# Momentary Switch circuit help!

Discussion in 'General Electronics Discussion' started by Kneumann, Dec 18, 2014.

1. ### Kneumann

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Dec 18, 2014
I don't have much background in electronics but am very mechanically inclined and would rather build something myself then buy it.

I'm trying to make a laser training pistol to use with the LASR (Laser Activated Shot Reporter) software. Which uses a web cam pointed at a target on a wall to detect the light from the laser and show where the shot would hit the target on the computer screen. This a great way to practice without having to go to the gun range.

So my problem is that I want to control laser module (Wave length: 650nm. Output power: Class IIIa - less than 5mW. 2.6~5 VDC) using a momentary switch. But no matter how long the switch is held down it would only cause the laser to turn on for a fraction of a second, much like a flash on a camera but without the dimming of the light. Just on and off very quickly. My first thought was to come up with a circuit that uses a capacitor and when the momentary switch is open the capacitor is charged and when the switch is closed the capacitor would dump the charge and flash the laser once. But I don't know how to make that circuit or what other components I would need.

The laser module would be fitted into the barrel of a real pistol and the rest of the electronics would be housed in the magazine well with the momentary switch placed right behind the trigger.

There is a pistol you can buy already but is way over priced. (SIRT Pistol \$200)

Any help would be greatly appreciated!

2. ### Gryd3

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Jun 25, 2014
In the most simplistic fashion, this can be done with a momentary button, a capacitor and two resistors... The draw-back is the there will need to be a resistor in-line between the capacitor and power source. This is so that when the momentary button gets pressed, the power source won't power the LED at the same time due to the additional resistor (It will, but the current flow ill be too low to keep it on).
Additionally, this means that the LED would function like a semi-auto, but would require a delay between shots to 'charge'.

Another option would be a 555 timer configured to be a 'one-shot'.
Once triggered the LED would stay lit based on the configuration of the 555.

Do you have any ideas on how long you would like it to be lit?

3. ### Kneumann

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Dec 18, 2014
Thank you for such a quick response!

I think the 555 timer might be the better option. I wouldn't want to have to worry about the capacitor not being charged because I pulled the trigger too quickly after the first shot.

Probably no more than two tenths of a second. Since the web cam detects light to record the shots, if the laser was lit for any longer the web cam would think there was more than one shot.

Any help on the layout of the circuit with the 555 timer would be awesome!!!

4. ### Gryd3

4,098
875
Jun 25, 2014
@Kneumann ,
We can help draw up the schematic or help draw up a finished product you could make on perfboard or stripboard (veroboard).
Do you have any electronics test equipment or prototyping equipment? (Like a breadboard)

prototyping equipment is less of a priority, but will let you build and test the circuit without soldering. Otherwise you solder away, and if you make a mistake it's wire tracing time

In the mean-time, take a look here : http://www.electronics-tutorials.ws/waveforms/555_timer.html
There are all sorts of great sources online covering 555s and projects you can make from them. This one will cover the details about building a one-shot (mono-stable) 555 timer.
It also covers sinking or sourcing the load, or switching the load with a transistor.

Let me know how you want to continue.

5. ### hevans1944Hop - AC8NS

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Jun 21, 2012
I was thinking about building one of those things that would slip into the barrel from the breech. On the commercial version, "O"-rings hold it in place and center it in the barrel. There is no extractor lip at the breech end, so the device is "extracted" by pushing it out with a rod inserted in the muzzle. There is a switch that the firing pin strikes to "light up" a small laser diode. I don't think an LED will produce a collimated beam without a lot of optical finagling. I shoot a .45 caliber ACP round while my wife shoots 9mm Luger. My.45 ACP and the 9mm Luger head-spaces on the rim of the cartridge, so my home-built laser light would have a little shelf to do that. It does need to be short enough to allow the device to be inserted and extracted by hand from the breech while long enough to contain the laser diode, circuitry, and a one or more small silver-oxide "hearing aid" type cell(s). The commercial versions cost about US\$100.00 each, so it may not be economical to build one.

6. ### Kneumann

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Dec 18, 2014
hevans1944 - The problem with using the firing pin is that you would have to rack the slide each time the trigger was pulled in order to reset the sear. So instead of focusing on your sight picture and continuing to shoot you have to stop after each shot and rack the slide, then shoot again, then stop and so on and so on. At first I thought about getting something like that LaserLyte Laser ammo cartridge where it uses the firing pin to activate the laser but decided against it because of having to rack the slide each time.

7. ### hevans1944Hop - AC8NS

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Jun 21, 2012
That is a real inconvenience, racking the slide for each "shot," but then this is just a "simulation" of real firing. The best way to work on focusing and sight picture, IMHO, is at the range with real ammo, preferably the type you will carry in your pistol. This may help explain why I have not "invested" in laser training devices. I would like to see a green laser pointer that fits in the recoil spring housing of my Colt M1911 .45 though. They are available (I think) for my wife's 9mm Ruger SR9c.