100 Day Countdown Timer.

[spw]

I have a small laser that I want to recalibrate once every 100 days, and want a hard down after 100 days of usage. I have limited room (about 1 inch and 3.5VDC input power).

I find many solid state timer relays for minutes, but nothing with that long of a range.

My thought is to control the circuit with a small cheap watch, and relay.

Has anybody built a similar relay?

 

[(*steve*)]

The only similar thing I've seen is a "how many sleeps until Christmas" counter (don't laugh). It can count from 99 down to 0 days.

It is essentially a clock circuit (you have to set the time on it) and it decrements a counter every 24 hours. I'm not sure that there's an alarm output at zero though, and the one I've seen has to be manually reset to the number of days each time.

Probably not very helpful...

Other than that, maybe an oscillator with *many* stages of division set to produce a low output after approx the period you want. -- tricky to test as it will take 100 days each time (although you *can* cheat)

 

[Laplace]

If you can find a small cheap watch that you can set the alarm for 100 days in the future, then you are all set. Otherwise the crystal oscillator in a watch is usually at a frequency of 32,768 Hz. So a counter for 100 days would need to count 32768*60*60*24*100=283,115,520,000. Now the closest power of two is 2^38=274,877,906,944 or about 97 days. You can get closer to 100 days by adding 2^38 + 2^33 = 283,467,841,536.

So build your timer with a 32.768 KHz oscillator and three CD4020 binary ripple counter chips wired in tandem. The 38th stage will go HI after 97 days so use that signal to trigger the shutdown. It will stay HI for the next 97 days or until reset. To get 100 days, feed the 38th and 33rd stages into AND gate for the trigger (it will stay high for 3 days unless latched). Be sure to design with battery backup or risk losing the count.