How to Make a Basic MOSFET Switch Unit

[DonP]

Update Sept 2009: Parts kit now available! See this post.

(This is also posted on the web here.)

I sometimes get asked about basic (non-computerized) MOSFET switches. Here is how to make a basic one. If you have at least basic soldering and electronic skills, you should be able to put one together without too much trouble.

Terry Fritz of extreme-fire.com provided the design and kindly released it into the public domain. I have expanded it with some step-by-step and instructions. Enjoy!



WHAT IT IS
If you’re reading this you probably already know what a MOSFET switch for Airsoft AEG guns is. (If not, then the short version is that it replaces the way a gun turns power on and off to the motor with solid-state electronics. A MOSFET switch unit controls power far more efficiently and reliably, especially for upgraded guns.)

WHAT IT ISN’T
It doesn’t have active braking, low battery warning, fire control, or any other of the fancy features that are part of computerized MOSFET switches such as my Trigger Master. It’s as basic as you can get while still being robust!

NOTE: Do not use a battery below 6.0V. You risk damage or destruction of the unit!

WHY CAN’T I USE LOW BATTERIES?
To work properly, the MOSFET needs to be switched on with a high enough voltage. Too low and the insides don’t work right. Under 6.0V is too low, and the unit might be damaged or destroyed. That sounds scary but it’s really not a big deal. Note that your average 8.4V airsoft battery is dead as a doornail at 7.0V so it shouldn't be an issue! What is really being said here is: “Don’t plug really small non-airsoft batteries into this. It won’t work anyway, and you might hurt it.”

A MOSFET is in many ways an efficient and tough device, but in other ways it’s quite fragile and needs to be protected.

ARE ALL THE PARTS REALLY NEEDED?
Short answer: yes.
You may have seen some other MOSFET switch designs out there with fewer parts, but this design includes safety and protective parts which frankly others should not do without. There are simpler designs but not necessarily more reliable or tougher.

Some other final questions to nip in the bud before we get started:

Q: Can I substitute part “X” for “Y”? For example can I use part XXXXXX instead of the MOSFET you listed?
A: There is more than one way to skin a cat. But if you know what you’re doing, then you don’t need ME to okay your substitutions! If you don’t know what you’re doing, then stick to what is listed.

Q: “Bolt on, do not solder” to the tab of the MOSFET? Why can’t I solder to it?
A: The part (like many MOSFETs) is heat-sensitive and it’s easy to damage or destroy it with excessive soldering heat. Bolt the connections on with a connector like the one shown. Feel free to ignore that advice but don’t come to me when your unit blows or doesn’t work at all!

Q: Do I really need the huge diode?
A: Yes, you should use it. That part eats up the large voltage spikes that are normal in this kind of application. This is not a “theoretical” just-to-be-on-the-safe-side thing, there are some scary voltage spikes that can easily damage and destroy the MOSFET otherwise.

Update Sept 2009: The fat diode can be installed in a different orientation to make the whole assembly overall a little thinner. Just be careful to insulate all the connections so they don't touch where they're not supposed to. Here's an example that moved the huge diode further "up". The way it's drawn in the diagrams below is the easiest to hook up however.

With that all out of the way, here is my illustrated assembly guide.

The Basic MOSFET design, redrawn


Illustrated Parts List


Cut off the middle pin on the MOSFET (we won't be using it)
The middle pin and rear tab are electrically the same. We’ll be using the tab and not the pin.


Connect the 2.2k resistor and solder it.
This part ensures that the MOSFET turns off properly. When soldering anything to the MOSFET, try not to use excessive heat – solder long enough to connect the part and no longer.


Attach the 100 Ohm resistor to the GATE pin. Make sure you put it on the right pin! Check twice, solder once!


Connect the other components and MOTOR (-) wire to the MOSFET.
It is absolutely critical that no metal parts touch each other other than where they should. Assemble it so that there are no shorts, and we’ll insulate with heat shrink later.




The rest of the connections are wires are made and insulated. Compare with the drawing.


You’re done! Just twist up the wires as shown (it really does make life easier for the unit, trust me) and wire up into your gun! When installing the unit, leave as much of the wire twisted as possible (it won’t be able to be 100% twisted of course, just leave as much twisted as you reasonably can.)



A DIY parts kit is available if you can assemble it yourself but would like a package of the basic parts to work with. See this post.

And of course, if you'd prefer to buy a pre-made high-tech unit - you can buy one from me here.

