This is a 4 Channel 3-24V Solid State Relay Module. A solid-state relay (SSR) is just what it sounds like; an IC that acts like a mechanical relay. They allow you to control high-voltage DC loads from lower voltage DC control circuitry. They accomplish this by using infrared light as the ‘contact,’ a solid-state relay  is really just an IR LED and a photo Triac sealed up into a little box. Solid-state relays have several advantages over mechanical relays.

A solid state is similar to a mechanic relay where it can be controlled through a digital signal. solid state relay generates no noise and has a much longer lifespan compared to the traditional mechanic relay.

One such advantage is that they can be switched by a much lower voltage and at a much lower current than most mechanical relays. Also, because there are no moving contacts,solid state can be switched much faster and for much longer periods without wearing out.

Input Parts :

1. DC+: connected to VCC (Supply power according to relay voltage)
2. DC-: connected to GND
3. IN1: CH1 Signal triggering the end of 1-channel relay module (active at the high level)
4. IN2: CH2 Signal triggering the end of 2-channel relay module (active at the high level)
5. IN3: CH3 Signal triggering the end of 3-channel relay module (active at the high level)
6. IN4: CH4 Signal triggering the end of 4-channel relay module (active at the high level)