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donderdag 19 juni 2014

LED spot light, part 2

In my previous post, I explained how to make a circuit board. Now we're gonna fill it with components and create an LED spot light!
A word about buying components first. If there's only one place you should shop for components, it's Ebay. Seriously, there are lots of Ebay stores that sell electronic components at unbeatable prices! Most of them even ship them for free. You'll have to wait a few days, but it's definitely worth it.

The circuit
The circuit is pretty simple. It contains 24 high powered green LED's, divided over 8 parallel sections, each with its own current limiting resistor. The power is controlled by a transistor using pulse width modulation. I'm not gonna explain what that is, google it, it's very easy to understand. For controlling the pulse width signal, I'm gonna use an Arduino microcontroller, but that's for another post.
In the future, I hope to build a DMX receiver to control my lights. DMX is a protocol used in professional lighting, and it would allow me to use DMX software to control the lights. For now, however, the PWM method will have to do.
The circuit requires 12 volts, and about 0.16 amps of current at full power. For testing, I used my lab power supply, but I plan to build a better power supply for outdoor use.
And it wasn't until after I created the circuit board that I realized I forgot an important component.  The base of the transistor, wich will receive the control signal, should also have a current limiting resistor! I'll have to fix that for my next spot light (I'm gonna make several), for this one I'll have to add a resistor off-board.

Part list

  • Circuit board (see previous post)
  • 24 high powered green LED's, 5 mm diameter
  • 8 resistors 120 ohm
  • 1 resistor 4.7 k
  • 1 transistor type BD139 or equivalent
  • 3-pole screw terminal
If you want to use a color other than green, you'll have to adjust the value of the 8 resistors. Again, I'm not gonna explain this in detail, just google "LED current limiting resistor". Really, it's easy.

Time to start soldering the components to the board! I started with the resistors and screw terminal, because once the LED's are in place, the board would be harder to reach.

After the resistors, I soldered the semiconductors, one lead at a time. Don't solder them both at once, semiconductors are quite sensitive to heat and you should give them enough time to cool down.

Voila, that's all there is to it! Sorry for the crappy photo, it was either that or an overexposed flash photo. I suck at taking pictures.

I found the perfect housing for the circuit at a local hardware store:

It's a coupler for PVC drain pipes, and a screw-on endcap. This is why the circuit board had to be cut into a disc shape! I glued it in using epoxy glue. You'll notice there's a bevel on the inside of the coupler, perfect for holding the board in place. On the next picture you see why I made the three notches in the board.

Next, the resistor I forgot! The base of the transistor uses a 4.7 k resistor. The exact value isn't that important, a few kilo ohms is a standard value when using a transistor as a switch. I simply soldered it to the wires, and then finished it with a piece of shrink tubing.

And finally, a connector to hook things up. There are three wires connected to the board: two for power, and a control wire. I thought the perfect type of connector would be a headphone jack. I used a panel mount connector, attached to the endcap.

The light is quite bright, at full power you can't look directly into it. I have already tested it with an improvised Arduino-based controller, and it works perfectly! I'm gonna make a few more, and then a proper controller.

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