This excellent tutorial was created by Rich Pyke, and has been posted on his behalf.
This Tutorial will go through the steps for making a TIP120 and TIP122 transistor switching circuit. These can be used much like a relay or switch for turning on and off a string of LED's for example, but have the added bonus of using PWM (pulse width modulation) so you can adjust the brightness of the LED's. This became an evolving tutorial with additional information and options available being added. The change log at the end of this post explains changes made. Any additional information will be added as and when discovered.
! 1 x TIP120 or TIP122 Darlington Transistor or IRL3103PBF
! Mosfet (see notes at the end)
! 1 x 1k ohm Resistor
! 1 x Small Piece of Strip Board (7x5 holes)
! 1 x Pin Header (1x2)
! 1 x servo Extension
! Soldering Iron
Extra parts needed (if inductive load):
! 1 x 1n400x Diode as required
Search for part numbers in google or eBay, or use your preferred supplier.