High Beam Latch Circuit

Back in the day when Mazda was building the RX7 they apparently thought it would be really clever to make their high beam dimmer ECU controlled instead of making it a toggle switch like just about everyone else in the world.  Well it didn’t look so clever to me when this dedicated latch circuit failed on me, leaving my vehicle without high beams.  I threw together a little circuit that uses a T-Flip Flop to toggle the Dimmer Relay.

UPDATED: Made some minor changes to the circuitry to deal with a high RPM flickering problem

Difficulty : MEDIUM
Time: 1-2 hours (including wiring)

Here’s what I threw together to fix this little problem.

Updated 4/8/2010 : I found that during high rpm operation I was picking up some inductive interference from the engine’s ignition causing the circuit to flicker the high beams on and off wildly.  Which is bad…. I found a solution in placing a 12V constant voltage regulator on the input voltage, and a pair of 100nF capacitors at a couple of points.  I’ve updated the schematic below to reflect my changes.  If you’re going to make one of these on your own, I’d suggest putting a temporary toggle switch on the 12V input of this circuit the first time you try it out so you can disable it if you have any problems like this.

In this schematic I have a D-Flip Flop setup in a toggle config controlling the action.  In the circuit that I actually built and implemented I used a NTE754 T-Flip Flop IC, however that isn’t necessary and I could have used any D-Flip Flop that can handle the 11-15V DC you typically see in automotive applications.

The really annoying thing about how RX7’s do their electronic systems is they’re normally ground switching, which creates a few design headaches.  This design could be adapted to switching +12V by correctly biasing a P-channel MOSFET on the output for a high output and using an N-Channel MOSFET to take high input on the input side of the circuit.  Notice I have a reverse biased diode on the input, this is to stop the flash to pass in the car from back feeding through the circuit and actuating that relay.  If you’re going to modify this circuit to switch high, you’ll need to omit that diode.

Most vehicles make it rather easy to pull the relay out of the vehicle for testing as you see me doing here.  I threw a little LED (with a proper resistor) to help me see when the relay was in the NO position. After that I thew the circuit on a hobby PC board and put it in a project box.  Since this is for automotive application, I made sure to put RTV around the seams and holes of the box so that I didn’t get any moisture in there.  once that was done I hooked the input up to the NO dimmer switch and the output up to a wire that went out to the Dimmer Relay.  Also I made sure to put the +12V input on an ignition on lead from the vehicle.  Putting this device on a hot lead could kill your battery if you happen to shut the car off when the high beams are latched on as the relay will be left in the NO state and will draw ~100mA all the time.

Download the schematic in pdf

.brd coming soon

It sounds like this problem happens more frequently than you’d think.  If there is enough interest I would consider making etched units ready for to wire in to the car.  If interested email me.