Quick and dirty solution to put on a battery to test a circuit for short circuit. the blue resistor is a PTC resistor or polyfuse
PTCs get very hot very quickly. even with an AA battery, you can get a nasty burn off a PTC. and if left alone they can melt plastics and start fires etc.
if the circuit is shorted, the ptc resistance will increase and the bicolour led will turn on. bicolour led lets the circuit work in reverse.
knocked this together because I was trying to fix a little toy and burnt myself on the tiny ptc while trying to find the short
