Abstract: A capacitance point level switch includes a probe adapted to be disposed in a material containing vessel, the material having a capacitance value dependent upon its level in the vessel. An oscillator circuit develops a periodic signal at a select frequency. A reference network is driven by the oscillator and includes a reference capacitance for developing a periodic reference signal at the select frequency and phase displaced responsive to the reference capacitance. A probe network is driven by the oscillator and is connected to the probe to include the capacitance of the material for developing a periodic probe signal at the select frequency and phase displaced responsive to capacitance of the material. An output circuit is connected to the reference network and the probe network for developing a discrete output assuming a first or a second state according to the relative phase displacement of the periodic reference signal and the periodic probe signal.

Abstract: An improved a.c. voltage powered warning device for detecting the occurrence of predetermined phenomena, is characterized by battery backup for providing power to the device in the event of interruption of the a.c. voltage. In the disclosed embodiments the warning device is a fire detector having circuitry for supervising the energy level of the backup battery and for providing a warning upon the energy level decreasing to a predetermined value, and for testing a combustion detecting and alarm generating portion of the detector.

Abstract: An improved warning device having a sensor responsive to predetermined phenomena to generate a signal which changes in value upon the presence of the phenomena, and means for comparing the signal with a reference potential and for generating an indication upon a predetermined difference therebetween, is characterized by integrated circuit components which may economically be manufactured and assembled, and which maintain substantially constant the sensitivity of the device to the phenomena despite changes in the level of power supplied thereto. Where the warning device is a battery powered fire detector, the phenomena to be sensed is products of combustion, circuitry supervises the power level of the battery, and means are provided for readily testing the entire combustion detecting and indication generating portion of the detector, whereby a user of the detector may be assured of proper operation of the detector in response to combustion.