Electrical & Audio Guides
Lights-On Warning Indicator
A simple circuit that needs only four wires connecting for it to work. The circuit draws it's power from the light circuit of the car and it is grounded through the door switch. To ensure that it is only operated by the drivers door switch a diode can be used after the point where the warning circuit is connected thereby preventing the circuit grounding through the passenger side switch. The third and fourth wires on the circuit ensure that the circuit will only sound when the ignition is in the off position. When the key is in the accessory position the transistor is powered and so stops the circuit from powering the sounder. When the ignition is off, the ground connection pulls the transistor base to ground and allows current to flow when the door is opened. This means that the circuit will only sound when the lights are on, the ignition is off and the drivers door is open, unlike a majority of devices which simply sound when the lights are on and either door is open. The circuit is also very small and will fit in the steering column surround so making it easy to mount inside the car.
Diagrams:
Diagram for the main circuit
Diagram for the door switch circuit
Stripboard layout for making the circuit
