Monday, May 13, 2013

Caltrans Love

Early last quarter I helped clean out a storage room in the ATL here on campus. The Advanced Technologies Lab is home to some supercomputing power, machine shops, car garages, and conference rooms. While cleaning out one of the rooms, I came across this beauty:

CalTrans traffic light, complete with sun-damaged ABS plastic and a decade of dust and dirt!

Find out what I did with it after the jump.

Donated by CalTrans back in the 90's, this was used for students to compute traffic algorithms and write traffic controller code for the light. Since then the hardware had been condemned to storage and gathering dust. I like to think I liberated it!

This particular unit had some peculiarities:
  • Plastic ABS body with a metal base
  • "One / two cars per-green" sign denoting it was for metering light use
  • One large 12" red LED array with built-in controller (12V DC, 3A draw)
  • One medium 8" yellow LED array with built-in controller (12V DC, 2.5 A draw)
  • One medium 8" green lens with an....AC lightbulb socket? what?!

You read that correctly. 2/3 sockets were LED and DC powered, while the third was a combo of an AC screw-type bulb socket and a aluminum reflector. Upon doing some research, the drop-in 12V LED modules for traffic lights are bloody expensive! I suppose they can rip off the cities and counties who buy them, but still....$200 per array? In any case this wasn't an option for me.

Another problem was deciding how to power this thing. With both lights on the drivers pulled nearly 5A...much more than any power supply I had could handle. Nolan Reker, a fellow engineer and friend of mine, suggested a laptop or Xbox power supply. That was the plan until I came across 12 or so Dell M8811's, or GX620 SFF desktop power supplies, at Cal Poly Surplus. Free from their garbage bin, these beauties output 12V with a max current draw of 18A. After opening them up to short the remote sense wire, I attached some 10 ga wire and a powerpole connector to the end (I now am madly in love with powerpole connectors by the way). Here is the end result (sorry for the potato):

Dell M8811 with new leads and powerpole connectors. Green means go! 18A of 12V goodness

Lighted up at full-power for the first time

Back to that pesky green light issue. I thought about converting an AC module from eBay for DC use, but at that point I might as well make my own array. I thought about 200 or 300 small green LED's, but directionality with these is poor. I wound up buying 3x 3 watt SMD LED's on aluminum PCB's from gybbshop on eBay. Each LED runs at 3.0 - 3.6V DC and draws 450 800mA. After some testing I determined that the best brightness / current point was around 600 mA. Since these LED's were in need of a driver, I decided to make my own. I slightly modified a constant current LED driver circuit I found on instructables, and after some fiddling with resistor values I got what I needed: 7V with an output current of 1.2 A max. 

Constant current constant voltage driver circuit. Some of the parts decisions were mostly because its what I had on hand. Dropping down to 7.2 V, the FET doesn't heat up too much, but testing ~2V the FET was too hot to touch. Beware. 

This will drive two LED's in series with another two in parallel. Before you say anything, I know this is generally a bad idea. Internal resistance of LED's can change over time and lead to fried boards. Since I am under-driving these to begin with and they are relatively low-cost, I decided the simplicity of the circuit was worth more than the risk of a blown LED.

Perfect fit! Yes I removed the blue masking tape after mounting.

Heatsink installed. I was able to use the original retention clip for the AC socket. Don't forget the thermal grease!

Final mounting in the bottom of the reflector. I couldn't have planned this better if I tried.

Finished driver circuit. 12V in, current limited 7V out. The zip tie is to prevent strain on the FET leads.

Reflector, LED array, and driver circuit mounted. I used a static bag to isolate the driver circuit from metal bits inside. Still LOTS of room in there though (rPi and relay board?)

All powered up! I admin the green is a little bright compared to the yellow, but it balances out the 12" red nicely.
All in all, I'd say that this project has been a huge success. My initial concerns about power and the green light were solved for less than $20 of parts and a few man-hours. Next up for this project is a relay board and RaspberryPi. I'm thinking VU meter, disco mode, and maybe some sort of network status indicator.

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