Solid-State Windshield-Wiper Delay

Posted on Aug 22, 2012

In the wiper-delay schematic shown, with the ignition on, Dl maintains regulated +12 Vdc. When SI closes

Solid-State Windshield-Wiper Delay
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, CI bypasses transients and passes this +12 Vdc to divider R2-R3, producing a TTL high at pin 3 of IC1, a 4013 CMOS dual leading-edge triggered D-type flip-flop. Filter R4/C3 keeps IC1 from triggering erroneously when the ignition is on. When SI is pressed, output Ql (pin 1) of IC1 latches high, turning on Ql, which conducts via R5, turning on IC2; LED1 indicates power, and R6 sets the current. Because IC2 depends on Ql for power, IC2 stays off until Ql turns on. The left half of IC2 is an astable, with its delay set by R7, R8, Rll, and C4. The right half of IC2 is a monostable, with its pulse duration set by R9 and C5. With the values used, you might expect Rll to vary the delay from about 15 to 84 seconds, with a 2.42-second monostable pulse operating the wiper blades on each cycle. However, the actual delay will range between 2 to 18 s, with a 1-s monostable pulse on each cycle. That discrepancy stems from the fact that IC2 is being fed from the emitter of Ql, rather than directly from the regulated +12-V supply. Transistor Ql acts as an active current source, charges and discharges C4 faster than it ordinarily would. The astable output (OUTi, pin 5) is tied to TRIG2 (pin 8). When OUT2 (pin 9) becomes high, Q2 is biased via RIO and current flows through RY1, with D2 dissipating back-emf when RY1 shuts off.

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