On/Off Delay Timer

KingKrak

Jul 28, 2010
2
Hey,
I need to make a circuit that acts like a combined on/off delay timer. i.e. input goes high then x seconds later output goes high, input goes low then x seconds later output goes low.

I know you can do this with a 555 timer, but the catch for me is that I need the time delay to be for 1 or 2 hours, not seconds or minutes.

I have thought about using a microchip, but I am trying to avoid having to program anything.

Thanks!

blocka

Jul 7, 2010
18
what sort of accuracy do you need.

RC into a schmitt trigger buffer would be simple but not very accurate.

KingKrak

Jul 28, 2010
2

I have thought about using a binary counter on a higher frequency oscillator. Only problem is it is more of an on-delay timer (I need a combined on/off delay timer, I guess you could even say a delayed output).

Maybe if I hooked two up together and did some fancy ORing or something.

The accuracy isn't that important, as long as it is can time around 2 or 3 hours, within 20 or 30 minutes accuracy.

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Two or three hours with 20/30 minute accuracy is easily within the limits of an RC oscillator.

If you're using an RC oscillator and a divider then you could use the output state to either switch oscillators, or to change the oscillator frequency, or perhaps to change the amount the oscillator is divided.

For example, if you're using 2 4040's to divide a 32768Hz signal, the first will divide it to 16Hz, and the second to 1/256 Hz (on/off for about 2 minutes in each state) when the output is Off, you could change the first divider to divide by 512 rather than 4096, so that the off time becomes around 16 seconds.

Clearly the delays you're looking at are longer than that, so you'd need either a slower clock rate, or more stages of division, but you get the idea.

logic like 4066 can be used to change the speed of an oscillator by switching in additional resistors or capacitors.

As another alternative you might consider a microcontroller.

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