Features:

Relay Shield (ie 5V Quad Relay Module) is a small current signal control power equipment commonly used electronic module, enabling high-power single-chip control devices are widely used in SCM system to make smart home project; and can be directly plugged into the Arduino compatible the use of various types of motherboards, eliminating the patch cable troubles.

Module features are as follows:
1. Standard Arduino Shield interfaces and shape
2. Can continue to stack other Arduino expansion boards
3. 3 M3 screw positioning holes for easy installation
4. High Drive (5V or 3.3V) normally open contact closure
5. Onboard relay indicator (red)
Special Note: When using the Arduino UNO board connected to it, because UNO USB interface is extremely high, should avoid green terminals are shorted its shell.

Relay Shield Interface: (the last picture show)
Relay: sky- 5V DC relay , model HJR-4102-L-5V, which Relay1 behalf of the 1st relay , the other and so on.
Power Interface : 5V DC power interface
Indicator: four lights are red when the relay is driven high , the corresponding indicator will light up .
Relay Terminals: Terminal high-power , high-current high voltage can be connected to the device . The 1st foot ( i.e. NOx) is a normally open pin , NCx pin is normally closed , COMx is common, i.e., when the relay is not driven , NOx and COMx open between two pins , NCx and COMx connection ; When the relay (x) control signal pin is high drive relays , NOx and COMx connection open between NCx and COMx.
Relay Shield v0.9b Schematics:

Package Included:

1PCS*4 Channel 5V Relay Module Board Shield For PIC AVR DSP ARM MCU Arduino UNO R3