If you are happy writing a bit of software, one of the really tiny micros will do whatever you want in this area.
PICs and AVRs come in 8 pin packages with a reasonably good internal clock so one chip is all you need. No xtal, no Rs and Cs for timing...
-- These are my opinions. I hate spam.
A flip flop and a one shot:
Modify it to stop after 24 hours. Your roll your own trigger circuit (FF + triac, relay, whatever) latches on, but you'll need to add something mechanical to the appliance timer to reset your circuit when the timer reaches 24 on the dial. Functional diagram:
----------- input pulse---|Jim circuit| ----- | | |Timer|---timer ac out---relay AC in---|ACin ACout|------| | | | | |---+ ----------- ----- | | | +---------Reset--------+
Input pulse to flip flop, one shot to reset it comes to mind, but I'll call it your circuit as it will include more than a flip flop.
The input pulse turns your circuit on, which sends ac to the timer, starting it. After the timer has produced AC out to the relay for the amount of time you set, it drops timer ac out, but keeps turning, because your circuit has not been reset.
If you can mod the timer to trigger a one shot when the dial hits 24 to produce a reset signal to turn your circuit off, that will leave the timer at 24, awaiting the next trigger pulse.
Your original post mentioned a duration of up to 23 hours, implying an interval between trigger pulses of greater than one day. If that interval is less than a day or you can't mod the timer, then this approach is no good for you.
Ed
-- Maybe, but hey, it's kinda slow around here... If you like, I'll build one for you, all hardware... 1. Power source: mains, batteries, ??? 2. Timeout: If fixed,period? If settable/programmable, min/max periods? 3. Accuracy? 4. Trigger/retrigger source? 5. Relay contact ratings? Email me if you're interested. John Fields
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