A small compact unit weighing only 1.5 kg power supply input is from a direct 12V DC rechargeable battery connected by a 240 VAC current limited & isolated transformer charger unit. The onoff switch is incorporated in the operator volume control with 5 watt microphonespeaker fitted with a momentary press to talk switch. A channel selector switch allows the selection of main lock entry lock or both. The master unit listens to the chamber until the press to talk switch is pressed. A headset with boom microphone and press to talk switch is available as an option. Separate send and receive volume controls are fitted neither volume control can be turned totally &ldquooff. Battery autonomy is about 24 hours depending on talk time and background noise.