Electrodroid is an Android app designed for electronics hobbyists, but it has uses for model railroaders too. Its LED calculator is invaluable when using LEDs to light buildings, cars, locomotive cabs or headlights, or for other projects. Knowing the input voltage, you can then determine what resistors to use to protect the LED.
The voltage drop calculator is useful too, if less obvious. Read more

David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.

