DIY: Low-Cost IoT Solid-state Relay

Ashwini Kumar Sinha

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Electromechanical relays are not reliable power switches. The electromagnet that they use needs to be energised till the plunger is at a specific position, thus consuming a lot of power.

Most of the  IoT devices use the solid-state relay as they have no mechanical switching system and there are nearly zero failure points. At the same time, solid-state relay save a lot of power as they do not require power the whole time. You can even turn the power OFF when switching is completed; it only requires power while switching ON the circuit.

Solid-state relays are also very silent and do not produce noises while switching unlike other switching systems. 

Solid state relays come with a lot of benefits and should be readily implemented. But their price is a major barrier. Electromechanical relays approximately cost Rs 60 while solid state relays cost more than Rs 800.  In order to bring the benefits of solid state relays to more people at an affordable cost, I have simplified its design that can be used with this highly reliable IoT project. This IoT system is operated by Alexa. So let’s begin our exploration with the shopping of the following components.

IoT solid state relay
Fig 1. IoT solid state relay

Bill Of Material

Designing 

First, design the solid state relay by eliminating the mechanical switching system that can be controlled using a small DC current. Despite having voltage as low as 2V to 5V, it can drive and switch the AC appliance and circuit having voltage of 22OV. But how can it switch the 220V AC appliance while using only a 3V switch? For that, isolate the two circuits electrically and at the same time, couple the two circuits using an optocoupler chip. An optocoupler coupler is basically a device with LED and a photodiode coupled together. This allows the two different circuits to share each other’s signals using light. Both do not need to be electrically connected; one circuit sends a signal to another to switch without any electrical connection. As the optocoupler is based on light which travels the fastest  among all other waves, it’s another advantage of optocoupler. 

Cost Optimisation

An optocoupler costs Rs 30 to 80. Because the device is a combination of photodiode and LED packed inside a chip, you can simplify your design with LDR and LED coupling instead of optocoupler

You can use TRIAC and LDR with a resistor to make the solid state relay. When an LED is coupled with the LDR, then the resistance of the LDR connected to TRIAC falls gradually and as a result of the opening of gate, the flow of AC current gets allowed that was blocked by the gate of TRIAC in the IDEAL switch off state .

Fig 4 Solid state relay design
Fig 2. Solid-state relay design
Fig 5.
Fig 3.

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