The Secret Date Between PLC and Arduino: How Two Platforms Collaborate for Home Automation

Hello everyone, I am Lao Liu, a veteran in the industrial control industry. Today, let’s discuss an interesting topic: how to combine industrial-grade PLCs with DIY Arduino to create some home automation gadgets.

This all started with my balcony vegetable growing system that I modified last year. At that time, I had a small, idle PLC (Siemens LOGO!) and a few Arduino boards. Using just the PLC would be a waste; using only Arduino would not be stable enough. So I thought, can these two “date”?

Why Let Them “Date”?

First, let’s talk about their respective advantages and disadvantages:

Advantages of PLC:

• Super stable, can run continuously for years without issues
• Strong anti-interference capability, resistant to electromagnetic waves and voltage fluctuations
• Industrial-grade input and output interfaces, safe and reliable

Advantages of Arduino:

• Cheap, a board costs only a few dozen yuan
• Various sensor modules are complete and inexpensive
• Networking and Bluetooth are easy to implement
• Open-source code, with plenty of tutorials available online

I thought: if we could combine the stability of PLC with the flexibility of Arduino, wouldn’t that be the best of both worlds?

How to Make Them “Hold Hands”?

Communication is key! I used the simplest method: connect the digital output of Arduino to the digital input of PLC, and connect the output of PLC to the input of Arduino.

Wiring Diagram:

Arduino D2 ----> PLC I0.0 (Arduino sends commands to PLC)
Arduino D3 ----> PLC I0.1
PLC Q0.0 ----> Arduino A0 (PLC sends status back to Arduino)
PLC Q0.1 ----> Arduino A1
Common Ground GND -----> GND

A Simple Collaboration Example

My balcony vegetable growing system works like this:

1. Arduino is connected to a DHT22 temperature and humidity sensor, a light sensor, and a WiFi module, responsible for monitoring the environment and providing remote control via mobile phone
2. PLC controls the water pump, fan, and supplementary light, responsible for precise timing and stable control

Arduino Code Snippet:

// When the temperature exceeds 30 degrees
if (temperature > 30) {
  digitalWrite(2, HIGH);  // Send signal to PLC to turn on the fan
} 
else {
  digitalWrite(2, LOW);   // Send signal to PLC to turn off the fan
}

// Read PLC status feedback
int pumpStatus = analogRead(A0);
if (pumpStatus > 500) {  // Water pump is working
  sendToApp("The water pump has started, watering");
}

PLC Ladder Logic:

// Receive signal from Arduino to turn on the fan
|--[I0.0]--|---(Q0.0 Fan)---|

// Watering at a scheduled time every day
|--[Timer 8:00AM]--|---(Q0.1 Water Pump)---|---(Q0.2 Feedback to Arduino)---|

How is the Actual Effect?

To be honest, this “mixed” system has been running stably for over a year. Arduino acts as the “eyes” and “mouth” (sensing and communication), while PLC acts as the “hands and feet” (precise control). I can see the condition of my vegetable garden at home using my phone while lying in the office, and I can also remotely adjust the irrigation schedule.

The best part is that this system is quite cost-effective—rather than using an expensive industrial control screen, I achieved visualization with Arduino and a mobile app; and I don’t have to worry about stability issues like in a pure Arduino solution.

Some Tips for Friends Who Want to Try

1. Make sure to do proper opto-isolation for signal interconnection to protect both devices
2. The protocol should be simple; using high and low levels or pulse signals is the most reliable
3. Test communication on a breadboard before formal installation
4. PLC should handle core control, while Arduino should manage peripheral functions; do not reverse the roles

Alright, that’s all for now. Friends who are interested can try this “cross-border date”; it’s actually quite fun! Next time, I will share some other DIY insights.