connected resistors 714 and 716 and through the pressed switch to ground voltage division by resistors 714 and 716 causes the power supply 720 to be switched on supplying power from battery 719 to microprocessor 715 memory 717 and an rf transmitter stage 721 initially microprocessor 715 enables a power on circuit 723 to cause a transistor 724 to conduct thereby keeping the power supply 720 active microprocessor 715 includes a timer which disables power on circuit 723 a predetermined period of time e g 10 seconds after the last key 701 713 is pressed to preserve battery life the row and column conductors are repeatedly sensed at input terminals of the microprocessor 715 so that microprocessor 715 can read each key pressed and store a representation thereof a human operator presses a number of for example four keys followed by pressing the enter key 712 the key 711 or the key 713 when one of the keys 711 713 is pressed microprocessor 715 generates a b 40 bit trinary code which is sent via conductors 722 to transmitter stage 721 for transmission the code is formed by microprocessor 715 from a fixed code portion and a rolling code portion in the manner previously described with regard to transmitter 30 the fixed code portion comprises however a serial number.