I'm modifying a Razor Ground Force Go Kart that my son has outgrown. The Razor was also eating batteries about every two months. I am replacing the electric motor with a 49cc two stroke engine. The go kart is finished and works great. (Instructable to come) Eventually, I want to include electric start, headlights and brake lights. I want the headlights to be hight/low beam and the brakes to also work as taillights. This is what i came up with.
Step 1: Easy Peasy
It is really simple. I will be using 12v LED bar lights. Red for Taillight and White for Headlights. Per the circuit diagram we just add a 9v 5W zener diode (1N5346B) in parallel with the brake switch or headlight switch. When the switch is open 9v goes to the LED. When the switch is closed, 12v bypasses the zener to the LED.
I upgraded my wife's car lights to LED a few months back and had problems with the taillights not having much difference between taillight and brake light. I didn't want to add a resistor because it would just negate the reasoning for using LEDs. It would just increase the power of the circuit. Google'ing the problem I figured out to put the zener in series on the taillight wire. Since the taillight is now LED, it draws very little power so the 5W zener is plenty enough to handle it. I checked the temp a few hours after using the lights and it never got too warm to touch.
Step 2: Revision
Well, I ordered my headlights and taillights. The headlights are great, super bright and compact. The taillights must be voltage limited with built-in circuitry. (Possibly even a zener) When I went to hook up my switch, there was no noticeable difference in brightness. My guess is the voltage is already limited to lower than my 9V zener. They are not very bright anyway. Really only useful as a marker light. Keep this in mind if you are attempting to do something similar.
An afterthought. If you have a circuit that is requiring more power you can double or triple the zeners in parallel to better handle the amperage. I know there is math to support all this but I'm a little too lazy for that nonsense. (My zeners is rated for 5W.)