Twin filament headlight bulbs... Fitted to any thing without separate main beam 
An obvious modern example would be a Ford Transit. The controller is there to prevent the dipped and main beam circuits from operating simultaneously
No it's not.
Dim/dip was fitted for only a short time ... SWMBO M reg Hyundai had it. It was to stop people driving on just side lights, when you had the ignition on & side lights, you got your dipped beam on too but at lower power via another box of electrickery.
http://en.wikipedia.org/wiki/Daytime_running_lampUnited Kingdom[edit]
UK regulations briefly required vehicles first used on or after 1 April 1987 to be equipped with a dim-dip device[20] or functionally dedicated daytime running lamps, except those vehicles type-approved to ECE Regulation 48 regarding installation of lighting equipment—this exception was made because ECE R48 did not require dim-dip or daytime running lights, and while countries signatory to the ECE Regulations are permitted to maintain their own national regulations as an option to the ECE regulations, they are not permitted to bar vehicles approved under the ECE regulations. The dim-dip system operated the low beam headlamps (called "dipped beam" in the UK) at between 10% and 20% of normal low-beam intensity. The running lamps permitted as an alternative to dim-dip were required to emit at least 200 candela straight ahead, and no more than 800 candela in any direction. In practice, most vehicles were equipped with the dim-dip option rather than the running lamps.[20]
The dim-dip lights were not intended for use as daytime running lights. Rather, they operated when the engine was running and the driver switched on the front position (parking) lamps. Dim-dip was intended to provide a nighttime "town beam" with intensity between that of the parking lamps commonly used at the time by British drivers in city traffic after dark, and low beam headlamps; the former were considered insufficiently intense to provide improved conspicuity in conditions requiring it, while the latter were considered too glaring for safe use in built-up areas. The UK was the only country to require such dim-dip systems, though vehicles so equipped were sold in other Commonwealth countries with left-hand traffic.[21]
In 1988, the European Commission successfully prosecuted the UK government in the European Court of Justice, arguing that the UK requirement for dim-dip was illegal under EC directives prohibiting member states from enacting vehicle lighting requirements not contained in pan-European EC directives. As a result, the UK requirement for dim-dip was quashed.[20] Nevertheless, dim-dip systems remain permitted, and while such systems are not presently as common as they once were, dim-dip functionality was fitted on many new cars well into the 1990s.[citation needed]
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Topic: Guess the plug (part 1) (Read 4276 times)