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Electronic Telephone Ringer

This circuit produces a ringing sound similar to that made by more recent telephones. It consists of three almost identical oscillators connected in a chain, each generating a squarewave signal. The frequency of each oscillator depends on the RC combination: R4 and C1 around IC1.A, R8 and C2 around IC1.B and R12 and C3 around IC3.C. The pairs of 100 k? resistors divide the asymmetric power supply voltage (between 5 V and 30 V) so that, in conjunction with the 100 k? feedback resistors (R3, R7 and R11) either one third or two thirds of the supply voltage will be present at the non-inverting inputs to the opamps.
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