I guess this works as a simple solid state warning light assimilator. I built one of these years ago and it's worked fine since then. I just soldered the parts together, shoved them into a piece of heatshrink with some dielectric grease - shrunk it up and then wired it up in the headlight shell. I found a nice chrome LED holder and mounted that to the handlebar pinch bolt. You should be able to get all the parts at Radio Shack - I doubt I spent more than 5 bucks on it.
Copied from Jon Weitzman from an old Airheads.org post-
- The bottom of the LED is always at 7.5v due to 7.5v zener
- The top is equal to battery voltage minus the 6 volts from the top zener.
- LED’s need about 2v to turn on and will only work when current flow is one direction (to the top as drawn)
- Resistors provide curentl limiiting for the LED
- Below about 10v, neither Zener is zenering so you don’t need to eval them
- Both zeners dissapate around 300mW,
- You need to put this on a switched circuit as it draws around 70mW when not lit and 25mW when lit
So by way of example
- If the battery is at 11v, the top of the LED is at 11-6 = 5v, the bottom is at 7.5v. This represents a 2.5v difference and the LED is ON.
- At 13 Battery Volts you get 13-6 = 7. This is only a 0.5 difference, light OFF
Other approaches
Here's a way of doing it with an opamp - http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm
Or you can buy one of these microcontroller based versions
- AO Services Battery Status Monitor - (which you can get from Paul Goff or Sterling Motorworks ) Ambient light detection is a nice feature.
- Electrical Connections Battery Monitor - MCN likes this one
- Signal Dynamics - Heads-Up Voltage Montior - Similar to the Electrical Connections unit, but has auto - dimming
Or you could cobble your own out of something like a PIC16F84 - it couldn't be that hard.
Of course it's all been discussed before - http://www.accessnorton.com/battery-status-monitor-t6555.html.