Another method is to measure the voltage drop across a short section
of the wiring caused by current flow to the bulb. A common voltage
comparator will have enough gain to allow this. Unlike the other
method mentioned, this one only works when  the bulb is ON.


RM


From: Matt Bennett <mjb@ARLUT.UTEXAS.EDU>


>At 02:25 PM 7/26/99 , you wrote:
>>In a discussion with someone else, I was asked if all a car's
>>"auxiliary" lighting could be replaced with LEDs, and/or how to
monitor
>>existing bulbs with a PIC (so you can tell when a turn signal bulb
burns
>>out, etc.)
>
>One feature of incandescent bulbs that you can exploit is that when
off, an
>incandescent filament is very low resistance- much lower than when
it is
>glowing.  You can trickle a small (fractions of a milliamp) current
through
>the bulb- if there is current flow, the bulb is good, none, the bulb
is
>burnt out.  This method can only tell you the health of the bulb
when the
>bulb is off.  Make the current small enough not to light the bulb.
I can't
>remember the issue, but this was detailed in one of the "design
ideas"
>columns in EDN or Electronic Design a few years back.
>
>Matt Bennett