led tester

2 Articles

LED Probe

LED Probe: A Smart, Simple Solution For Testing LEDs

June 14, 2025 by 13 Comments

If you’ve worked on a project with small LEDs, you know the frustration of determining their polarity. This ingenious LED Probe from [David] packs a lot of useful features into a simple, easy-to-implement circuit.

Most multimeters have a diode test function that can be used to check LEDs; however, this goes a step further. Not only will the probe light up an LED, it will light up no matter which side of the LED the leads are touching. A  Red/Green LED on the probe will indicate if the probe tip is on the anode or cathode.

The probe is powered by a single CR2032 battery, and you may notice there’s no on/off switch. That’s because the probe enters a very low-current sleep mode between uses. The testing intelligence is handled by either an ATtiny85 or, in the newest version, an ATtiny202, though the basic concept and design are compatible with several other chips. All the design files for the PCB, the ATtiny code, a parts list, and a detailed explanation of how it works are available on [David]’s site, so be sure to check them out. Once you build one of these probes, you’ll want something to test it on, so explore some of the LED projects we’ve featured in the past.

LED Tester Royale

January 29, 2016 by 33 Comments

What do you get for the geek who has everything and likes LEDs? A tricked-out LED tester, naturally. [Dave Cook]’s deluxe model sports an LCD screen and two adjustable values: desired current and supply voltage. Dial these in, plug in your LED, and the tiny electronic brain inside figures out the resistor value that you need. How easy is that?

An LED tester can be as easy as a constant-current power supply, and in fact that’s what [Dave]’s first LED tester was, in essence. Set an LM317 circuit up to output 10mA, say, and you can safely test out about any LED. Read off the operating voltage, subtract that from the supply voltage, and then divide by your desired current to figure out the required resistor. It only takes a few seconds, but that’s a few seconds too many!

The new device does the math for you by adding an AVR ATtiny84 into the mix. The microcontroller reads the voltage that the constant current supply requires, does the above-mentioned subtraction and division, and displays the needed resistor. So simple. And as he demonstrates in the video below, it does double-duty as a diode tester.

This is a great beginner’s project, and it introduces a bunch of fundamental ideas: reading the ADC, writing to an LED screen, building a constant current circuit, etc. And at the end, you have a useful tool. This would make a great kit!

Continue reading “LED Tester Royale” →