# power cycling source

Discussion in 'The Projects Forum' started by timgor, Jul 19, 2013.

1. ### timgor Thread Starter New Member

Jul 19, 2013
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0
I have 4 v power source.
Does anybody have idea how to make it off for a short period 1...5 sec, 1...2 times per day periodically? there should very fast and simple solution of it...

2. ### wayneh Expert

Sep 9, 2010
12,087
3,027
Really? You don't know how to do it and yet you assume it's fast and simple?

Sorry to complain to a new visitor but it amazes me how many people come here thinking their idea is just a tiny step from reality.

Anyway, if I was doing this, I would employ a \$5 digital lamp timer from the hardware store to handle all the timing chores. Since you need just a brief (and adjustable?) pulse, I'd also use a 555 timer IC-based circuit to produce whatever pulse you want when it is triggered by the lamp timer.

3. ### timgor Thread Starter New Member

Jul 19, 2013
6
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Thanks for solution. I was thinking about digital timers but I didn't know how to control 1...5 second interval.
Actually some problems intuitively seem to be very simple even if you don't know details.
I think that it is possible somehow to apply \$1 alarm clock for this purpose. But I don't know how. Thanks again.

4. ### wayneh Expert

Sep 9, 2010
12,087
3,027
Look at the 555 monostable multivibrator or "one shot" circuits. You could arrange one to trigger for x seconds when powered on by the lamp timer.

There may be relays you could buy with this capability built in. Never saw or used one but sort of remember one mentioned in this forum.

5. ### timgor Thread Starter New Member

Jul 19, 2013
6
0
Thanks. May I have a question about the oscillator scheme
http://www.bowdenshobbycircuits.info/555.htm
The calculator gives me R1=10000 K Ohms, R2=0.3 K Ohms, C=10000 Microfarads
(T1 = 20 hours, T2 = 2 sec)
Will it work for me with this parameters or this scheme has practical limitations?

6. ### wayneh Expert

Sep 9, 2010
12,087
3,027
20 hours is a very long time for a 555 timer and I'm not sure I've ever seen it used for that long a cycle. But I cannot think specifically why this would not work. The timing will not be precise, maybe differing by 2% (wild guess) from day to day.

I would increase R1 to 100K or more so that C1 can be reduced (and R2 increased). Big capacitors can be expensive and physically large, if that matters.

7. ### wayneh Expert

Sep 9, 2010
12,087
3,027
Oh wait, there's the problem. 10,000 KΩ is 10MΩ. That's almost impractically large already. If you used a 1MΩ, the capacitor has to be 10X bigger and R2 smaller. That passes too much current into the 555, I think.

I don't think it will work, but I'd like to see if any 555 wizards out there feel differently.

8. ### timgor Thread Starter New Member

Jul 19, 2013
6
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I don't need precision. 10% is absolutely perfect. I see that 1F, 5v capacitor is about \$3.
The only reason not to work is that capacitor will leak faster than charging through the 100k resistor. What do you think?

9. ### timgor Thread Starter New Member

Jul 19, 2013
6
0
Oh, you mean the capacitor will discharge too much current into the 555 during the T2 cycle and it will burn it down?

10. ### wayneh Expert

Sep 9, 2010
12,087
3,027
Yes I think that's a concern. I'd have to re-read the 555 data sheet.

11. ### timgor Thread Starter New Member

Jul 19, 2013
6
0
The data sheet may not help because the 1-2 A current will run for a short period (1-3 seconds) and may not damage it by heating.

12. ### wayneh Expert

Sep 9, 2010
12,087
3,027
I've just read that the discharge rate is internally limited so that any size capacitor can be used safely without concern about R2. However this will mean that the discharge time will be longer than you expect, and R2 may not be needed at all to get a few seconds (or more) of discharge time.

I also see recommendations that the discharge current should not exceed 200mA, and the overall heat dissipation cannot exceed the limits of the package either.

So I don't know if you'd get away with it or not. It may depend on the particular 555 you get.

And, you should read this, see #6

Rather than going for extremely long times using only the 555, it would make far more sense to build a 1-hour timer, say, and use a counter to divide that down. I like the 4017 counter for instance, which could easily give you a ÷10 function after your timer, which produces the clock signals. You can even cascade multiple timers to divide down by any factor.

Last edited: Jul 19, 2013
13. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,766
928
You could remain highly accurate and achieve 12/24 hour delay rates by utilizing the hands of an analog clock(remember those?)

let the hands passage between/by a sensor of your choosing provide the trigger.