Feedback on time-delayed TRIAC design

Question

I would appreciate some feedback regarding the design I came up with. While I am not unfamiliar with electronics, this is the first time I actually designed something myself.

The purpose is to solve an issue with my Land Rover / travel trainer. The car sends out every 2 seconds a short electrical pulse of 100ms length, to check if a trailer is attached and if the trailer lights are still OK. With incandescent light bulbs this works just fine, as they need a moment to light up. Our trailer however has LED light, which cause the following problems:

• The short pulse makes them flash briefly
• The car does not recognize that a trailer is attached due to the LED lights high resistance.

My design is supposed to do the following:

• V1 is the car's brake light output
• R1 simulates an incandescent light bulb, so that the car 'sees' the resistance it is expecting
• R2, 3 and C1 delay the TRIAC to turn on by around 150 ms.
• The delay of 150 ms exceeds the duration of the pulse, the brake light will therefore only turn on when the brake light is actually activated.

I simulated the circuit in LTSpice, everything seems to work just fine.

What are your thoughts?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

you could add a series capacitor to R1, to not ruin the power saving of the led over an old bulb.

but the thing gets complex. you want a reliable brake light, so you got led. who cares if the car does not detect the trailer?

Answer 2

you could try a simpler solution. just connect an inductor in series with the led.

Answer 3

As this is an electronics forum, I'd propose to avoid bulky inductors or capacitors by going to a more digital solution. Use power mosfets or darlington transistors to switch the lamp imitation resistor R1 and the led. Use some digital stuff to do the timing of 150 ms and control the fets/transistors. An Arduino board may be easy. You'd power it from a free 12V wire in the trailer cable. If you find this overkill, just look at what is inside a regular led lamp for 120 or 230 VAC: a lot of led controller and transformer stuff in one cubic centimeter for under 5 euro/dollar.

But because the application is a car to be used on public roads, I'd hesitate on this solution. How much testing do you need to guarantee safety and reliability? How about insurance coverage? How about regulations?

There are two other options, that are safer.

1) Replace the LED brake lights with old style Bulb

2) Inquire at the car dealer if the car computer can be configured to recognize the led. Perhaps an OBD setting.

If you insist on a fun electronics project, you should investigate the detection pulse system. Perhaps R1 can be larger and still be recognized as a "lamp". Perhaps the detection takes place in less than 150 ms. Perhaps the car is so smart that it looks for a resistance that increases after some milliseconds, just like a heated filament.