A very simple circuit to supress 12v pulses from illuminating some LEDs

Question

I have recently acquired some Cree LED headlight bulbs for my ageing 5-series. They are amazing enough to actually make the vehicle safe to operate at night. The stock incandescent bulbs were woefully inadequate.

Unfortunately, and legal grey areas of the usage of such lighting technology aside; the vehicle's light control module sends short pulses to the bulbs when the lights are off to determine if they are blown. These pulses aren't adequate to illuminate an incandescent bulb; but are completely adequate to make LED bulbs periodically flash like crazy, making people believe that I'm driving...an inferior but also ageing vehicle.

Anyhow, I'm trying to design a circuit that will achieve the following:

• Be very, very simple to make.
• Supress the short bulb-test flashes from illuminating the LEDs.
• Not affect the brightness of the LEDs when the lights are on.
• Not make the light control unit think the bulbs are blown (I'm happy if the circuit bypasses this function altogether and always fools the LCU into thinking they're not).

The only thing I've been able to come up with, via my limited and often dangerous electrical engineering knowledge, are expensive time-delay relays. Attaching the largest capacitors I have (2200uF) across the bulb connectors has no discernible effect (though I'm willing to buy larger ones if necessary).

Can anyone suggest a design, or where I should be looking for information?

I'm also not averse to a circuit that will apply a simple transient over time to full brightness. It'll make the lights behave more like incandescent lights and therefore look less of a legal grey area!

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Something like this may work. The R1, R2, C values in the diagram has a time constant of around 1 sec, so it should reject pulse of a few 100ms. I really have no idea what is the duration of the pulse you need to reject, so you need to adjust the values.

Choose R3 with just low enough resistance to pass the check. Choose M1 with low RDS-on for low loss when fully on.

An issue to look out for is that M1 will have extended transition time which fits in with your slow turn on criteria but could dissipate significant power during the transition. But the transition should only happen at very low frequency, i.e. when being switched, so may not need special attention.

Answer 2

Credit to @Andrew Morton for this answer; which he is happy for me to submit.

I simply used PA Soft to disable the checks in the Light Control Module (LCM) by going into "module coding" (or similar?) and unchecking the checks, then writing the coding data to the module. Make sure you read the coding data from the module first!

Consequentially, this means the module will NEVER check or report headlights again unless this is undone; so bear this in mind if you may revert back to incandescent bulbs in future.

Answer 3

I would suggest a resettable fuse or polyfuse installed across the bulbs. This may delay your actual turning ON of your headlights.