Build: Turn Signal-Brake light converter

Here is the schematic to allow you to use the rear turn signals for both brake lights and turn signals, without changing or affecting how the turn signals operate.

Tail light converter

The front turn signals can power the rear turn signals straight through diodes D5 and D6. Anytime the flasher unit puts 12V on either front turn signal, it will also power the rear.

The rear turn signals act as brake lights as follows:

when the turn signals are not flashing: FETs Q1 and Q2 act as switches to apply the 12V from the brake light switch to the rear turn signal bulbs. Op-amp U1 is wired as a comparator, and the outputs at pins 1 and 7 pull down to turn on switches Q1 and Q2 when the brake light switch sends 12V in to power the circuit. R4 and R5 form a divider which puts 1.1V on pins 2 and 6, so the outputs of U1 are low (turning the FETs on) since the voltages at pins 3 and 5 will always be 0V until the turn signals are turned on. As long as the turn signals are off, the brake light switch powers both rear bulbs through this circuit.

Note: D1 and D2 are required as blocking diodes. Without them, either rear turn signal turning on would force the other side on as 12V was conducted through the body diodes built into the FETs (shown in schematic).

R1, ZD1, and C1 are to protect U1 from system voltage peaks.

R2 and R3 make sure the FETs stay off until Q1 turns them on. R10 and R11 are to shunt off state leakage current from diodes D5 and D6.


When the turn signals are turned ON:

If either turn signal is turned on (while the brake is applied) the rear light on the side away from the flashing turn signal will be steady ON, and the other rear bulb will flash with the front turn signal due to the action of diodes D3 and D4. For example, if the left turn signal is flashing, D3 will charge up C2 to about 11V. That will cause the output of pin 7 to go high, turning off Q1. As long as the voltage on C2 is above 1.1V, Q1 is held off and the right rear turn signal flashes along with the right front as it is powered only through D5. R7 allows C2 to discharge down to about 1.1V in about 0.5 seconds. While the turn signal is flashing, it refreshes the C2 voltage on each cycle. Once the turn signal is turned off, the voltage on C2 decays down below 1.1V (in about ½ second) and the brake light comes back on if the brake light switch is still depressed. In this way, the turn signal operation is the same regardless of whether the brake light is used.

Post edited by: bountyhunter, at: 2008/03/03 17:20

Post edited by: bountyhunter, at: 2008/03/04 12:10

Trying again:





Sorry it looks so ragged, that's all the resolution allowed.

Post edited by: bountyhunter, at: 2008/03/03 19:42

Here are the noes that go along with it:

Here is the PC board to build it on:

With componenets mounted;

Notes for PCB page: