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I'm at a loss with this circuit


I built this simple circuit on a breadboard and it works perfectly but when I soldered the components onto a perf board the same circuit doesn't work.
The 12 volt buzzer just chirps a millisecond and then quits.
I rebuilt the circuit again changing and testing the resistors and capacitors prior to assembly and get the same result.
I have traced everything out over and over checking continuity and all looks good.
The supply voltage is 12vdc.
I'm at a loss as to why it isn't working?

one shot.png one shot perf.jpg one shot image.jpg


Hard to see from those pictures, but it looks to me as though you have wired pin 5 to + supply, when pin 8 goes to + in schematic.

Check the LM555 datasheet to check pin numbering diagram


Well-Known Member
Most Helpful Member
I'd try adding a good size capacitor directly across the power connection in to the board.
The current the buzzer takes could be causing noise on the supply and upsetting the timer.

It's always a good idea to put a decoupling cap across the supply on anything like that. Try 100uF or larger.


Active Member
Yes, use good bypassing on the power supply. Also, tie pin 4 to Vcc and add a diode across the 'buzzer' with the anode to ground and the cathode on pin 3. And a 0.1 uF on pin 5 is a good idea too.
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Well-Known Member
Maybe you have put a wrong resistor altering (shortening) the monostable period to "a milisecond".


hex, thanks for responding, 5 is vacant and 8 is on +.

atferrari, resistors are good, thanks for responding.

rj and Ylli, the decoupling capacitor made it work, I'll wire in the rest tomorrow and retest it.
Thanks very much, I've been looking at this for three days and was wondering why the breadboard model worked and this one didn't?
I didn't want to bother anyone but I was stumped. Thanks again.

dr pepper

Well-Known Member
Most Helpful Member
1 m and 100k are high values for a 555, a cmos 7555 might be more reliable.

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