The Relay should be on a separate board from other DC Electronics. There should be a fuse on the AC and the DC if the voltage is high."
You can sometimes connect a relay coil directly to an Arduino output pin, if the relay coil draws less than 40mA. You will still need a back-EMF protection diode."
Relays wear out and duplicate parts should be obtained to be able to service artwork sold. Repair concerns should be addressed at time of sale. It is perhaps smart to use a relay with a socket or removable plugs.
When a relay coil is de-energized a "back EMF" can be created. This is a high voltage spike that travels back through your relay signalling electronics, in our case our Arduinos. This can fry the electronics. A Flyback diode is placed across the lines to suppress the spike. http://en.wikipedia.org/wiki/Flyback_diode
What is a DPDT Switch? Scroll down to the chart. Note that the terminology also applies to relays.
SPST Single Pole Single Throw. Switches one line between off and on.
SPDT Single Pole Double Throw. Switches one line between one connection (pole) and another.
DP (Double pole) works like two independent SP swithces with one shared toggle.
SPST off- (on) the parenthesis indicate that the on condition is momentary. That is when you are holding the button down the switch is on or closed but when you release the pressure it is off or open.
