Posted on Apr 27, 2012

You can set the interval from about 5-30 seconds. A relay operates the slide-change mechanism. Op amp Ul forms a sort of Schmitt trigger. Resistors Rl and R2 bias the noninverting input at pin 3 of Ul to half the supply voltage. Feedback resistor R3 increases or reduces the bias to pin 3, depending on whether the output of Ul is high or low. When power is first applied to the circuit, C2 has a zero charge and the inverting input of the op amp is at a lower voltage than its noninverting input. When the output of Ul is high, C2 begins to charge through R5 and D 1.

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It takes about one second for the charge on C2 to reach the same voltage as that at the noninverting input of Ul. At that time, the output of Ul begins a negative swing. Because of the positive feedback through R3, the voltage at the noninverting input is reduced and the output becomes more negative. The voltage at the noninverting input is about lf. of the supply voltage, and C2 begins to discharge through the resistor bank. The timing is controlled by R6. The resulting pulses are fed to the base of Ql, configured as an emitter-following bufferstage, which is used to activate relay Kl. Transistor Ql is necessary because op amps usually have an output current in the 20-mA range, which is too low to activate the relay.

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