Title: Electronic alarm circuitry

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Abstract: An electronic device for indicating an abnormal condition in a rotatable member. A typical use for such a device would be for the detection of a low pressure condition in a tire on a vehicle. In the preferred form for this typical use, a reflecting surface, such as a mirror, is affixed to the wheel and associated with a pressure sensitive device. A light source reflects light off the mirror upon each revolution of the tire which is received by a photo-transistor. A timing device in the form of a retriggerable monostable multivibrator is triggered by the photo-transistor and puts out a high signal for a predetermined period of time. As long as the timing device receives triggering signals at time intervals less than that predetermined period of time, the output of the timing device will remain high and no alarm will be given. If a low pressure condition exists, the pressure sensitive device will move the mirror to misalign it with the light source such that the phototransistor does not give a triggering signal to the timing device within the predetermined period of time. Then an alarm, such as a buzzer and/or light, will alert the operator of the abnormal pressure condition. Means can be provided to prevent false alarms should the wheel be moving at a very slow speed such that one revolution thereof would exceed the predetermined time period of the timing device. In addition, means are also provided to prevent false alarms should the reflective surface coincidentally line up with the light source when the vehicle is stopped.


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