Do you connect transistors properly?

This tutorial is more for digital guys who usually control things with microcontrollers and other digital circuits. Simple motors, relays, LEDs cannot be connected directly to microcontroller pins because they require more current that I/O can provide and in other hand this isn't safe way to do so. Take a look at this great video tutorial how to use simplest pnp transistor properly when controlling electromagnetic air-valve.

Probably you've done tons of similar circuits with same relays. What you usually do is connect about 1kohm resistor in transistor base circuit ant things work just great. But if you want to understand how actually transistor currents depend on each other and how to select proper transistor and right base current limiting resistor spend some time and make things clear. Completing this you will be able to build more safer and energy efficient circuits. [via]