A Relay is a digital switch to control much higher voltages and currents than your normal Arduino boards does. When inputs a logic voltage, the relay will switch to allow current to flow or cutoff, depending on your wiring. A Relay usually consists of a coil, 1 common terminal, 1 normally closed terminal and one normally open terminal. When the coil is energized, the common terminal and the normally open terminal will have continuity.
Why do we want to use a relay and do we really need to? Anytime you want to switch on/off a device which draws more current or works with a high voltage, you'll need to use a relay, such as controlling of headlights, parking lights, horns, etc. There are also the 2-Channel Relay Module and 8-Channel Relay Module for your option. Please Note that Connect the VCC to JD-VCC with the jumper when using.
it was great with raspberry pi. easy to use that connect VCC to +5V and connect GND to ground and connect any of IN1/IN2/IN3/IN4 to the GPIO/control pins.
if you want to do more work with the raspberry pi, you should use a combination of USB voltmeter and ammeter that convenient you to monitor the electric and the power usage.
Plug and play with just work and you don't have to worry about building interface circuits, the load calculation to mA or any too complicated. (Posted on 5/5/2017)