That could be done by a chip called a window comparator, witch is capable of
monitoring the current or resistance depending on how its set up, of each
light, but do people really go to this complexity these days?
Brian

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Harry Bloomfield; "Esq." wrote in message
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After serious thinking williamwright wrote :
I don't know but mine has the same thing except it sounds a buzzer.

My fancy system indicates nothing at all if everything is working
properly - I'm not sure it is entirely legal to the letter of the law.

Rather it monitors all of the trailer lighting constantly and puts an icon
up on the screen, indicating which bulb has failed. Once on, it stays on
screen, until the engine is restarted - so no chance of it missing an
intermitant problem.