The buttons on the left control the target current and the voltage of the destination system. Once an LED is connected it will light up and the display indicates the LED’s forward voltage, the LED current, and the calculated series resistor value to obtain the same result at the selected target voltage. It’s a handy way to empirically dial in LED brightness values without needing to actually set up any particular test environment.
On the inside there’s little more than a handful of passive components, an Arduino, an LCD display, and a few buttons. This kind of tool reminds us of the highly clever component testers that hit the hobbyist scene years ago, showing what kind of advanced tricks a modern microcontroller is capable of with the right programming. (Here’s a look at how those work, if you’re interested in some deeper details.)
Source: LED TESTER ALSO CALCULATES RESISTOR FOR TARGET VOLTAGE