Re-triggerable Monostable 555 Timing Circuit Issue

Thread Starter

Matthew Whittingham

Joined Jul 23, 2015
Hi guys, this one is really scratching my head and I'm pretty sure it's simpler than I'm cooking up in my head!

I'm designing a trigger circuit for a 'laser tag' system. S1 represents the laser blaster's trigger, R1 is the Current limiting resistor for the output diode (Actually a Transistor output driver but for simplicity just a resistor), and R3 is a preset potentiometer to adjust the output pulse duration.

It needs to output a pulse from ~50 - 120ms, and not be able to re-trigger until the trigger switch has been released.

I have designed the monostable oscillator circuit without issue, the output is exactly what I need it to be:upload_2019-7-29_22-7-24.png

However, if the 'trigger' is held down (ie: low), the output remains high. This is registered by my windows based system as a constant input and your 'blaster' then acts as a mouse:

How do I go about preventing my circuit from re-triggering until the trigger switch has been released (ie the input pin has gone HIGH), so that the user can shoot as fast as they can pull the trigger?

Thanks in advance!


Thread Starter

Matthew Whittingham

Joined Jul 23, 2015
Welcome to AAC!
Connect the R2/S1 junction to the Trig input via a capacitor (say 10nF). Connect a 10k pull-up resistor from the Trig pin to Vcc.
Thank you! I knew it would be something simple!
Thanks for the welcome, I have been a member for years but I've completely forgotten my login :D


Joined Mar 30, 2018
Although not the answer to your question, if resistor R3 is adjusted to zero ohms, the 555 timer is likely to be killed – therefore you should include a 1kΩ resistor in series to prevent this.