Building An Led Driver Array

RoboGarden Build Log: May 18, 2013

My awesome Chinese 10 Watt LED's arrived in the mail Friday, so I was clearly excited to test them out. Some cursory research had revealed this schematic:

Simple regulator schematic

Which I then adapted into a rough prototype:

Example regulator wired up on the breadboard

Which was powered by a very simple hacked 12V 1.0 Amp wall wart that I found in a parts box. According to the LED specs (which I grabbed off ebay). The forward current should be 900mA at 12V DC. The regulator has a control voltage of 1.25V, which according to this article, could be used to calculate the required resistor for the power regulator:

V = IR or Voltage = Current * Resistance

Rearranged it is V/I = R or in my case, 1.25V / 0.900mA = 1.3889 Ohms.

I opted for a 1.4 Ohm resistor, but realized I probably shouldn't have picked a single resistor, because the only ones available had a lower tolerance of 5%. If it turns out to be an issue, it would seem I could add an additional resistor to fine tune each of the larger ones.

As you can see below, it worked, and it's hella bright:

Very bright regulator test

So, the proof of concept worked. Now, it was time to neatly organize all of the power regulators (LM317) on a bread board. Each of the power leads could be cleanly bent back into the power busses. Similarly, with a little cajoling, so too could the resistors:

Eight regulator circuits wired up on a breadboard

Figuring Out the Relay

I got a SainSmart 2-Channel Relay Module with the thought that I would use it to control both the light array and the little water pump. Figuring out how it was suppose to work was a little tricky until I found the link to the instructions for the relay:

Relay schematic

This certainly is not what I thought of when I was getting a relay; rather something like "oh you just put some small volts here to get large volts here…" Thankfully, the Amazon reviews provided more help:

The connections to your Arduino (or whatever) are: VCC - supply voltage. 5V from my Arduino. IN1 - set to HIGH to set the relay to its "default" state, set to LOW to switch the relay to its alternate state IN2 - same as IN1, but controls the second relay on the board GND - ground

I decided that for now, pins 1 and 2 from the Arduino would control the relays, which would switch main power to the breadboard busses. Here is a picture of the stuff loosely patched together with a little more order:

Arduno, relay, and LED regulators all hooked together

After verifying this worked, I needed to affix the LED's to their respective heat sinks and solder on the main leads. Here is one:

10 Watt LED on large heat sink

Then, I soldered all the main leads together joining all the grounds in one series, while keeping the power leads separate so that each could go to a driver circuit. It occurred to me that maybe it is over kill to have a driver for each LED; I could probably get away with either one, or a few drivers. I'll experiment with that in the future. Here is the array of the grow lights:

Eight LEDs wired together with heatsinks

(Notice the bourbon glass in the background…)

Finally, it was time to wire everything up to the power supply. I'm using a Mean Well 12V x 8.4 Amp or so model, which appeared on paper to have enough juice. But, when I tested all the lights, they were noticeably dimmer than when I had simply jammed in a wall wart at 12V and seemingly 10A (even with the regulator). More surprising, was that running just one LED off of the Mean Well was still rather dim. I think I may source a 150 Watt supply for the final build, or further experiment with the driver circuits. Here is the array on:

All eight LEDs turned on for test

REALLY COOL! I'm excited about the build progress. Next up, I'm going to be building a rough prototype frame, hopefully fabricating some mounds for the watering carriage, and finishing up the electronics testing.

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