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- 

Voltage_Monitor

  • 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

  1. 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.
  2. 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

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

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