Backdraft Airsoft says that the Backdraft Catalyst FCU is the cheapest FCU in the market... "The backdraft catalyst is the cheapest and smallest FCU on the market, sports 5 different fire modes, is easily adjusted with 2 potentiometers, and is compatible with any single solenoid airsoft HPA engine on the market!

For 65 dollars you get your choice of wire harness, trigger board, and one catalyst FCU with semi auto, full auto, 3 rd burst, 2 rd burst, binary, semi locking, and 2 potentiometers to control the cycle time of the solenoid

There are 2 different lengths of wire harnesses, a short version which stores your FCU and battery in the handguard or grip, and a long version which can store your FCU."