detecting car lamps faults

[jumpjack]

How can I build a circuit which detects if a bulb in my car burns out?

[torbjorn]

The easiest way is to install a lamp supervision unit from a scrapped car, you don't have to build one from scratch.

Such units have been used on nearly all Volvo from 1974 onwards, and on larger cars of all brands from mid-eighties onwards.

The Volvo unit uses the principle of balanced currents. Each pair of lamps - i.e. left and right parking lamps and rear lamps - is connected through a pair of coils around a reed switch. When the left and right circuits draw equal amounts of current, the magnetic fields from the coils cancel each other and the reed switch stays off. If the currents are not equal, then the reed switch goes on.

Most other units, i.e. the one fitted to Ford Scorpio from the eighties, use a small resistor in series with each lamp circuit and comparators measuring the voltage drops across the resistors.