The FRED can be powered by a 9 volt battery, fixed voltage (6-20 volts) AC, DC, DCC, or by track power. The electronic circuitry produces a high intensity flash of an ultra bright red LED and is usable as an "end of train" device, on top of water tanks, towers, ambulances, fire trucks, locomotives, caboose's or other applications. They will fit HO and larger cars. The FRED is furnished with double sided mounting tape and includes wire leads with a 9 volt battery clip installed. For other power sources simply cut the battery clip from the wires. If an on/off switch is desired, item #524 does the job! F.R.E.D. stands for Flashing Rear End Device. The board size is 0.84" X 1.65"