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Basic Light Test


James Collins

Jan 1, 1970
I am looking for a way to test the continuity of a simple circuit with
a 12v light bulb. An example is that I would like a LED on the dash to
light up with a brake light bulb has blown, simple to do when the
brake light is activated but can it be done continuously without
lighting up the bulb?

Henry Kolesnik

Jan 1, 1970
Not quite what you want but a LED will light up with less than 20ma so you
if you put a 700 ohm or so resistor in series with the LED, connect one end
to the cold side of the brake switch, and the other to battery and the LED
will light while the brake light won't even glow. A flashing LED might even
be cooler. Or you use an RC circuit with whatever time constant you want to
flash the LED. Since brake light bulbs are in parallel, the light will not
tell you you if one is out! That would require more thought! Another way
would be to get a strand of fiber optic cable and run one to each lamp and
to the dash but it'll only light when you step on the brake.
Perhaps someone else has a better idea.
hank wd5jfr