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how build cheap clock timer with on/off trigger at two different times per day?

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LandMime

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What's the cheapest way to build a clock timer that runs on 120 AC, which can be set to activate a relay for one or two seconds, then turns off. I need this to happen at least twice a day.

I'd like to be able to use a cheap hardware store 24 hour on/off timer but the shortest "on" cycle is over a couple minutes. Is there a way to limit the "on" time to just a few seconds?

Thanks
 
Read this entire thread. You can use a variant of the CMOS circuit I posted there. To shorten output pulse, use a "differentiator" (series C, shunt R) to create the output pulse width. Place the CR network between the counter bit and the base (gate) of a transistor which drives your relay.
 
Or something like this. On-time will depend on the relay coil resistance, the relay's drop-out voltage, and the value of the capacitor.

Ken
 

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I like the Moffett design because it's cheap and simple...very simple...and it will work.
 
Thanks KMoffett,

Now let me see if I have this right.

The timer is just a generic AC timer set by pulling in or out on time tabs.
The transformer, a generic 12-volt, is plugged into the timer. How many milliamps?
The circuitry is connected to the plus and minus of the transformer outputs.
The relay’s energized time is determined by the amount of time it takes to "fill" the capacitor?
When the relay energizes, then turns off, but the wall timer is still on, is the circuit protected from frying?
How is it I build the relay output circuit? This circuit will need a separate power source, correct?

Thank you,
 
The relay coil is powered from the wall-wart. The duration that the relay is pulled in is just a fraction of a second (determined by the capacitor, relay coil resistance). It happens as many times as the timer switches on in a 24hour period.
 
The timer is just a generic AC timer set by pulling in or out on time tabs.
Yes
The transformer, a generic 12-volt, is plugged into the timer. How many milliamps?
It should be greater than the coil current of the relay.
The circuitry is connected to the plus and minus of the transformer outputs.
Yes. Make sure the + terminal of the capacitor goes to the + output of the wall wart.
The relay’s energized time is determined by the amount of time it takes to "fill" the capacitor?
Yes, as Mike said.
When the relay energizes, then turns off, but the wall timer is still on, is the circuit protected from frying?
It is safe as long as the capacitor's voltage rating in higher than the measured, no-load output voltage of the wall wart.
How is it I build the relay output circuit? This circuit will need a separate power source, correct?
The relays contacts act as a switch for your external circuit. The contacts need to be rated for the maximum voltage and current of your external circuit. What are trying the momentarily switch on and off?

Ken
 
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