Electronic car burglar alarm - uses Doppler effect on ultrasound signal within car to detect intrusion and trigger alarm via RC time delay circuit

摘要

The alarm includes an ultrasound transmitter and a receiver positioned inside the vehicle for example on the dashboard. The ultrasonic control circuit is sensitive to movement within the vehicle creating Doppler effect disturbance of the received ultrasonic signal. Whilst the disturbance continues, a capacitor is progressively charged until it reaches a certain threshold voltage. When the threshold is attained the voltage on the capacitor is used to activate the audible alarm. The system is not sensitive to transitory effects. The size of the capacitor used, and its associated resistance determine the time delay - typically 5 or more seconds. Typically, the capacitor has a value of 100 micro F and the resistor is of approximately 500k ohms.