# What happens when the switch is closed in a 555 timer and left there?

1. Feb 24, 2013

### Solarmew

Here's my circuit

The question at hand is what happens when the trigger switch is closed during the timing cycle (after being closed once already to initiate said cycle)

here's the waveform i'm getting, but i'm not sure what to make of it.

2. Feb 24, 2013

### Solarmew

Re: What happens when the switch is closed in a 555 timer and left the

is it because the capacitor doesn't have time to charge, so when it tries to trip, it's forced to go back immediately?

3. Feb 24, 2013

### jim hardy

Re: What happens when the switch is closed in a 555 timer and left the

10K X 100 uf = 1 second time constant.
You're showing a pulse every 14 microseconds.

I'd guess C1 is more like 100 picofarads.
Have you a photo of it, or is this a simulation?

Try bypassing the supply with a few uf physically near the 555... that's always good practice because 555 takes a gulp of current when it switches.

old jim

4. Feb 24, 2013

### Solarmew

Re: What happens when the switch is closed in a 555 timer and left the

this is just a simulation, i didn't get a chance to do this on a breadboard because we ran out of time :< but i wanted to see what happens if i do it in MultiSim ... and this is what happens (see pic of the waveform) ... and i have no idea why

5. Feb 24, 2013

### jim hardy

Re: What happens when the switch is closed in a 555 timer and left the

From the Phillips datasheet available here,
http://www.doctronics.co.uk/pdf_files/555an.pdf [Broken]

I dont trust that simulation.

old jim

Last edited by a moderator: May 6, 2017
6. Feb 25, 2013

### Staff: Mentor

Re: What happens when the switch is closed in a 555 timer and left the

I haven't scrutinized the operation in question here. But I can point out that not all 555's are the same. For input combinations outside those encountered in the timer's myriad designed applications, chips from different manufacturers can and do behave consistently differently.

So for configurations outside the norm, you may find yourself confined to using a particular manufacturer's 555 to provide the i/p o/p characteristic you want.