Yoinks ago, Robbie Cutler and I knocked up these dinky little low battery voltage alarms for our slope planes, using a little buzzer instead of the LED. I always have to ferret around for the now ancient December '94 issue of the Quiet Flight International Ken Vincent article every time I wish to revisit the project, so the blog becomes a useful reminder place.
The recent availability of ultra bright LED's for day time visibility and a battery scare for Ziggy with his immaculate new Gulp has prompted a revisit to the project. Given that the IC is still available and the two resistors, LED and servo lead are simply soldered to the legs on the back of the tiny IC (about 10 mm long), it is simple, tidy and very compact (thumbnail size) to assemble to the end of a servo lead, which is plugged in to a spare receiver socket and installed in a craft, as against other more busy circuits on the net.
The original article is detailed and offers a bunch of resistor values with various trigger voltages but I have settled on 4.6 volts as useful for four cells. R1 is 30K and R2 is 10K. Think layman drawing for pin orientation.
Can't say you were not given a useful plane saver!