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From the Q&A

With TJ Byers

Solid-State Interval Timer


I need a controller that will run a compressor at a timed interval of 5 to 60 minutes, then off for the same interval. The compressor has a surge of 10 amps starting and 5 amps running. The device needs to do this constantly.

Dean Freeland
via Internet


The best circuit for this application is a pulse triggered flip-flop. The pulse is generated by a 555 chip which has a timer range of about three minutes to 60 minutes. Each time the flip-flop is triggered, it changes state, which either turns on or off the solid-state relay. The relay has to be rated for the load involved — in this case, the relay should be 280 VAC at 10 amps or better, like a CC1028-ND from Digi-Key. Don't forget the circuit breaker, it's a vital safety device. For readers with lighter loads, the RadioShack 275-310 solid-state relay will handle up to 300 watts.