Monostable to an Astable

I have a monostable 555 circuit running off 9v and I want to link it to an astable one so that the astable runs only when the monostable has a high output.

Could I use the output of the monostable as the positive power supply for the astable? Or would I be better using a transistor as a switch with the monostable output attached to the base, this would mean that the astable is running off the 9v.

Thanks In Advance

Posted by Mike on 28/07/2003, at 13:44 GMT

Linking 555s

What you suggest is pretty intuitive, and I think it would work, but there is an easier way - use the 555 Reset input.

Connect pin 3 (Output) of the monostable to pin 4 (Reset input) of the astable.

When the monostable is off, pin 3 is low and the astable output will stay low. When the monostable is on, pin 3 is high and the astable is free to run.

Posted by Tim Surtell on 31/07/2003, at 19:48 GMT

THANK YOU!!!

Wow, what a good idea, I tried it and it worked great too.

There is one small problem, I'm using a 100uf capacitor and a 560k resistor for the monostable so using the formula it should stay on for about a minute. It works perfectly before I linked it to the astable but now it times out after about 28 seconds, is there a reason or should I now use a bigger resistor?

Thanks again.

Posted by Mike on 31/07/2003, at 23:24 GMT

Try extra smoothing capacitors

You should not need to change the timing components. The problem is probably electrical noise from the astable upsetting the monostable. You should already have a large smoothing capacitor across the supply (100uF or more) but it is a good idea to also add a 0.1uF capacitor across the supply to remove the brief glitches that occur as 555s turn on and off. They tend to create a significant glitch on the supply even without a load on the output!

Posted by John Hewes on 04/08/2003, at 13:09 GMT

fixed it!

I tried a fresh battery, the one I was using was vey worn out. With a new battery it works perfectly now and times out after one minute exactly.

Thanks

Posted by Mike on 06/08/2003, at 21:30 GMT