*** UPDATE *** It appears the IRF 3709 is no longer available through most suppliers but can be replaced with something like the STMicroelectronics STP90NF03L. ***

I have been swapping the stock electical system from my AEGs after the contacts on one of my G3’s trigger assembly had been severly damaged after thousands of rounds had been put through it. I do a lot of semi-auto fire which seems to wear them out quicker due to the continual arcing across the contacts.

The first solution I tried was a single MOSFET firing circuit that moved the main electical load to a semiconductor instead of the trigger contacts, then added a second MOSFET to the setup after researching the benefits of activly braking the gearbox motor. A little more reasearching turned up some existing designs so I made some minor changes to the layout and figured I’d share the wealth. Here’s what you’ll need to make one, should cost less then $10. I usually order all my parts through Digikey.

Step 1.

Start with both MOSFETs, bend the gate pin on the 80P06P (the guy on the left) so that it will reach the gate pin on the 3709 when they are sitting back-to-back.

Step 2.

I like to apply some thermal paste to the back of either unit along with a dab of super glue, then place both MOSFETS back-to-back and let the glue dry.

Step 3.

Solder the gate pins together, for reference the 80P06P is on the bottom.

Step 4.

Clip the drain (middle) pins off both MOSFETs, then solder the 30k ohm resistor between the two outter pins on the 3709, I also like to put a dab of super glue between the resistor and the MOSFET to keep it in place.

Step 5.

Solder the 100 ohm resistor to the gate pin on the 3709 and clip the lead.

This is going to change depending on the layout of your gearbox, this is the way I set these up for my G3s but you can change the position of the resistor to point in whatever direction fits best.

Step 6.

Now is the fun part, you may need to open your gearbox to re-route the wiring. On my G3s I pulled the wiring that was running to the trigger assembly and the motor out and ran the motor wires outside of the gearbox. My G36 kept the stock wiring and I was able to just pull the wires off the trigger assembly from the outside and was able to use the stock motor wiring. Also, be careful with super glue, as you can see I used way too much and it made a mess on this one, oops.

Once everything is wired up, place the wire from the motor through the MOSFET holes, then insert the screw and tighten it down. Also, make sure you shrink wrapped the new connections + the MOSFET leads (try and keep the leads as short as possible, solder the wires close the the MOSFET and clip the excess,) then put a chunk of shrink wrap over the whole thing. I didn’t include a fuse in the diagram, but you definatly want to keep one in there, place it as close to the battery as possible or keep the AEG’s stock fuse position.

So how does this work? When you pull the trigger and send some juice to the MOSFET’s gate pin it opens a wide path between the motor and the battery, then when you let go of the trigger the gates on the MOSFETs start to close. The 80P60P’s gates close much slower and they’re biased towards positive flow, so a bit of positive juice squeeks through and gives the motor some reverse thrust, which keeps it from winding the gearbox up again due to inertia and the force of the piston bounching off the front of the chamber. This ensures that your gearbox is always at absolute rest and your spring isn’t left compressed. The biggest effect you’ll notice is that semi-auto fire is super crisp, but the added benefit of storing your AEGs at full rest is always nice.

Now a quick disclaimer, I’m not an electrical expert and this firing system hasn’t seen any battle testing, but it’s a proven design and I have put a good 3,000 rounds through my mule G3 without a hitch. You can also purchase similiar circuits pre-made from Extreme-Fire for ~$30. They also make some crazy firing circuits that do burst fire and add some additional goodies for handling lipoly systems (although my little circuit should handle them without a hitch as well.)