While there are calculators out there that tell you the amount of resistance to connect in series with your LED, you'd still have to guess the operating voltage and current. LEDs don't normally come with a datasheet and whatever specifications they come with may very well be inaccurate. This little circuit will allow you to determine the exact voltage and current to supply to your LED.
The LED tester isn't my original idea. I came across it here. I was pretty much testing my LEDs as he did before he made the tester; hooking up an LED, a potentiometer, a power supply, and a multimeter. Not the most elegant of methods and often very troublesome. A current regulator circuit wasn't new to me but it never came across my mind to use it as an LED tester. I do however, consider my board to be neater with the test pads/loops arranged in a more intuitive manner. And while its no rocket science to produce the PCB layout from the schematics, I am supplying my layout for your convenience.
If you check out the original author's website you'll notice that I have something extra in my tester. He used a double-sided board, hence he can afford to solder the components on one side and have the large flat pads on the other side. I ran out of double-sided boards at the time I made mine. At first, I thought of just having an extra small piece of board back to back with the main board and soldering the two together to get a partial double-sided board. Then I thought maybe I could make a socket so that the large test pads are removable and can be plugged into a breadboard for other uses. Imagining how it would look like, I realized it would have a rather high profile and was thinking of a solution to reduce the height. It then came to me that I could probably make use of the space underneath and add a magnet so the LEDs (both through-hole and SMD) will stick to the pads without me holding it there. I quickly test the idea out with a magnet and some components and it seemed to work.
It only occurred to me to write up an Instructable on the LED tester when I saw the Get The LED Out! contest. I was already using the LED tester for quite some time so this was documented after its completion and may lack photos of the project in progress. If there's anything that needs to be cleared up or explained please do not hesitate to post a comment.
I am assuming the reader will have at least basic electronics knowledge and sufficient skills in soldering and PCB fabricating.
This project has three sub-Instructables because I feel that each part deserves its own guide:
- Another Quick PCB Prototyping Method
- Magnetic Surface Mount Device (SMD) Adapter
- Trimpot Knob Turning Tool
Step 1: List of components
1x 9V battery
1x 9v battery clip
1x 2-pin female header connector (pins & housing)
3x 1-pin SIL socket
1x 2-pin male header
1x 2-pin right angle male header
1x Shorting block
1x 100nF capacitor
1x 1N4148 diode
1x LM317LZ positive adjustable regulator
1x 39 ohm resistor
1x 500 ohm square horizontal trimpot
1x Female header
1x 8-pin IC socket (needed only if you're making the adapter)
1x 50mm X 27mm copper clad board
Materials for the magnetic SMD adapter (optional):
2x 4-pin male header
1x 12mm X 27mm copper clad board
The capacitor and diode are not crucial to the operation of this circuit. I used them to make my board look more populated.
I reduced the value of the resistor to 39 ohms (can be harder to find) instead of 47 ohms so that my tester can output a maximum of about 32mA. David Cook's version can output up to about 25mA. I do use some high power LEDs and 25mA is not enough yet 32mA for short durations should be relatively harmless for weaker LEDs. You can use a 47 ohm resistor if you're happy with 25mA max.
You can determine the max and min output current by dividing value of the reference voltage on the LM317LZ (1.25V based on my datasheet) over the value of your sense resistor (trimpot + resistor to be correct).
Min output current (trimpot set to max of 500 ohms):
1.25V / (500 ohm + 39 ohm) = 0.0023A = 2.3mA
Max output current (trimpot set to min of 0 ohms):
1.25 / (0 ohm + 39 ohm) = 0.0321A = 32.1mA
Use the equations above to make an LED tester with a different current output range if you desire. Just remember that the LM317LZ is limited to a max output current of 100mA.
You will also need soldering equipment, some double-sided adhesive tape (for attaching the PCB to the battery), and PCB fabricating tools and materials (depends on the method used). You should already have all this available if you had ever done any home brew electronics.