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.
External connections for connecting an LED for testing.
10K multi-turn potentiohmeter.
Cable and plugs for connection to an LCD multi meter.
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
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.