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Old 07-07-2016, 09:39 PM   #1
Bus Nut
Join Date: Feb 2012
Location: Salt Lake City Utah
Posts: 915
Year: 2000
Chassis: Blue Bird
Engine: ISC 8.3
Dimmable LED outdoor/work lighting

Several times I've noticed enclosed cargo/car hauler trailers with a flood light recessed into the exterior wall. It appeared to be a neatly integrated 500 watt halogen fixture. I've wanted something similar on my bus for lighting up a camp site, for investigating when something outside goes "bump" in the night, and so on. Except:
  • don't want halogen -- too hot and too much power
  • needs to be dimmable because excess light could be annoying to myself and the neighbors
Searching online I found the Pit Posse PP2695 Recessed Trailer Light Bucket. So those lights I've seen aren't a single integrated product but an assembly built around an off-the-shelf light fixture. Those buckets are fairly big, though, and I didn't find any good LED alternative to the halogen -- especially when dimmable is in the equation.

However, there are inexpensive LED work lights all over eBay. They are not dimmable, but the housing looks like it might be disassembled easily enough, they're advertised for dc-powered automotive use and likely have a simple power supply circuit, and the price is cheap enough (US$15 for two) for an experiment. Let's see whether they're easily hacked into what I want..

Upon their arrival I tested them for all of 30 seconds before commencing disassembly..

Given the 9-30 volt input spec and the low price I guessed there must be a very simple buck (step-down) power supply inside. As I studied the single-sided circuit board I looked for info online for the chip marked PT4115 that clearly had to be the power supply controller. I had hoped maybe it'd be an adjustable voltage regulator with a feedback loop I could hack into. That's when things took a turn for the dead-simple easy.

It turns out this chip is a purpose-built LED driver and natively features dimming control via a dedicated input pin. Even better, the circuit board was laid out with that pin floating so I could tack on a wire and inject a control signal without even having to cut a trace. It's quite flexible, too: the dimming can be controlled with a dc voltage 0-2.5 volts or according to the duty cycle of a PWM signal on the same pin. Even just an adjustable resistor works (the dim pin has a built-in pull-up). I was about to write a quick Arduino sketch to source the PWM, but then remembered there's a function generator on the shelf I could use instead!

This generator is limited to 20-80% duty cycle so I couldn't see the full dimming range of the controller, but it's looking promising. Super-easy to hack: I'll just replace the two-conductor power cable with three-conductor and build a little microcontroller circuit to send PWM down the third conductor. I thought about superimposing a signal on the power wire, but adding another conductor will be so much simpler.

These lamps are just 18 watts each so my plan is to make a sheet metal bucket large enough to hold two of them, and place two or three of these buckets down each side of the bus. The rough equivalent of 500 watts of halogen will be spread over multiple point sources to reduce shadowing, it'll produce less heat, run directly from the house battery, AND be dimmable! That's coming up next... but first I need to get some sheet metal hung on the bus!

I'm hopelessly mired in re-skinning the whole thing and somehow allowed myself this tangent because of course I "need" to get this light situation worked out so I can weld it into the walls before painting. "Might as well" and "before I do that I must do this" are the death of me... Stay tuned, maybe in a month or two or three, for the continuation of the recessed LED light hack!
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Old 07-07-2016, 10:21 PM   #2
Bus Nut
Join Date: Jun 2016
Location: Garden State (rural NJ)
Posts: 383
An inexpensive LED fixture with on-board (yet unused) dimming circuitry. Nice find! I wonder why there aren't more inexpensive dimmable fixtures with a simple rheostat.
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