This circuit demonstrates the use of a NPN-bipolar transistor to actuate a solenoid (the relay being an example) using the logic output from a control device such as a PIC or Arduino.
A couple details to note:
1. When powering a DC solenoid it is important to add a diode in parallel with the coil, with reverse polarity to the input power. The magnetic energy stored in the solenoid when the power is removed will generate electricity as it deactivates. The diode will act as a short circuit for that energy and it will absorb it rather than allowing it to feed back into the transistor or the Logic circuit that drives it.
2. The voltage coming in to the base of the transistor can be anything logic-driven. (1.5v, 3.3v, 5v, etc.) But you will need enough amperage to drive the transistor and allow it to conduct accordingly. Most only require a few hundred micro-amps. Be sure to look up the details of your transistor first. On that same note, be sure your transistor will handle the amperage of the solenoid. Don't use a 200mA signal transistor to control a 5A solenoid!
