Hello, I need a timer circuit that can latch a 12V DC relay for 20 seconds every 2 hours. It can run off of 120V AC or 12V DC. If necessary, it can run off of a voltage lower than 12V by using a voltage regulator, but I would perfer to avoid that if possible. Can someone help me please? Thanks.

Rain

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The obvious answer is a PIC, which would give a single chip (very cheap) solution, but requires 5V for it's supply - the relay could still work off 12V though.

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Oh.. a couple of 1 shot timers will do it but you might pay around a dollar more(in final total) for those than a pic. Pic is obviously more flexible but also more time consuming to get started (esp if no prog exp.)

And since its only a couple of digital parts plus a relay coil trans, just put 6.8V zener diode off the +12 supply to the VCC (plus a resistor to gnd for >1 mA bias _if_ needed)

If you use 1 shot timers, a 2 hour time constant will not be accurate (maybe you dont need it to be) but you can always cascade stages to make smaller (more accurate times).. then of course there is crystal controlled PIC at the other extreme..

Which way do you want to go?
Dirt cheap? I need it yesterday fast?
How accurate & repeatable do the times have to be?

 

Another way you could do it would be electromechanical... you can get these mechanical timers for water softeners, that you could mess around with to get it to flip a switch every two hours. This switch would trigger a one-shot to activate a relay for 20 seconds.

 

That's exactly what I was going to say. I'm going to come up with a circuit and ask if it will work. Thanks guys.

Rain

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You can also do this with two 4060 chips and one 555. 60Hz mains frequency as time-base give 128 min. (If i calculated correctly...)

 

Don't forget switch noise. You'll want a big RC filter on the trigger line so it doesn't trigger the one shot when the switch is triggered AND when the switch is let go.

 

Schematic is attached.


Description:

P1 is a standard 2-prong plug that will plug into the outlet on the timer at this url: http://www.allelectronics.com/cgi-bi...-11&type=store This timer is the type used many older, non-digitally controlled outside sidewalk lights or sprinkler systems. Since it only comes with 3 sets of start and stop pins, I can old turn the device that I need to control on three times a day, rather than every two hours, like I wanted at first. But this is okay now. The "start pin" and the "stop pin" will be placed right next to eachother. So it will only be on for about 1-2 minutes. However, I need it to be on for only 11 seconds. (I know, I said 20 seconds, but I need it to be 11 now.) Therefore, I've added a 555 timer operating in monostable mode that, with the combined R&C values, will give a one-shot output lasting for exactally 11 seconds. The trigger is wired to the relay that is wired to the timer which turns on at whatever time I want it to. The contacts of the relay who's coil is wire to the output of the 555 is going to be connected to the circuit that needs this timing. Sound good?

Rain

P.S. Um, the output of the 555 is the voltage of V+ right?

Attached Images

untitled_552.jpg (24.5 KB, 657 views)

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When life gives you lemons... make a battery.

 

Um... RC filter? I uh... don't know what that is... ops:

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Question: Where's the switch in that schematic? I can't find it.

Here's what an RC filter looks like. It's a low-pass filter, it smoothes things out. Try 1K and 1uF maybye?

Attached Images

rcfilter.png (1.1 KB, 646 views)

 

Close enough to it.

 

So a 12V relay will latch from it then?

What switch? Oh, btw, heh...... ops: it seems as if I forgot to post the url to the timer... :lol: http://www.allelectronics.com/cgi-bi...-11&type=store That timer is the timer that the 120V relay is going to plug into. That's probably the switch you were talking about. Oops! :lol:

Rain

Edit: Also, I think I can only place the clips a half hour apart. Therefore, it can be on for a minimum of a half hour. This would pose a problem with the 555 will it? What I mean is, the relay that is going to be pluged into the timer is going to HOLD the trigger of the 555 on for a half hour. So will that be a problem? Will it restart as long as the trigger is held low? (It's low right? Because it going to ground? I'm still a newbie. :lol: ) Thanks again.

Rain

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When life gives you lemons... make a battery.

 

It should, a 555 is designed to drive considerable current. An LM555 should be able to handle 50ma without problems. You might want to put a 100 ohm resistor in the way of the coil, or it might short, damaging relay and/or timer. No, the switches in your circuit. I see absolutely no switches in your schematic diagram whatsoever, not even on the relay.

 

Do you see the symbols above both relays that look like caps but larger? That's the symbol for a NO SPST relay contact.

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Oh, I see. ops: Not the symbol I'm used to.

I'd modify your circuit a bit, to add a 10K resistor between pin 2 and +V... On the TTL version of a 555 chip, you can generally count on it being a logic 1 when left unconnected, but not always, and the CMOS versions could go bananas if you leave a pin open-circuited.

And I'd put that RC filter between the switch and pin 2, and a 100-ohm resistor between output and the relay coil.

And you'll have to modify the mechanical timer, of course. They're designed to turn on and off AC-power devices directly, not just open and close a circuit.