Monostable 555 Triggers when trigger button is lifted

Thread Starter

Reed00112

Joined Nov 3, 2019
3
Hello all, this is my first post so please be gentle with me.

I have a 555 timer in a monostable configuration. I needed it to be able to not retrigger itself if the trigger button was held longer than the pulse the 555 generated. So i incorporated an RC circuit to do this. It's a very common way of doing this from my research. You'll see a diode also on the trigger line in series. This is required in my setup to prevent high voltage spikes blowing up the 555. It works well for this purpose, i haven't blown one up since incorporating this diode.

My issue then is that sometimes when lifting off the button after holding it down longer than the 555 output pulse, the 555 will trigger again. In my circuit this can not be allowed to happen.

I have tried a lot of little things but sadly i am simply out of electronics knowledge to be able to fix this issue by myself.


Electrical help pic.png
 

eetech00

Joined Jun 8, 2013
1,894
Hello all, this is my first post so please be gentle with me.

I have a 555 timer in a monostable configuration. I needed it to be able to not retrigger itself if the trigger button was held longer than the pulse the 555 generated. So i incorporated an RC circuit to do this. It's a very common way of doing this from my research. You'll see a diode also on the trigger line in series. This is required in my setup to prevent high voltage spikes blowing up the 555. It works well for this purpose, i haven't blown one up since incorporating this diode.

My issue then is that sometimes when lifting off the button after holding it down longer than the 555 output pulse, the 555 will trigger again. In my circuit this can not be allowed to happen.

I have tried a lot of little things but sadly i am simply out of electronics knowledge to be able to fix this issue by myself.


View attachment 190521
Hi

Change R15 to 1MEG. Move D7 so its between switch contact and junction of R15/C8.

eT
 

Thread Starter

Reed00112

Joined Nov 3, 2019
3
Welcome to AAC!
Try adding a pull-up (say ~5.6k) from pin 2 to Vcc so that the trig input isn't floating when the diode isn't conducting.
Hey thanks!

I've realised as per eetech00 solution I can move the diode to provide a pull up to pin 2 via the existing resistor. I forgot adding the diode there would mess up that. Thank you.
 
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