johnny

Hi, I'm an amature trying to make a timer using a 555 cmos in astable mode to light an led at a specific time say 10 hours and then run for around 3 hours and stop. The Vcc should be +6 volts, any help on resistor and cap values would be great as I'm having trouble working out the math. thankyou.

pebe

Sorry, you won't get 10 hours with a 555 timer on its own. The periodic time depends on the CR product, (seconds = C in microfarads x R in megohms x a constant) and the values for 10 hours are far higher than the 555 can handle.

You need to drive a divider chip from your 555. Your question has been asked before, so do a search for previous answers.

bogdanfirst

i think that one hour is doable.......so use a 4017....but this way you have 1hour on time and 9 hours off time....

pebe

I'm facinated that you find an hour 'do-able'. Out of curiosity, what C and R did you use?

kinjalgp

May be in Giga Ohms and capacitor in Farads If you use C = 10,000uF then using formulae:
T_on = 0.693 * (R1+R2) * C
T_off = 0.693 * R2 * C
We get R1 = 4.08 M and R2 = 4.6 M
This I think is practically impossible. I don't think the circuit will be reliable at all.

Well, it can be done using long duration timer IC from EXAR. Its XR22?? along with 555 serving a clock to this IC. I don't remember the number of this IC since I studied it quite long back and have never used it practically. May be some on here knows it.

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Chippie

How about using a 4060, on board clock with RC timing and then binary counting........ timing can be set quite accurately if you use a preset....... or else, a 4047, and 4040 combination......


just a suggestion of course, unless you need to use a 555 8)

kinjalgp

Even that is a good solution.

If more accuracy is required use Crystal Oscillator.

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