Happy New Year all:

I've searched through the forum without luck at this time. I am in the planning stages of building a project requiring a 12 hour timer. This timer will activate a relay for about 2 seconds every 12 hours. I was thinking about using a 555 timer, but not sure if the timer and caps can handle the 12 hour 0V time with any decent degree of accuracy. I don't need to-the-second accuracy, but would like the timer to be within 5-10 minutes. However, at that rate, the activation time may end up being waaaaay off after a week or so.

The other alternative would be to use a mechanical timer. As the mechanical timer would have an on cycle of no less than 10-15 minutes, I would need something that would only pulse the relay for no more than 2 seconds and then go back to 0V after the timer supplies power.

Thanks,

John

 

I would recommend a microcontroller solution. If you want I will be happy to elaborate.

 

I am open any possible solutions.

Thanks.

 

OK, here's a relative simple circuit that will do what you want with a wall plug timer. It forms a monostable pulse generator that pulls the relay on for about 2 seconds when power is applied to the wall wart. Set the time to go ON at two 12 hour intervals for the minimum time...15 minutes for a motor driven one or 1 minute for a digital timer.


Ken

 

I liked that last one, but if it is too simple (and you really want to roll your own) you could use a 4060 with a crystal time base.

 

KMoffett: I like that one, too! Brilliance in its simplicity. Thank you. I think that will work perfectly! You guys rock!

John

 

KMoffett: I like that one, too! Brilliance in its simplicity. Thank you. I think that will work perfectly! You guys rock!

John

Yes it is but you may want to consider using a programmable plugin digital timer available at Home Depot ~ $35.00. Ken's clever circuit would remain unchanged.

 

Nice solution, Ken!

My .02: select a DC plug supply aka "wall wart" and relay pair that have similar ratings.
If a relay has a coil with 250 Ohm windings, try to find a wall wart rated for 12v @ 50mA.
12v / 250 Ohms = 0.048A=48mA

You might also find that a wall wart rated for 10v @ 400mA will work just fine.

Wall warts are generally not regulated. They will output within 10% of their rated voltage at their rated load current.

Use a capacitor rated for at least twice the expected voltage in the circuit. Higher is OK.

 

The wall wart I bench tested the circuit with was rated at 12VDC/1A...~16VDC open circuit. Smaller would be more convenient. I noticed Radio Shack had mini 12V relays in with 200Ω, 300Ω, and 400Ω coils.The 400Ω relay would allow you to reduce the capacitor size to 470uF/25V...a lot cheaper than a 1000uF/25V.

Ken