Interrogation of multiple switch conditions

Abstract

Arrangement for interrogating multiple switch conditions employs a single input microprocessor for a plurality of switch elements that are coupled in series with an operating element and a power source. A variable oscillator has a control input and an output coupled to the microprocessor input, providing a periodic signal to the same. A plurality of impedance elements, e.g., resistors, each with a respective value, are coupled between a respective junction between two successive switch elements and the control input of said variable oscillator, such that the rate of said periodic signal varies as a function of the open or closed condition of the respective switch elements. For each switch being tested, the microprocessor looks for one predetermined frequency or pulse rate corresponding to an open condition and another corresponding to a closed condition. The switches can be tested to ensure that they both close and open when directed, and to ensure that none of the switches is locked or frozen into a closed condition. If a given switch does not behave as it is supposed to, a service light will come on to indicate the problem.