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AUTOMATIC_VEHICLE_DOOR_UNLOCK_CIRCUIT

2017-01-06 06:19  
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AUTOMATIC_VEHICLE_DOOR_UNLOCK_CIRCUIT
When the ignition switch is on, the +12V and IGN lines connected to the circuit are within half a volt or so of each other. Resistor R3 reverse-biases Darlington Q2 and keeps it cut off. Transistor Q1 is in parallel with Q2's emitter and collector; Q1 is ON as a result of the large voltage drop across Q2's emitter and collector. When the ignition switch is turned off, voltage is removed from the IGN line, effectively placing R5 at ground level. Capacitor C1 provides a path for current to flow through R3 to R5 and the vehicle components to ground. That flow will forward-bias Q2, driving it into saturation and lowering its emitter/collector voltage to less than 0.5 V. As a result, almost full battery voltage is applied to the vehicle unlock relay; transistor Q1 is cut off at the same time. The voltage divider of R3 and R4 prevents false triggering during voltage sags on the IGN line.