Wiper control circuit IIV

Posted on May 27, 2012

U1 is configured to operate in the standard astable mode, providing a form of relaxation oscillator. When power is applied, C2 initially charges through Rl, R2 and R3 to two-thirds of the supply voltage. At that point, U1 senses that its threshold voltage at pin 6 has been reached, and triggers the timer, causing its output at pin 3 to go high. That high, applied to the base of Ql, keeps the transistor in the off state. Now C2 begins to discharge through R2 to pin 7 of Ul. When C2 has discharged to about one-third of the supply voltage, Ul is toggled back to its original state.

Wiper control circuit IIV
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C2 starts to charge again, as pin 3 of Ul goes low. The low at pin 3 causes Ql—which serves as an emitter-follower buffer stage—to turn on, allowing current to flow through the coil of relay Kl. That, in turn, causes Kl's contacts to close, applying power to the wipers. The charge time of capacitor C2 is determined by the setting of potentiometer R3. Capacitor C2 should be a tantalum type, and actually, almost any 12-volt coil relay with sufficiently heavy contacts should serve well.

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