Arduino Tutorial: Interfacing Arduino to arbitrary electronics (work in progress)


  • Bipolar (2N2222)
  • Darlington (TIP120)
  • N-channel (MOSFET) /NPN (bipolar) vs. P-channel (MOSFET) /PNP (bipolar)
  • Sizing: Voltage and current
  • Pros:
    • Speed (PWM OK)
    • High gain (i.e. small signal current controls much higher load current)
    • No moving parts – highly reliable, long life
    • Inexpensive
  • Cons:
    • DC only
    • Not really isolated
      • VCC or GND only
      • High voltage


arduino5VRelay_schem5V relay 9007-05-00 datasheet


  • Combines LED with phototransistor
  • Absolute isolation
  • Otherwise same pros and cons as transistors

Solid State Relay (SSR)

  • Opto-isolator built with special transistor-like device capable of carrying AC
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