Help designing an 'reset' hourly timer... 555??

Sure sounds low for max collector current. You sure about that?

Depending on how the reset circuit is designed, you may be able to use an optoisolator instead of the relay. Cheaper, easier to build. Guessing that the reset might be a closure to ground, again depending on the value of the pullup, you may be able to use a diode for the reset and power the thing off the reset current...depending... mike

--- Here ya go...

The one Meg pot should be set about half-way through its range, to about 500k, to set the oscillator frequency to 2.275Hz. The 0.36µF cap should be polyester film. With a 5V supply for Vcc, base current into the 2N4401 will be about 10mA, so for a forced beta of 10, you'd like a relay with 100mA or less of coil current. No big deal, though, just be aware that the 2N4401 is rated for a maximum collector current of

-- John Fields Professional Circuit Designer

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Russ,

Their are more ways to skin a cat. If you want to stay in the TTL-like components you can use a 556 (dual 555) and two 4-bit counters like the LS163. Connect the latter to make an 8-bit counter. Use one timer as a clock by making an astable multivibrator with a period of about 14s. Use this to clock the 8-bit counter. The ripple carry output of the counter should trigger the the second timer which is a monostable with a time of the 2s you need for the reset.

I've heard about monostables that can be programmed for an hour or so but I never saw them.

A low component count solution would be an eight pins PIC or AVR, but you will need some skills and (little) equipment to program them. If you need - let's say - twenty of them it may be worthwhile to have a look at them.

The best solution of course is to find out what causes the problem and remove that cause. However, I know all too well that it's not always possible.

petrus bitbyter.

As a suggestion.

There are plug-in electrical timeclocks available with 4 segments to the hour. By pressing in each 4th segment you would achieve a 1 hour timing. This pulse although 15 minutes long could then energise a 12V transformer with the output going through a diode, a 2000uF cap and a 12V relay in series. Should give a couple of seconds, connect a bleed off resistor across cap.

Cheers.

Looks like Q3 is activating the relay every 3.5 seconds-I think you want Q13 driving the 2N4401 base and this will last until Q4 goes H again about 7 seconds later, when the whole thing MR's and starts over.