Do not let its extreme simplicity deceive you — this thing is useful! I have made many over the years and have even given some away as gifts. Yes multimeters have a Vf function, but mine barely lights a green, blue or white LED. This is especially useful in checking SMD LEDs that are very easy to otherwise get reverse. It may also be used to check a string of LEDs (one at a time) to make sure that all are installed correctly.
3V is sufficient to turn on any LED, but is within its reverse break down voltage rating (that is why I did not use the common 9V battery). Short circuit current is approx. 30mA.
LED Tester Construction
I simply soldered (2) 47Ω, ¼W resistors to the two battery terminals and insulated the leads with Teflon tubing — PVC insulation stripped off wires will also work fine. I put a kink toward the end of the lead to keep the tubing in place. I have not had problems with the leads breaking — of course they will eventually fatigue and break, but it is a very simple repair — just avoid bending them sharply.
I used a 3V lithium battery that I salvaged from a circuit board. Good ones have a voltage that exceeds about 3V. Alternatives are the CR123 or DL123 photoflash battery—or you may simply tape two 1.5V AA or AAA batteries together and connect in series.
Soldering to battery terminals
Soldering directly to battery terminals can be risky. Sometimes it can damage the internal battery connection, so keep this in mind and be willing to sacrifice one in the process. My recommendation is to use a hot soldering iron to get it done quickly. Sometimes solder does not stick—in this case, carefully scrape the terminals with a knife to remove oxide and/or unfriendly plating.
How about making a single probe with stranded wires and a pair of spring-loaded test pins?
I keep mine in a small plastic “jewel case” to prevent accidental discharge.
LED Tester Photos