The article explains how to build a simple car lights ON warning circuit for enabling the driver to learn about the a car light accidentally left switched ON after a preset delay from the time the ignition was switched .
Anyone who has ever forgotten to switch the car lights off, after having driven in poor weather conditions, and returned later in the day to find the car battery flat will appreciate this circuit. The circuit sounds a buzzer when you turn off the engine and have forgotten to switch off the head lights. When required, the circuit can be reset with push button Sl. This may be necessary, for instance, when you have your headlights on, get stuck in a traffic jam ·and turn the engine off to keep the carbon monoxide level down. When you turn the ignition key and start the engine, the dynamo or alternator runs, relays are operating, the electric fan comes on, the automatic radio aerial extends. . . All these produce large voltage spikes on the car's electrical system and any electronic circuit must be protected against these.
The proposed car lights ON warning circuit is connected to contact breaker S3 and the lighting switch S2. At first sight the use of the contact breaker as an indicator for ’engine running' may seem surprising but, during the testing of the circuit, it became apparent that if the ignition switch is used instead, the circuit will not work properly. The cause for this was found to be variations in the battery voltage. The pulses from the contact breaker charge capacitor C1 via resistor R1. Once C1 is fully charged, there is a stable dc voltage at the collector of transistor T2: the trigger input (terminal 2) of the timer IQ (555) is then not driven. When the engine is turned off, Cl discharges, T1 is cut off and the base- emitter voltage of T2 rises. This causes the collector voltage of T2 to drop to 0. This voltage change is converted into a trigger pulse for the timer lC by differentiating network R9/C4. The lC is then operative and causes the buzzer to sound; with values as shown the warning tone will last for three seconds. The time can be lengthened increasing the value of C6. The circuit does, of course, not operate with the car lights switched off as it is powered from the car lights switch!