Cheap Light Meter and LED Tester for Less Then $5

Ive recently been playing with high brightness LED's for an LCD LED projector. The problem i found was its very difficult, so see if all of the LED's are the same brightness, and so difficult to compare individual LED's. Also if you quickly wanted to check an LED for colour, polarity and brightness, it meant messing around with a power supply, and leads etc.

I wanted a simple but accurate device that fulfilled all of the above requirements, had to be simple to make, and didnt cost an arm and a leg. The most expensive component was the solar light!

Out shopping one day, i spotted a cheap solar powered garden rock in one of our discount pound stores, so i invested the huge sum of £1 and sat down to play.

For those of you that havent played with a solar garden light, the vast majority consist of the following components. A solar cell array, usually consisting of 4 or 5 individual cells connected in series and encapsulated as one unit. Depending on the amount of internal cells, the output voltage is usually 2 to 2.5 volts at a low current. There is usually just one single AA Nicad battery rated at 600 Ma, mine had a 1/3 size AA cell rated at 80 Ma. Then last of all, the heart of the unit, is a small circuit board that contains a circuit called a BOOST CONVERTER. This circuit steps up the low voltage from the Nicad, which is 1.2 volts, to a higher voltage of up to 2.5 volts to light the LED, it also turns off the LED when the solar cell starts to see daylight, and then controls the charge to the battery during the daytime.

The LED tester plugs into a standard cheap LCD multi meter, which is set to the 2000 mv or 2 volt scale, but a meter isnt required for the LED testing.

The switch and the 10K pot serve 2 purposes. In the 'UP' position the full voltage from the photo cell, comes out of the tester to the multimeter, at the same time, the internal battery is connected, allowing it to charge in the usual way from day light. In the 'DOWN' position, the 10K pot is switched into circuit, and after setting the pot, reduces the voltage from the solar cell by 50% (my cell gave an output of 2.5 volts, which caused the meter to go out of range in bright light, the meter has a 2 volt range. The other function of the switch is to disconnect the battery when not in use.

So with the switch up, you measure the full output of the cell, you charge the battery, and allow LED's to be tested, in the down position, you reduce the voltage by 50% and disconnect the battery.

The second picture shows the Solar light stripped down and the extra parts used in this project,, which are:-

Solar light, to re-use battery, solar cell and circuit board.
Small case.
External connections for connecting an LED for testing.
10K multi-turn potentiohmeter.
Cable and plugs for connection to an LCD multi meter.
Cable grommet.
Double pole, double throw switch (DPDT)
Sundry items, tools, glue and solder.
some spare time, and a few cups of coffee! 

Step 1: Mounting the Hardware

Right, with all the parts laid out on the bench, decide where you want to mount the various components. As solar lights vary from make to make, its difficult to give concise assembly instructions, you will have to use a little ingenuity and common sense, but just make sure, that, however you mount your hardware components, the parts dont get in the way of any case screws or other things you wish to mount afterwards.

In mine below, top left shows the DPDT switch, with the circuit board soldered to a spare pin for mounting. Centre top shows the multi-turn pot. I drilled a tiny hole in the case, to allow access to the trimmer screw, in case i wanted to adjust the setting, or change the scaling from 50% to some other value, and on the right you can see the pins of the connector for the test LED sticking through the case. The Led was the original which i left in place for testing when i had finished.

Step 2: Connecting It All Up

As you can see, everything is now wired up, i moved the position of the circuit board (yes, i didnt follow my own advice, it fouled on the case) and connected the battery. The battery and the Pot were held in place with super glue, make sure you dont get any glue around the adjusting screw, or you end up with a fairly useless fixed resistor!!!

Step 3: Final Assembly

With all of the components in place, make 2 holes in the case front for the solar cell, check that the cell sits nicely flush to the case, as it will be glued in place later.

On bit of advice, when you dismantle the solar cell from the light, LEAVE the wires connected to the cell, and extend them if they are too short. Many a time ive tried to solder wires to a solar cell and even the quickest of heat, can remove the silver connection pads from the cell.

Now clip the back in place and test the unit, when you are 100% happy, screw the case together and glue the solar cell into position, dont apply glue too close to the edge of the cell, because when you press it into place, it will spread to the edge and get all over the place, also when super glue dries, it leaves a whitish stain around it
(I had to glue my cell AFTER screwing down the lid, because the single case screw was under the solar cell)

Step 4: Setting Up and Calibration

And here is the finished unit with an LED in the test socket.This socket had little buttons that depressed allowing you to push the leads of the LED into the connection holes, just like some connectors found on HIFI's for the speakers.

With the switch in the 'UP' position, plug the leads into you LCD multi meter, set the range to 2000 mv or 2 volts, and measure the voltage from the tester. Then switch the switch into the 'DOWN' position and adjust the pot to read 50% of the first reading (or any other % that means something to you, ie 1/4 1/3 2/3 3/4 etc)

Then assuming the internal battery is charged, connect an LED for testing to the connectors.

3 other bits of advice:-
                                        This tester can be fairly accurately calibrated against a proper light meter, just compare the readings between this and another meter and prepare a simple chart or graph, comparing voltages from this to light level on a proper photographic light meter.

If you are testing individual LED's Make a tube that fits snugly in position over the solar cell, block off one end of the tube. Take 2 plastic LED mounting clips, one for 5mm LED's, and one for 3mm LED's (you could also fit a 10mm mounting clip, if you have the need to test many of those!)
Fit both clips into the end of the blocked up tube, and paint the inside of the tube matt black. Now when comparing different LED's, place the tube, over the solar cell and push the test LED into the mounting clip. Each LED will be exactly the same distance and angle from the cell, keeping readings consistent, and more comparable.

Accurate LED calibration.
                                             The brightness of LED's is measured and quoted in mcd's low output devices are often less then 1000mcd's and high power devices 20 to 30,000mcd's

Taking a handful of different brightness LED's, ie 10 or so rated at 1000mcd and another lot rated at say 10,000mcd and another lot rated at 30,000 and finding the average reading for each group would allow you to calibrate the readout in mcd units. When you then test an LED of unknown brightness, this tester would give you a fairly accurate reading of how bright the LED actually is. Use the tube construction as described above, and use a stable current/voltage source for all readings.

Have fun, i hope this instructable has interested you, feel free to post comments and pics of your own version, for others to see.


After posting this, i bought  some LED's from the local store, i purchased 6 white LED's in 3 brightnesses, less then 1000mcd  less then 5000mcd and less then 10,000mcd, tested them with the tube and holder, noted the results, and then went through my bin box of salvaged LED's....I save every scrap component

it was interesting,