Electronic – Timed one-shot circuit to drive laser module

555multivibratorone-shottrigger

I'm trying to adapt my Match Guns MGH1 (Air pistol with electronic trigger) to use trigger a laser when shooting. Currently the trigger circuit is driving a solenoid. The circuit board seems to have the trigger signal passing through a monostable multivibrator (NXP HEF4538BT) that drives a LL024N MOSFET, which in turn drives the solenoid. I don't want to use the trigger circuit to drive my laser directly as I don't want to introduce any noise in it and affect trigger performance.

I want to create a circuit, sharing power and the trigger signal, that upon trigger press (positive edge, trigger is pulled down), generates a pulse with time T, regardless of the time I press the trigger. I designed the following circuit:

enter image description here

The idea is that as long as I'm not pressing the trigger, the capacitor charges up through R2 and M5. When I press the trigger, the capacitor can discharge through R3 and M2. Doing an AND operation with the trigger signal and the voltage on the capacitor seems to produce the signal I want.

With the premise that this works as I see it, I would like to get rid of the NOT and AND gates with some 555 magic, or some simplification of this that I'm not seeing!

This all-FET version seems to also work, replacing the logic gates since the circuit wont have logic-level voltages.

enter image description here

Best Answer

Several options:

  • A solenoid driver circuit is probably pretty robust. You need to worry about the strong currents driving the solenoid, and the flyback voltage generated when the solenoid is discharged, affecting your laser driver circuit much more than you need to worry about a laser driver circuit disturbing the solenoid driver. So a simple option is just connect a buffer to the solenoid driver signal, make sure any overvoltages can't get through it, and use that to drive the laser.

  • Use an off-the-shelf monostable multivibrator like 74LV1G123 (for example). The datasheet will clearly explain how to trigger it and how to control the output pulse duration.

  • As mentioned in comments, use a tiny microcontroller. This is a good solution if you want the output pulse to last more than 0.5 s or so. If you want a shorter output pulse, I expect the multivibrator solution will have less total complexity.

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