>> ISOLATING INDEPENDENT CIRCUITS WITH OPTOCOUPLERS

Yet another handy electrical component everyone should be aware of - so many uses!

An optocoupler (aka optoisolator, photocoupler) is quite an cool electrical component - it allows to independent circuits that may be operating on different voltages to allow one circuit to act as a switch within the other. The hardware utilizes an LED on the initiating circuit to light up and allow a photosensor on the receiver to trigger and close the circuit when triggered. Why does this component exist and how can it be used in an IoT project?

There are two reasons to use an optocoupler:

• to isolate one circuit from another, each having different voltage
• prevent electrical noise or other voltage transients from interfering between circuits

In the majority of IoT projects; the first reason will most likely be the reason for the decision to use an optocoupler. You may need to integrate with an existing hardware solution that is simply not capable of using the same voltage of your micro-controller. Remainding independent also means you are in no way interfering with the various electrical certifications the hardware may have.

It doesn't take much to setup a test environment to see an optocoupler in action.

The key thing is to isolate pin 1 (normally denoted with a dot) and then wire your signal source on the controller circuit to it; and pin 2 to the associated ground signal. On the receiving circuit; isolate where you can break the circuit in two - connect one end to pin 5 and the other to pin 6. Keeping in mind the optocoupler has an LED - it doesn't hurt to throw a resistor on the circuit so it doesn't fry when set HIGH.

When the optocoupler is set HIGH - it will effectively close the circuit on the receiving side; setting it LOW will open it again. A very simple sketch can make the LED blink and you can verify that the optocoupler is functioning correctly - a sample sketch is provided below:

#define PIN_OC 11

void setup() 
{
    pinMode(PIN_OC, OUTPUT);
}

void loop() 
{
    digitalWrite(PIN_OC, HIGH);
    delay(500);
    digitalWrite(PIN_OC, LOW);
    delay(500);
}

To give you a real-world example; I am working on a project that has a "blackbox" that operates at 12V and has four digital out signals on it that get triggered when certain conditions are met. Unfortunately; the specifications for the Photon state that the operating voltage should be between 3.6 and 5.5 volts - sharing the power circuit would fry the Photon as soon as it powers up!

I had a faulty component (was going crazy); the sketch and wiring is a great test harness!