Second look at simple 555 timer circuit

Thread Starter


Joined Apr 30, 2009
First I'd like to say I don't have much experience with electronics, and a reasonable portion of this design was pieced together from other miscellaneous projects I found online, so it may not make all that much sense.

I have a little project I'm working on to use as a prop for a game called airsoft, it's similar to paintball. I've got the circuit drawn up, however would appreciate a second look at it from someone more experienced than myself.

Basically I'm trying to make a mock 'bomb' with an alarm, not an explosive device that after started, will go off after a certain amount of time. There are 6 main buttons on the front, 3 of them pressed simultaneously will deactivate it, while the other three, pressed individually, will set off the alarm. There will be an LED that will indicate the time is running down, and of course the obligatory start button and power switch. There is also a potentiometer to adjust the time it takes to go off.

Anyways, any helpful comments/suggestions will be appreciated.



Joined Mar 24, 2008
Don't think it will work, though I see what you're trying to do. The big problem is you are trying to use the same 555 as an oscillator and a monostable. That dog don't hunt.

You need two 555's, one for each funtion. Try reading these articles.

555 Monostable

Pay special attention to pin 2, your setup will not work as is.

555 Hysteretic Oscillator

A basic oscillator, which is also what you're trying to do. Pin 2 is connected to pin 6, which is the only way it will work as such, while pin 7 is NC.

Thread Starter


Joined Apr 30, 2009
Alright well -- this is what I took the majority of my circuit from:

And pin 2 is connected the same way as mine, save for the button, which I now realize that since I'm not too sure of what I'm doing, I probably should have put in directly after the 33k resistor, however I'm not sure if that would affect it anyways. Insight please?

Other than asking about that, I'm essentially lost, because I don't believe I'm trying to use any part of this circuit as an oscillator.