When you are building lots of circuits and using surface-mount LEDs, there are many times when you have to test a LED to determine the cathode and identify the colour.
You may also need to compare the brightness as some LEDs are TOO BRIGHT and others are very weak.
I have even had the case where one end of a surface mount LED was not soldered correctly and did not illuminate. I had to determine if the LED or the joint was faulty.
That's why this little project was born.
The tester delivers only about 5mA to 7mA and this will help compare high bright LEDs from poor-quality LEDs and also any LEDs that have been damaged due to over-heating/soldering.
One of the big problems with surface mount LEDs is their susceptibility to damage when being soldered. They are more-sensitive than transistors!
I have never damaged a transistor but some LEDs fail after the best attempt at soldering.
That's because the light-emitting chip is only a millimeter from the lead being soldered.
This tester will identify all these problems and it only costs a dollar.
Some of the old LED testers only tested loose LEDs with a current of 1mA, 10mA and 25mA, but this tester will test LEDs in-circuit as well as surface mount LEDs.
This is the first handy piece of TEST EQUIPMENT for LEDs and there will come a time when you wish YOU HAD IT.
The tester also has two more features.
It will test a shorted LED and continuity of tracks.
The LED on the tester will illuminate when the probes touch each other and this proves the batteries are connected. This feature can also be used as a continuity tester and will also illuminate LEDs very faintly through the driver-resistors on the circuit you are testing. So, it has many uses. Just add it to the range of TEST EQUIPMENT designed by Talking Electronics and you will be ready for designing and testing all sorts of projects.
Digital Multimeters (DMM's) will illuminate all types of LEDs (with positive coming out the positive lead) but analogue meters using a single cell will not illuminate any LEDs. But they will not illuminate two LEDs in series and many of the projects we design have two LEDs making a segment of a 7-segment display.
The earth lead is negative and the tip of the probe is positive.
That's why you need all types of test equipment.
When a LED on the PC board is illuminated, it will drop between 1.7v and 3.6v due to the characteristic voltage drop across the LED (according to the colour). This leaves only 0.9v to 2.8v for the voltage across both the 470R and 330R resistors, as they are in series. Since both resistors are about the same value, the voltage across each resistor will be about 1.4v and this is not sufficient to illuminate the red LED. A simple but clever way of turning off the red LED when a LED is being tested. It only turns ON when a short-circuit is present.
This tester can also be used to test continuity.
It was used recently to determine the connections on a PC board from a surface-mount 8-pin PIC chip to the 5 programming pins. The LED on the tester illuminated when connected to the ends of a track. The adjacent pins were also tested to see if any tracks were shorted to other tracks.
Just another use for this handy tester.
A kit of components is available from Talking Electronics.
All the components are included in the kit and everything is identified on the board.
Fine tinned copper wire is fitted through the centre holes to make
contacts for the negative of each cell. These wires are carefully
soldered and the ends trimmed close to the underside of the board.
The cells are then held tight by drawing the two wires together
and joining them with a tiny dob of solder. They must not
move or they will lose contact.
(buy a number of kits and pay
(buy a number of kits and pay