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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

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  • \$\begingroup\$ Welcome to EE.SE and +1 for the timing diagram. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Apr 4, 2020 at 15:50
  • \$\begingroup\$ Does this answer your question? Is the NE555 the IC I need, and if not, what do I replace it with? \$\endgroup\$ Apr 4, 2020 at 16:13
  • \$\begingroup\$ I'm most certain that this will be easiest if realized with a microcontroller, even a very small cheap one. \$\endgroup\$ Apr 4, 2020 at 16:14
  • \$\begingroup\$ @MarcusMüller yeah, it would, but i wanted to do something that is purely based on ICs and has no code. I do code every day, e forgot everything about electronics :D \$\endgroup\$ Apr 5, 2020 at 11:22

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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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  • \$\begingroup\$ Hi ! Thanks for getting back ! I redesigned the first circuit to use all FETs. Since my circuit is not "logic level", the trigger has like 8-9 volts when pressed. I also wanted to use as less "special" components as possible, since with the Coronavirus quarentines they are harder to get. \$\endgroup\$ Apr 4, 2020 at 17:03

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