Solid State switching in AEGs

So I have been messing today with MOSFETS, and a 555 timer circuit to produce a 10th of a second pulse, which should fire the gun just once for each trigger pull. I had this all working fine the gun was cycling once for each time I closed the contact.

As soon as I attached the circuit to the trigger mech it started to go wrong. I think the problem is that the contacts on the trigger mech are so grungy that it sometimes fires several shots each time I pull the trigger having triggered the 555 timer multiple times. Sometimes it also triggers the gun when the trigger is released... it's just not reliable to be used.

So I'm going to give up on the electronics (except the mosfet as that'll save the trigger contacts) and get a new cut-off lever installed. Wolf Armouries have them.

Any chance of sending me details of what you did as I'm interested in doing a similar thing but wasn't sure where to start

Which, the mosfet thing?

Yes please

This is the guide I used...

http://www.airsoftmechanics.com/home/co ... 23/30/1/0/

Thanks for that, I've had a quick scan, it looks good but I couldn't see anything about a 555 timer so sorry to be a pain but can you give me some info on that, thanks.

Ah the 555 timer... heh... that's the bit I couldn't get to work!

I combined the mosfet stuff with the monostable 555 timer stuff from here.

http://www.doctronics.co.uk/555.htm

I used the circuit in part 8 "more about triggering". Unfortunately it triggered far too often! The contacts on the trigger are rubbish.

A microswitch may be the solution, but I'm not at all sure where it should be mounted.

Also the use of a timer is flawed as unless you get the timing spot on you can occasionally get no shot when you pull the trigger and at other times you could feasibly get two shots. Also because the rate of fire of the gun will vary with the power in the battery, the timing will not always be correct.

However as I said before the main problem was that the trigger contacts are too "bouncy". Even a big decoupling capacitor didn't help that much.

TBH, as an electronics engineer with a hobby interest in microcontrollers, I'd use an 8 pin PIC rather than a 555.
It'd need some different parts as AEG battery voltages are too high for the PIC, but it could be programmed to debounce (yup, that is the proper engineering term) the trigger switch and only fire the MOSFET once.
In fact, such a device could be interface with a selector switch for safety, full-auto, and burst control.

What you do is tell it to wait for a trigger, then wait 1ms to see if it's still triggered.
If it is, turn on the MOSFET for the required time (or use a sensor to tell the PIC the gun has fired).
Then check that the trigger has been released. Wait until it has.
Sit there waiting for the next trigger.

This is something I've thought about many times WRT making my own AEG control system.

Yeah... I'm a qualified electronics engineer as well. It was my degree way back in 1990. However I've not done any for quite some time now and also have never worked with PICs.

I didn't want to get into programming with this instead just a simple circuit that I could build up in a few hours.

Which is exactly what I managed, except for the flakiness of it all.

I'm completely aware of debounce in programming. I've designed microcontroller circuits professionally, but moved 100% to PC programming about a decade ago.

Another thing that could be done with the PIC is to count shots fired in a hi-cap gun, then stop after a predetermined number until the gun is "reloaded" by the cocking mechanism. e.g. stop a support MG after the equivalent of 1 ammo belt of firing, or 8 rounds in a Garand, and so on.
Of course, this would only be any use if all such guns are limited in this way, or as mitigation for using a hi-cap in a realistic-cap game.

Hmmm. Having talked about this, I'm already forming the code in my head.

I'd like take this opportunity to apologise if I/we start to confuse the non-electronics minded readers of this thread.

There are non-retriggerable monostables out there. Such as 74HC221 which can be rigged to trigger once, then the second half resets the first.

Any idea what the voltage drops to under motor load? The only real concern I'd have with the PIC idea is a brown-out reset.
What's the min/max battery voltages used in AEGs?

That's an interesting concept. I have no idea how big these components are and it sounds very complicated for regular home builders or AEg bodgers but if it were a tiny commercial module that could be wired to the trigger and would fit in any AEG then I could see this being more than a novelty!

If it can be made to work, the microcontroller part could be around the size of the MOSFETs shown in the referenced article.

Well there is a company that makes PIC controllers... http://extreme-fire.com/

All I wanted to do was get my garand firing semi-auto again! lol

Nice. They, even have the C source code, but it's Atmel AVR microcontroller based, not PIC.

When you look into the job of doing such programming, you start to realise just how clever (and warped) the AEG designers are to do it mechanically with switches.

Yup. Agreed.

It's a major project.

Plus anyway their system uses real semi-auto (which I don't have right now) to get the timing right for the electronic semi-auto, so it's no use to me... grrr...

Does yours have some kind of sensor to tell the motor to stop after a cycle?
If it has, or if one can be fitted, digital control is possible.

It does... but it's worn away...

Thing is once that's fixed there's less need to come up with solid state method, so I'll probably not bother, lol!

Is there a simple way to help the switch bounce problem with a capacitor?

Is there a simple way to help the switch bounce problem with a capacitor?

I think the trigger input on the 555 is too sensitive.

yup, that's what I found.

A cap seemed to help a bit, but it still wasn't perfect.