One of the most common questions asked on Airsoft groups is ‘should I fit a MOSFET’? However this can be more complicated matter than it first appears. These devices are also known by a variety of names such as micro-processor MOSFET, ECU and FCU. In this edition of Airsoft 101 We have a look at what it all means.


MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. To put it in simple terms a MOSFET is an electronic switch that on receiving a low input current from your trigger allows a much higher current to flow to your motor. They are in every microchip that we have in our devices and around the home. You will have billions upon billions in your phone for example!

Traditionally MOSFETs have been used to protect trigger contacts from arcing and burning and to improve trigger response. This didn’t used to be much of a problem but with todays high discharge rate batteries this is increasingly an issue. These days what we call a MOSFET can actually be a sophisticated unit with a whole heap of features. 

Plug And Play

The simplest way of installing a MOSFET is the plug and play method where the device sits in between the battery and the rest of the wiring in the gun. Whilst the easiest method this leaves a lot to be desired. Whilst the MOSFET still acts to protect the trigger contacts the full motor current runs through the trigger. This can actually slow down response as the MOSFET doesn’t feed the full current until the trigger is fully closed. Whilst a plug and play MOSFET can offer some additional features these are limited when compared with a wired in unit. 

Wired in MOSFET

In this case the battery current doesn’t pass through the trigger. Thin signal wires are soldered to the trigger contacts along which is passed the low current voltage. When the trigger circuit is closed a separate high current circuit is opened to the motor. This means that the full power of the battery can be unleashed virtually instantaneously allowing for a faster trigger response. As this involves some technical knowledge we recommend that it be carried out by a qualified technician unless you are sure of what you are doing. Wired in MOSFETs whilst more efficient also allow for a richer feature set than a plug and play device. 

Micro Switch Triggers

Some MOSFETs don’t play very well with microswitches, Microswitches can bounce when applied causing an on-off-on-off etc signal to go to the transistor. More advanced MOSFETS with additional processors can detect this with software and ignore it. 


Once you start adding more features then the devices stops being a simple MOSFET, for the sake of sanity we will call them Electronic Control Units (ECU) if they are outside the gearbox or FCU (Fire Control Units) if they are mounted inside and have trigger and cycle detection. 

Battery Protection

One of the first additions to a MOSFET was battery protection. This looks at at the voltage of the battery and if it drops below a nominated value will shut off and/or produce an audible or vibration alarm. This isn’t a problem with older NiMh packs but is with LiPo and LiFe batteries that in extreme circumstances become damaged or even catch fire if too deeply drained. Most AEGs aren’t really efficient enough for this to be a big issue but it’s a good idea to have battery protection if you are using modern batteries. 

Active Brake

An active brake ECU works by reversing the polarity of the current going to the Motor when a firing cycle is completed. This stops the motor faster than just stopping the current completely. When firing in full auto this will be when the trigger is released whilst in semi when one shot has been fired. This can be quite hard on a motor. 


With Pre-Cock the ECU will continue to cycle the gearbox partially after a shot has been fired, holding the piston and main spring back in a compressed position. When the trigger is pulled subsequently the gears don’t have to turn very fire to complete a shot thus improving trigger response. In most MOSFETs with this feature you will have to tell the device how long to cycle with a process of trial and error. Starting a gearbox under load can also put a greater strain on the system and shorten the life of your parts. You will generally have to turn Pre-Cocking off after playing to make sure that you don’t effect the spring power over time. 

A combination of Active Brake and Pre-Cock can allow for extremely fast trigger response rivalling that of HPA and GBBs. It’s especially popular with DMR users and players that shoot mostly in semi auto. 

Burst Fire

This allows for number of shots to be fired on a single trigger pull. This is normally achieved by allowing the motor current to flow for a predetermined amount of time. Commonly the length of time can be adjusted by the user by programming it but in some applications this is set at the factory. Most ECUs don’t actually know how many shots have been fired unless they have cycle detection…

Cycle Detection and Lock-Up Prevention 

Some ‘in gearbox’ FCUs such as the GATE TITAN and BTC Spectre have cycle detection. This means that the FCU unit counts how many times the gearbox turns over. In most cases this is done with a sensor on the cut-off lever but this can be also done optically with the device looking at the teeth on the sector gear. This feature can be used to assist Pre-Cock, Active Brake and Burst Fire. 

Cycle detection has the huge additional advantage of preventing Lock-Up! I’m sure we’ve all had the problem where on semi auto our AEG stops firing when (hopefully) with a quick switch to auto solves it. In the worst cases this can mean having to manually release the anti-reversal latch! With a cycle detection feature you won’t have to worry about this again. 

Trigger Sensors

Within gearbox FCUs also replace the traditional trigger contacts entirely with either a microswitch or an optical sensor. This can have the added benefit of reducing the amount that a trigger needs to move before firing a shot and the reset between shots. The Gate TITAN has multiple optical trigger sensors so the position at which it fires can be changed to suit the user and application. 


Some ECUs and FCUs come from the factory preset and can’t be altered by the user. The FCU in the ASG Scorpion EVO 3 for example uses an FCU for it’s three round burst and other features, this is part of what makes the EVO such a fun platform for many players. 

Other units can be programmed by the user, this is done in a variety of ways such as trigger presses and a button that is clicked a number of times. These methods require you to either remember the specific sequence that is required to change a feature or to have the manual close to hand. Some more advanced devices have a visual programming card with the available options on them. The newest and easiest to use FCUs can be plugged into a computer or even your phone via a cable and simply programmed that way. Some of these have a whole host of features that we couldn’t dream of a few years ago and even keep track of statistics! 

Having used a number of different MOSFETs, ECUs and FCUs over the years I’ve now fallen in love with the convenience of a unit that I can programme on my laptop!


Before you fit a ‘MOSFET’ of any sort think about what features you would like to have available. More advanced units can cost a similar amount to a starter AEG even before you pay for installation whereas some new trigger contacts would cost just a few pounds if they do indeed ever burn out! If you are the sort of player that wants instant trigger response and plays a lot with semi auto fire then it’s more likely to be a product for you. If you play more on full auto and volume of fire is your thing then it may be worth giving it a miss for now. 

If you do decide you want a MOSFET it’s probably worth having it wired in to maximise the benefits of using one and if your budget can stretch the new generation of in gear box FCUs can really make a huge difference to how your Airsoft guns feels to use. When I’m buying or reviewing a new AEG compatibility with FCUs is a major consideration and some manufacturers are calling that a feature even if it doesn’t come with one as standard. 

I hope one day all AEGs will come with an FCU as standard and we can all enjoy them, until then we have to consider whether it’s worth it or not. 

We had a chat about MOSFETs on Airsoft Action TV, check it out at:

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