In Fig.1 see a DC current ? sensing switch, in which the current is applied from an 8 to 16 Volt supply. The R1 value is chosen so that it generates roughly 100 mV at the required trip current. In Fig. 2 shows how the above circuit can be made to act as a car lamp ? failure switch which turns on if any of the monitored lamps burn out. R is wired in series with the wiring harness feed to a selected ??cluster? of up to four similarly rated lamps.
R1-8=*See text R9-10-11-12=10Kohms RL1-2=12V >120R RELAY
R2-3-4-5=10Kohms RV1-2=1Kohms trimmer IC1-2=LM741
R6-13=2.2Kohms D1-2=1N4001 LB1-4=12V lamp