The wiring and electronics systems are the veins of an airsoft AEG. The right wires can be the difference between a working gun, and one that can't supply enough power to make it work. With the introduction of mosfets and computerized systems, all sorts of firing modes and customize-able settings are now at the fingers of airsofters.
AEG wiring systems come in a large variety of different configurations. The main part of an AEG's wiring system is the knife switch. This switch gets pushed into place by the trigger to complete the wire's connection between the battery and the motor, firing the gearbox. In full auto, the trigger will hold the switch on until it is released again, continuously driving the motor and firing the AEG. In semi auto, the trigger push the knife switch into place, but just after the AEG fires and the sector gear spins to start another cycle, a lever is hit by the gear that pops the knife switch out of connection and stops the gearbox from firing again. When the trigger is released, it hooks back up with the knife switch and can fire again. Different wiring harnesses correspond to a specific version gearbox, and at times a specific model of airsoft gun like an M4 or M16.
Most electric airsoft guns use Tamiya connectors to connect the battery to the wiring harness. Tamiya connectors have a positive and negative connection and come in 2 sizes, large and small. Large Tamiya connectors are less common that small connectors, and found on older airsoft guns and large RC car batteries. Though Tamiya connectors are found on almost all airsoft guns, they have a low flow rate and make a poor connection. This low power connection can starve your motor from receiving the battery's full power. Deans connectors are a common alternative, popular among many airsoft enthusiasts because they have a much higher flow rate and are a great upgrade for airsoft guns.
Most airsoft guns have a fuse connected in the wiring harness. The fuse is there to prevent the battery from over heating the motor and forcing too much power to flow through the airsoft gun and damage it. Blowing a fuse often occurs when the airsoft gun gets locked back, and the motor draws as much power as it can to fight the lock back. Airsoft fuses are easily replaced at a local hardware store and run in the 20A to 40A range (Amps). There are 3 major styles of airsoft fuses, the bullet (most common) that has two metal C clips that attach to the fuse to complete the circuit and a plastic housing that holds it together, the 'car' type fuse that uses a car fuse, and the VFC or micro fuse that has a locking plastic capsule and requires a fuse to complete the connection.
Low Resistance / Wire Gauge
Different airsoft guns may have different wire gauges (thickness). AEGs with thin wires often cannot supply enough power to the motor and frequently lock back. AEGs with too large wires all the battery to dump too much power in and fry the knife switch's contacts or harm the motor. Most airsoft guns have a wiring harness that runs somewhere in between. Low resistant wires don't change the gauge of wire but do allow the power to flow more smoothly through the circuits and get more out of the battery. These wires are often plated in silver or gold because of the metal's conductive properties.
Mosfets were originally a means to fix issues with Lipo batteries. When a lipo was attached and the trigger pulled, strong 'arcing' and carbonization would occur on the knife switch contacts. In some cases the lipo would melt the wires, or fry the contacts so badly the wiring harness would need to be replaced. It could also overheat motors and make them loose efficiency and power. Arcing occurs when a ground and a conductive element with lots of power gets close to each other and the electricity jumps from the element to the ground much like lightning. This overheats the contacts of the knife switch and frys the grease and carbon to cover the contacts with a grit of carbon the knife switch cannot conduct through. To solve this a mosfet would be used where two wires would connect to the knife switch with a little power tot the mosfet and another two to the motor with no power. When the knife switch connected the power, the mosfet would dump all the power from the battery electronically, into the motor. Switching from physical knife switch contacts to an electronic switch increases the rate of fire of an airsoft gun, the trigger response (how long after the trigger is pulled for the gun to fire), and eliminates arcing.
Some mosfets use computer chips to adjust different aspects of the gun's cycle or firing modes. Computerized mosfets can be used to create electronic 3 round burst functions, restrict sniper platforms to semi only, or be used to optimize the firing cycles of airsoft guns.