Repurposing (and PWM dimming of) 12v LED "auxiliary/offroad" lamps


Flashlight Enthusiast
Nov 3, 2009
Ottawa Ont. Canada
Not sure if this is exactly the right forum, as this is not for automotive use.

I have acquired a variety of 12v LED "auxiliary/offroad" lamps with one aim to use with solar-charged battery
for low-level lighting around the house, or outside. These lights are not meant for main automotive lighting,
however are rugged and quite bright and efficient. Commonly they are spots with 5000K or greater CCT.

I have opened several to look at the LED and driver configuration, and also observe build quality and
suitability to do minor mods. Some are not meant for disassembly and servicing. One 5-LED lamp has
one-piece metal body with plastic lens attached, and took some prying, with a bit of minor damage to the
PCB, but was still OK. Each LED has its own lens insert; by removing these and covering with flat
clear plastic, light becomes more of a flood than spot.

PT4115 is popular buck-driver chip. This lamp had five dual (2 series) SMT LEDs driven in parallel, total
power about 10W. I want to reduce brightness and current consumption for some cases, where practical,
but still allow full brightness to be set.

PT4115 has dimming pin for analog voltage 0-5v, or PWM input. CMOS 555 timer could be used but for
settability, repeatability and stability I used digital PWM. There are several ways to achieve this using CMOS
counters, or even using a small PIC micro-controller. I wanted something very low cost and easy to attach.

In local "dollar" store was found a battery-powered "bulb" with 3 LEDs, running from 3xAAA. It has
off-high-low-flash modes. For $1.25 plus tax, this could work. Bulb itself works quite well providing
low-level light.

With scope probe across one LED in low-brightness mode, 50% duty cycle drive was observed. LED
current is not adjustable, controlled by a small chip. Replacing LED with resistor, voltage swing should be
compatible with PT4115 input.

I removed LEDs from PCB. Due to LED drive being a current sink, resistor in place of one LED would
work, but the on/off sense to high-power lamp would be reversed. Driving circuit would need to be driven
"on" to turn LED lamp off, which would drain batteries of the smaller device faster.Therefore a PNP 2N3906
transistor and resistor was added.

After connecting modified PCB to the high-power lamp, with 2 wires, it works! Battery drain of the controller was
<1mA and might be lowered further. The only features I wish would be lower duty cycle (25%), and no flash
cycle (not really useful), but the price was right!