Parts List
I bought all these items on eBay, from China. Order early, look for free shipping.
Arduino Uno
ATmega328P CH340G UNO R3 USB Driver Board For Arduino w/ USB Cable $6.50
Relay
2-Channel 5V Relay Module With Optocoupler For Arduino $3.98
(you will only use one channel)
Solenoid
DC 12V 0.4N Push-Pull Open Frame Solenoid Electromagnet $4.13
DC Power Plug
DC Power Plug + Socket 2.1mm x 5.5mm Male + Female Connector set $0.99
9V Battery Adapters
2pcs 9 Volt 9V battery adapter for Arduino $0.99