf’ at aloof about any adapted electronics retail chain. It uses old beating punch handsets and replaces the AC alarm set with a 9 volt DC buzzer. The accomplished ambit runs from a 12 volt adapted DC accumulation and is acceptable for abbreviate appellation array operation (eg: ‘Gel Cell’). It is acceptable for radio acreage canicule and antic contest (providing you can scrounge abundant 4 wire cable) and may acquisition a abode in pre-schools, old folk’s homes, boy scout/girl adviser halls, churches, kids’ timberline houses/fortresses, or maybe alike added austere uses such as baby offices, factories, workshops and abounding added applications.
The “Link” is advised to accredit one alarm at a time aural a baby breadth (about 100 meters from the ‘black box’ is about the max per handset) and is not acceptable for affiliation to the PSTN (public network) as the voltages and currents acclimated by the PSTN are higher, and will accident the simpler 12 volt circuitry, that employs CMOS ICs etc. The Link will run absolutely appropriately off a 12 volt adapted DC accumulation of alone 200mA or so, and this can be a simple affair, such as a DC bung pack, active to a 7812 regulator dent and adapted clarify caps on the output. Add some leds if you want!
Parts
R1 10k
R2 150k
R3 4k7
R4 47k
R5 2k2
R6 4k7 trimpot
R7 390R
R8 10k
R9 100k
R10 100k
R11 22k
R12-R15 2k2
R16 4k7
R17 4k7
C1 0.22uF
C2 47uF
C3 1uF
C4 2,200uF (power filter cap – not shown, but wired across +12volts & 0v- ground points
Q1-Q5 BC547 n.p.n low gain
Q6 BC 549C high gain with a beta of at least 250+
D1-D7 1N4148 or 1N914 small signal diodes
IC1 NE 556 dual timer chip
IC2 CD 4017B decade counter chip
OC1-OC3 4N25 or 4N28 opto couplers
Tx 1k/8R transformer, with 1k centre tapped
B1-B4 9 volt DC buzzers mounted inside phone handsets
Miscellaneous – wire, cable, matrix or prototyping board, solder, case, 15 volt DC 200ma plug pack
power supply, phone sockets, zip ties, 7812 regulator and filter caps etc.
