555 timer one-shot 30 second light LED strip, TOTALLY CONFUSED!

Thread Starter


Joined Dec 22, 2014
Hello all!
I'm working on a personal project and I am at my absolute whits end here. I have an N.O./N.C. switch (http://www.nteinc.com/switches/pdf/magnetic_alarm/alarm_01.pdf) so I'm not too concerned with which way I use it.

My goal is as follows:
When the two parts of the switch are separated from each other an LED strip turns on for 30 seconds and then turns off until the switch pieces are brought together again and then pulled apart again at which point it would then turn on for another 30 seconds...repeat indefinitely.

What I've tried:
I've calculated that I'll be using 1Mohm resistor and 22uF capacitor to generate a 24.2 second pulse (which is close enough to 30 for me right now). I've attempted to hook up the following circuits
but they do not perform as expected and either only turn on/off the led strip by the switch without the timer or they run indefinitely.

I'm not entirely sure what I'm missing here but I would greatly appreciate an education as well as reference circuit. I think this has to be very common because it's almost like an intrusion alarm where if the window is opened an alarm sounds for a period of time so I'm not sure why I'm unable to get this to behave the way I want to.

Thanks in advance! You'll be saving my sanity here, I've been at this all day.


Thread Starter


Joined Dec 22, 2014
I forgot to add:

Power Supply: 12v worth of AA batteries
LED Strip: 9-12v (preferably closer to 12v)
LED Strip: 30mA


Thread Starter


Joined Dec 22, 2014
See that's what's confusing. That's one of the ones I listed and yet, for some reason, that circuit doesn't behave as expected either. It looks like it's perfect for the job but I think it needs a momentary button.


Joined Jun 22, 2012
The output will remain high as long as pin 2 is triggered low, pin 2 needs a pulse, and pin 4 needs connecting to pin 8, i would use an NpN transistor on pin 2 with a capacitor from its collector, use the NC switch across the base/ emitter.



Joined Dec 22, 2014
You could use another 555 monostable of short duration ( say 1 sec ) to trigger your existing circuit. I agree, pint 4 goes to pin 8. Finally, add a 100nF cap from ping 5 to 0v line.

Thread Starter


Joined Dec 22, 2014
@Dodgydave i tried that but the circuit doesn't turn on. Is the issue because the reed switch doesn't provide a pulse? It goes low or goes high depending on configuration but then stays there.

I was originally investigating a circuit that was just one shot at power on and i was just resetting the whole thing with the reed switch.

This seems so simply...


Joined Aug 23, 2012
You could using multimeter to measure the contacts when the two parts of the switch are separated from each other and they are brought together again.


Joined Aug 7, 2008
After the first cycle, if second cycle started when SW reclosed, would that be a problom? On some ckts, when SW re closes , SW bounce re triggers 555. If the SW were to reclose during the 30 sec. on time , then there would be no problem as 555 makes a good de-bouncer.
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