I use LEDs in almost all of my projects. For those small circuits that just need a couple of indicators, it's easy to just put the LED in the circuit itself and replace it if it is faulty, but what about those projects that call for dozens or even hundreds of LEDs? It's for those types of projects that I came came up with this super simple LED tester.
It should take about 10 minutes to build with parts most any hobbyist probably has lying around. This thing has saved me countless headaches from removing and replacing faulty LEDs in large arrays!
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Step 1: Proof of Necessity
So you may be thinking, "Can't I trust that the parts I order will just work?" Realistically, we should be able to do just that, but the truth is there will always be that one part in every so many that just doesn't work right - LEDs are no exception. I'm not talking about long term testing or failures here. This is just a simple test to see if the LED turns on when it is supposed to do so. The number of faulty parts per batch order will increase with very large orders, older products, or unreliable manufacturers or distributors.
I buy cheap LEDs. It's no secret, and I'm not ashamed of it. The 10¢ difference between a 10¢ and 20¢ LED from two different distributors can really add up when you buy a few hundred at a time. Even if 10% of the cheaper LEDs are faulty, there is still a savings of 45%. The caveat is that each LED needs to be tested before it is used to ensure it actually works. Otherwise, you may be spending a lot of time disassembling something just to replace a couple of LEDs that don't work - this is no easy task in a large array of LEDs that have been soldered together and sealed in some enclosure. It is much less time consuming and frustrating to just test them all first.
Step 2: Gathering Materials
LEDs - What sorts of LEDs are you testing? You should know the minimum forward voltage needed to turn them on. I once worked with an ultra bright white LED that turned on at around 5V DC, but most of the LEDs I work with turn on somewhere between 2V and 4V.
Battery - The best option here is a single 9V battery - alkaline or rechargeable, but anything (or any string) producing at least 4V should work OK for most LEDs.
Battery Holder / Connector - Whatever will work to connect your battery to the circuit.
2 x Resistors - I use 1000 ohm through hole resistors for this project. For an LED forward voltage of 3V with a 9V battery, there should be 6V left - 3V across each resistor. That means 3mA [ 3V / 1000ohms ] should flow through the LED - plenty enough to see if it is working. If you want more current for a brighter LED or you are using a lower voltage battery, use smaller resistors.
Cardboard - This forms the body of the device. I used a 1" x 8" strip.
Scissors - To cut the cardboard.
Electric Tape - To hold the cardboard together.
Hot Glue with gun - To secure the resistors in place.
Soldering Iron and Solder - To resistors to the battery cable.
Heat Shrink and Heat Gun - Not entirely necessary, but protects the solder joints.
Step 3: The Build
- Cut the cardboard into a long strip and fold in half. The ends can be folded up again for more support at the base.
- Wrap the cardboard with electric tape to hold it together.
- Solder one end of one resistor to the positive battery cable.
- Solder one end of the other resistor to the negative battery cable.
- (Optional) Shrink heat tubing over the solder joints.
- Fold the unsoldered lead of one resistor into a 'U" and glue it down with a section of exposed lead running along the top of the cardboard.
- Repeat step 6 with the other resistor on the other side of the cardboard.
- Connect the battery, and test an LED by inserting the top of the tester in between the LED leads, minding the polarity of the battery and LEDs.
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