He came up with Ohm Sense, an iPhone app that will take a picture of a resistor and calculate the value based on the color bands. It’s a great tool that we wish we had when we were starting out.

To calculate the resistance of the series resistor required, we need to look at the current and voltage values recommended by the LED manufacturer. Suppose a manufacturer recommends a maximum ...

and calculate resistor values for driving LEDs. If you’re going to keep a knock-around calculator on your bench, why not build the thing yourself? Given the dual nature of this product ...