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Electronics for a trigger mechanism

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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Electronics for a trigger mechanism

Unread postAuthor: evilvet » Sat Apr 02, 2011 1:02 am

Evening all,

I am using this circuit as the trigger for my latest experiment.
The idea is that the relay oscillates and will both pulse a solenoid to cycle the ammo load and to switch the valve for air flow at the same time. Right now it gives me about a 2Hz pulse which is ok but not ideal.

What I want to achieve is to have the relay close for 0.5 seconds then stay off for 1.5 seconds, an asynchronous oscillator.

Google is not helping me today, anyone here an electronics engineer ?
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Unread postAuthor: PVC Arsenal 17 » Sat Apr 02, 2011 1:22 am

You can accomplish this with a 555 timer IC. If you want maximum versatility, though, you can program the delays to a microcontroller. You'll get far better precision and the ability to make adjustments with just a few keystrokes rather than fiddling with resistors and capacitors in a dedicated circuit (a serious advantage as you'll likely find out with this sort of application in which timing can be very tricky).

I'd suggest PICAXE 08M. It's dirt cheap (minus the programming hardware/software) and uses a very simple language with convenient libraries for many functions.
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Unread postAuthor: Technician1002 » Sat Apr 02, 2011 8:35 am

The circuit posted will very quickly burn the relay contacts as it constantly closes power on the capacitor with no inrush protection. That type of circuit has a very short lifespan.

A uP application would work fine. A much cheaper solution is to use a pair of 555 timers or a dual 555 timer, the 556. A pair of potentiometers can set the rate of fire and the dwell time.

The circuit will need an output current driver to drive your solenoid valve, etc with either solution.

http://www.circuit-innovations.co.uk/555.html
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Unread postAuthor: evilvet » Sat Apr 02, 2011 3:43 pm

Thanks guys, the 555 running astable is exactly what I needed, time to head to Tandy later this morning.

Cheers
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Unread postAuthor: ramses » Sat Apr 02, 2011 5:53 pm

You actually only need 1 if you change the duty cycle and use a diode across one of the resistors(?). Just google for 555 duty cycle less than 50%
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