In the need of a single relay controlled by an Arduino or form-factor compatible board? Here's the Simple Relay Shield to the rescue!
The Evil Mad Science Simple Relay Shield is an Arduino add-on that lets you use your Arduino (or shield-compatible board) to control a single electromechanical relay, capable of switching loads of up to 24 V DC or 40 V AC, at 5 A of current. This relay is an "SPDT" type, meaning that it can be used either as a normally-open or normally-closed relay, so that you can choose to either switch on or switch off your load when the output signal from your Arduino is high. When operated as a normally-open relay, it is capable of handling loads up to 10 A (see note on safety below).
The Simple Relay Shield is sold as an easy-to-build soldering kit. The on-board relay is powered directly from the 5V power supplied by your Arduino board and typical power consumption is under 70 mA when the coil is energized. The relay is controlled through a transistor that is connected to Digital pin 4 on the Arduino headers. To create a demonstration sketch for the Simple Relay Shield, open the "Blink" example sketch and change the pin number from 13 to 4.
When output Digital 4 on your Arduino is low (or if the Arduino is powered off), the "Common" (middle) pin of the screw terminal is electrically connected to the N.C. ("Normally Closed") pin of the screw terminal, and is not connected to the N.O. ("Normally Open") pin of the screw terminal. When the output Digital 4 on your Arduino goes high, the relay turns on, and the Common pin is disconnected from the N.C. pin, and is instead connected to the N.O. pin. When the relay is on, the "Coil On?" LED on the circuit board also lights up to indicate this.
Special note on safety:
As (a) the Simple Relay Shield can handle moderate voltage and current and (b) the there are any number of Arduino and Arduino-compatible boards that the Simple Relay Shield can attach to, it is the user's responsibility to ensure that there is a safe distance between the bottom of the shield and any protruding components on the host board.
While the relay itself is capable of operating with AC loads as high as 120 V, we have derated the Simple Relay Shield to 40 V AC for two reasons. First, there is often very little spacing between components on the host Arduino board and pins on the bottom of a shield board. Second, the physical setup when using Arduino-type boards typically does not provide any mechanical means to prevent accidental touching of the wiring. If you choose to exceed our rating (at your own risk), be certain that you have thoroughly addressed these two points and to take any additional measures as necessary to ensure that the shield can be operated safely.
The relay shield is labeled with a current rating of 5 A. However, as per the relay's datasheet, it can be safely operated as high as 10 A (AC or DC) if configured strictly as an N.O. (normally open) relay, with nothing connected to the N.C. terminal.