Many cars are not factory-fitted with fog light and consequently, there is often no provision for switching. optionally added ones. However, since in most European and North American countries, it is now illegal to drive on sidelights and many cars still have a switch for these. the fog lights may be operated with this. The only trouble is that the fog lights are then on also when the headlights are on. This difficulty can, however, be remedied with two relays and two diodes.
For motor vehicles, an automatic fog light cutoff controlled by the headlights is described. Two current leads are connected to each other via a normally-open relay in the switching circuit for the automatic fog light cutoff. When the fog lights are turned on, the relay connected to the headlight current lead’s ground closes the fog light current lead, activating the fog lights. When the headlights are turned on, the relay’s current flow between the fog light current lead and the headlight current lead is interrupted, causing the currentless normally-open relay to open the fog light current lead and turn off the fog lights.
When the sidelights are switched on relay Re2 is energized and the fog lights are on. When the headlights are switched on, relay Rei is energized via one of the diodes. Relay Re2 is then no longer actuated and the fog lights go out.
Relay Re1 may he any 12-V type with a break or change-over contact, but for Re2 a car type must be used owing to the heavy current through the fog lights. Car typer(days can normally easily’ handle 10 A direct current: they are not normally available from electronics retailers, but rather for car accessory shops.
There is a fundamental difference of opinion between the designer and most car manufacturers, who make it impossible rightly— for the fog lights to be on when the headlights are not. Note that It is illegal in some countries to drive with fog lights on only; even during daylight fog, headlights (dipped) must be on also.