The electronic displacement detector of the invention is an electronic circuit combined with a mechanical position or motion state detecting element, which, after being activated, gives an alarm signal for a few seconds in the form of a current increase on its signal line connected to a power supply line, and then returns to its resting state. The electronic displacement sensor according to the invention achieves this property by the fact that the mechanical switching element, which performs the direct detection, connects a resting trigger element to the input of the circuit after the changeover, which trigger element has no further influence on the duration of the alarm signal. In the loop, the limit signal is generated by the individual sensing elements drawing orders of magnitude more current from the central power supply during the duration of the alarm compared to their quiescent current. After an alarm signal, which can be set for a few seconds, the activated electronic sensor element returns to its resting state and enables itself and the other elements of the sensor chain to continue monitoring. These sensing elements are intrinsically protected against both a trip short circuit and a direct short circuit of the signal line. Another great advantage of this unit with only three transistors is that, as in the case of complex central alarm systems, it protects not only against wire cutting and wire short-circuiting, but also against manipulation of the end-of-line resistor.
Simple additional circuitry makes the electronic displacement detector suitable for use in AC loops. If a capacitor or inductance is used as a limiting element instead of a resistor, the sensor loop can be fully protected against overfilling with a high value capacitor, manipulation with various series and parallel resistors, and disabling by external foreign voltage interference. In this loop design, also known as the phase-shift method, it is not possible to determine the direction and magnitude of the phase shift between the voltage applied by the central alarm system at the start of the loop and the current flowing through a point in the loop by simple measuring instruments at the sensor element mounting location. The recording unit of the central alarm system shall continuously monitor the phase shift of the voltage applied to the loop and the current generated in the loop by the line resistance and the termination element as R - C or R - L circuit and shall trigger an alarm in case of any phase shift in either direction. The electronic displacement sensor of the invention can also be used for on-site indication of the alarm. This feature makes it possible not only to use this new type of electronic sensor element for complex central alarm systems, but also to create a very simple and inexpensive alarm device.