Before we start, we should first go over the difference between a Solid State Relay (SSR) versus an Electromechanical Relay (EMR). A SSR is able to turn on or off a switch (such as the power switch) for a piece of machinery or equipment using a control signal from a digital circuit, microcontroller or computer (such as a Raspberry Pi) through use of non-moving electronics, typically a silicon controlled rectifier.
An EMR does the same function, but uses movable contacts that are mechanically operated by magnetic force. EMRs are most common and you can hear them "click" on and off as they operate when control signal causes the magnetic force inside them to physically move a set of contacts to complete the electric circuit or to open the circuit.
EMRs are most common in electronics since they are cheaper to manufacture and can be used in harsh environments. SSRs are more common in industrial use, such as in Programmable Logic Controllers (PLCs) on a factory floor, as we are simulating in this blog series, since they are have no moving parts, are faster for frequent switching, and can be easily controlled by digital circuits and computers (such as the Raspberry Pi).
It's important to start out with the right parts to meet the requirements of the use-case, such as the IoT Industrial Use-Case we will address in this blog series. Next, we'll look at having multiple SSRs in the 8-channel SSR we'll use in our prototype, and how they can all be controlled with one embedded computer (like the Raspberry Pi).