From: "Spehro Pefhany"
Subject: Re: Timer circuit for relay?
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Tue, 01 Oct 2002 15:23:59 GMT
NNTP-Posting-Date: Tue, 01 Oct 2002 11:23:59 EDT
In sci.electronics.design DaveC wrote:
> Posted elsewhere in these groups is my original question, "555 Timer
> Well, I thought that there might be another way to approach this circuit.
> I need to turn on a relay once an hour for a few minutes. The "once an hour"
> needs to be adjustable -- maybe between 45 and 75 minutes, and the "few
> minutes" should be between 5 and 20 minutes.
> How about running a 555 to generate a clock signal as in input to a counter,
> which, once an hour, outputs a signal (ie, goes low). This triggers another
> 555 which closes the relay for the shorter period.
Yes, you could do that (with the trigger signal AC coupled to the 555).
> If the clock signal 555 is adjustable -- say, plus and minus 40 percent, the
> "once an hour" output from the counter will change accordingly (ie, happens
> earlier or later).
> The relay-controlling 555 will be simple, adjustable 555 monostable circuit.
> Constructive criticism? Ways to improve on this?
20 minutes is a very long time for an analog timer. You could use a second
divider chain such as a 12-bit counter to inhibit the second 555 to make a
one-shot, then just reset the counter to restart the period (with an AC
coupled signal just as with the 555 trigger).
I'd be remiss if I didn't mention the obvious way to do this, with a PIC
12F629 (a single $0.99 (100's) 8-pin DIP, bypass capacitor and your two
trimpots, plus your relay driver). Some programming required ;-) so
probably not practical for your one-off (?) application.
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