I have a transistor connected to my Arduino so that I can switch a relay on and off. I followed the diagram here: http://www.arduino.cc/playground/uploads/Main/relays.pdf
The transistor is an NPN type NTE123AP, (2n2222 equivalent), the relay is a R22-5d16-12 (12v coil), and I am using a 1k ohm resistor between the transistor base and the Arduino. When the arduino pin is active, the relay switches on, but when the pin switches back off (~0v on the arduino pin) the relay does not switch off. I poked around with my multimeter and found that the side of the current limiting resistor closest to the arduino is reading ~0v, while the other side (connected to the base of the transistor) is reading 0.73v. If I short that to ground, the voltage goes to ~0v and the relay will then switch off. Do I need some kind of pull down resistor in the circuit somewhere? I have used this exact same design before, with the same code, the same parts, and no problems. I have purchased a new transistor, and I still have the same problem.