Abstract: Pulses of a carrier signal supplied to an input circuit of a codable electronic device, e.g. a transponder tag, are divided by a chain to provide a logic signal, the value of which depends on the length of the pulse, the logic signal only being entered into a data store upon termination of the pulse. The logic signal is entered by a reset signal from output of the input circuit.

Abstract: An active transponder of an identification system comprises a memory for storing coded information and supplying it to a transmitter output, and data means independent of the memory for supplying further information to the output. The transponder has a battery and the data means can comprise detectors which detect when the voltage of the battery falls below respective predetermined levels. The data means can also comprise input connections arranged to receive signals representing the updated values of respective parameters. These signals are supplied to respective inputs of a data selector independently of the memory.

Abstract: An active transponder device comprises a CMOS integrated circuit including means for receiving an input signal including coded information, a memory for storing said information, and transmitter means for transmitting an output signal including said information, the input signal and the output signal being both low frequency induction communication signals.Normally, the output signal is emitted in response to an interrogation signal received via the receiving means, and is formed by modulation of that signal with said information after readout from the memory. The transponder operates inherently synchronously, using a single master clock signal which is generated either in an interrogation transmitter or in the programmer for injecting the coded information.The integrated circuit has a second receiving means used in the injection of the coded information. The injection of a clock-pulse which is in synchronism with the input signal enables the memory to receive the coded information.

Abstract: A presence sensing system comprises a detector having a transmitter for transmitting an interrogation signal to an area to be kept under surveillance and a receiver for receiving a presence indicating signal. A receiver/transmitter device the presence of which is to be detected is adapted to receive the interrogation frequency and to transmit a presence indicating signal in response thereto. A phase locked loop circuit has a first input coupled with the output of the receiver a second input for a reference signal, and an output for providing a phase locked signal which is indicative of phase coherence between the signals on the two inputs. A frequency generator has an output coupled with the second input of the phase lock loop and an alarm is actuable in response to the phase locked signal.The invention also includes the presence sensing detector described in the above system.