[Goge]

Very helpful! I've seen lots of hand drawn diagrams but nothing with adequate explanations or as in this case very helpful photos. Thanks DonP! Did you think about maybe selling all the pieces together as a DIY kit?

I didn't want to double post so I edit. YES, I would be interested in a DIY kit where I wouldn't have to hunt down anything.

[DonP]

I have considered putting together a part kit for it. It would still be DIY but it would be all the parts together in a bag so you don't have to hunt anything down. Would anyone be interested in that?

[13Fido13]

Depending on what the price difference was between your assembled version and a kit version I would be interested. I've always been meaning to make one, but just haven't had the time.

Of course the issue would be in the instructions needed to be supplied with the kit.

[m102404]

Yes.

Thanks Don,

Tys

[DonP]

Originally Posted by 13Fido13

Depending on what the price difference was between your assembled version and a kit version I would be interested. I've always been meaning to make one, but just haven't had the time.

Of course the issue would be in the instructions needed to be supplied with the kit.

Just to be clear and make sure we're talking about the same thing, the "basic" MOSFET unit as shown here and the ones I sell are two very different things. The ones I make and sell are fully computerized and have far more features, (edit) and can handle more power.

The parts kit suggested would just be a bag of the parts for a basic MOSFET unit as shown here -- so you could buy the parts all together in one shot. Assembly instructions? They're in the first post!

[PoFF]

I for one would be interested, as I would like to have MOFSET switching on all my AEGs, without having to pay for features that I would not actually need such as speed control, burst mode and such...

[Skruface]

I too would be interested in a bag o' parts to make my own MOSFET switches. I'd be more interested in buying a pre-assembled lo-tech version as my soldering skills are ass, though.

[PrIeSt]

very nice little design. Im still trying to find a nice plug setup for the trigger wires. as i dont like having them soldered permenently like that?

[DonP]

You could use a micro-deans plug, or make the connector shown in the pictures. I have all the parts. It's cheap, secure, and effective if you don't mine making it yourself. Same (female) plug as AEG Motor connectors.

It's insulated by heatshrink in the images, but the heat shrink is clear and doesn't show up well. It's completely insulated though. No exposed metal when closed up.

I AM PUTTING TOGETHER A FEW KITS SINCE THERE IS INTEREST. I will update later when ready - in the meantime, would people prefer that wire and AEG Motor connectors be included in the kit (increases the price), or not?

[PoFF]

I'd say to put wire & connectors as well, speaking for myself, I don't have the time to run to town to search and buy all those little accessories, might even end up being more expensive gas-wise that shelling a few extra dollars to you.

[Skruface]

Originally Posted by DonP

in the meantime, would people prefer that wire and AEG Motor connectors be included in the kit (increases the price), or not?

This is always a good idea IMHO - nothing sucks worse than buying a product and then realizing you now need to go out and source another bunch of stuff to make it work.

If it's not too much trouble, you could offer a basic (no wiring/connectors; just the mosfet and some shrink wrap) and deluxe (wiring + connectors included) kits to meet the demands of the 2 groups I see paying for this: the newbie to upgrades who wants to try a MOSFET system without dropping $100 on a TriggerMaster, and the more advanced/gun doc types who already have spools of wire and connectors lying around and just want a simple MOSFET trigger.

[Primus]

I say make them as plug 'n' play as possible Don. Put me down for 3 basic mosfet set when you come out with them.

*EDIT* will these work properly with a 7.4V lipo setup?

[DonP]

Originally Posted by Primus

*EDIT* will these work properly with a 7.4V lipo setup?

Good question!

EDIT: The units will work just fine with a 7.4V Li-Po. (Originally more testing was needed and I thought under 7.0V would be a risk to the unit but that is not the case, happily!)

[DonP]

Looks like pricing can be expected to be as follows:

• Bare bones parts kit (only the loose main electronic components and the ring connector with nut and bolt, no wire, no other connectors, no heat shrink, etc): 30$
• Full parts kit with high-grade (milspec) wire, motor connectors, dean's plugs, connectors as shown a few posts above, heat shrink... just about everything you might need: 45$
• Assembled kit is same as above, but assembled main unit with wires attached (other connectors, etc still loose, to be used in your final assembly): TBD

These units require skillful assembly. Not only do you need to open up and rewire your gun, but you will need to wield a soldering iron as well.

I'll post full details and contents of packages when ready, doing some testing. Will post in my Retailer thread once they're ready for sale, and announce here!