Miniature ATtiny LED Probe

LEDs, one might argue, are the most fundamental of all modern display devices. As single units they can show if an Ethernet cable is (really) plugged in, while an array of millions can power a gigantic display appropriate for entertaining large crowds. Although most multimeters have an LED test function, and a battery and resistor will do in a pinch, neither is optimal if you need to go through a lot of these devices.

David Johnson-Davies (DJD) needed to test a large number of LEDs for a recent project, and to speed things up he made his own little LED probe device. The unit takes the form of a teardrop-shaped PCB, with a flying lead connected to one side of the LED-under-test (LUT), while a second probe, made out of a dressmaking pin, sticks out of its pointy end.

The probe can test most LEDs, indicating the LUT’s direction via an onboard red and green indicator that shows the device’s anode/cathode orientation. Connecting the probes will normally light up the LUT, but the onboard indicator shows if it’s operational, even when testing infrared or ultraviolet LEDs that aren’t visible to human observers.

The design comes in an ATtiny85 version, as well as one that works with an ATtiny202. Both test LEDs via pulses that alternate their positive/negative orientation. An analog input that measures the voltage drop across the current-limiting resistor to infer the direction/operation of the LUT. The ATtiny’s low-power operation, plus the use of sleep mode, means the unit can function without a power switch, even with its limited CR2032 battery power supply.

As usual, DJD includes lots of detail on his project writeup, along with design and firmware if you want to build your own!