Headlights Indicator

It is never advisable to leave a car"s headlights on for long periods when the engine is not running. Yet, especially during the winter months, many of us in- advertently do this. The indicator de- scribed here helps to prevent you suf- fering the consequent and inevitable flat battery. In its simplest form, the indicator con- sists of a d.c. buzzer and a diode as shown in Fig. la. With the headlights on (S3 closed), the interior light (Li) does not come on until one of the front doors is opened (when either Si or S2 closes). At the same time, the buzzer is ener- gized and sounds. Either closing the door or switching off the headlights (S3) turns the buzzer off. The diode in series with the buzzer is necessary because B is normally at + 12 V via LI and A at ground via the headlights (L2-only one shown here). The buzzer should then not sound, of course. A slight modification to the circuit, en- abling it to operate only on the switch in the driver"s door, is shown in Fig. lb (whe...