Basic Projects #4 -Start/Stop with Two Buttons on Siemens PLC 🟢🔴

In industrial automation, one of the most fundamental control circuits is the Start/Stop latching logic. It’s a simple yet powerful concept that’s used to control motors, pumps, lights, and many other devices. In this article, we’ll show you how to implement this logic using two pushbuttons and a Siemens S7-1200 PLC in TIA Portal.

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🔧 What You’ll Learn

• How to create a latching (memory) circuit using two buttons
• How to use Set/Reset coils
• How to control an output like a motor or lamp

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🧰 What You’ll Need

• Siemens S7-1200 PLC (e.g. 1212C or 1215C)
• 2x Pushbuttons:
  • Start button (normally open) 🟢
  • Stop button (normally closed) 🔴
• 1x LED indicator or motor contactor coil 💡⚙️
• 24V DC power supply ⚡
• TIA Portal (V16 or newer)

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🔌 Wiring Overview

• Start button: Connect to %I0.0
• Stop button: Connect to %I0.1
• Output device (LED/motor): Connected to %Q0.0

Make sure the Stop button is normally closed (NC) so the system defaults to OFF if there’s a fault or wire break — a common safety design.

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🧠 How the Logic Works

• Pressing the Start button energizes the output and keeps it ON via a latching contact.
• Pressing the Stop button interrupts the logic and turns the output OFF.

This is called a latching or holding circuit, similar to traditional relay logic.

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🧑‍💻 TIA Portal – Ladder Logic (LAD)

Here’s the logic broken down in three steps:

Network 1 – Start/Stop Latching Logic

| %I0.0 (Start) |——[ ]——+——[ ]——( )—— %Q0.0 (Output)
| %Q0.0         |       |
| %I0.1 (Stop)  |——[/]——+

Explanation:

• %I0.0 is the Start button (NO)
• %I0.1 is the Stop button (NC) – used as a normally closed contact in logic
• %Q0.0 is the Output (e.g., LED or motor)
• %Q0.0 is also used as a holding contact to keep the output ON after releasing the Start button

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💡 How It Works

• 🟢 Press Start → %I0.0 is TRUE → Output %Q0.0 turns ON
• 🪢 Output %Q0.0 „latches“ itself through the branch contact
• 🔴 Press Stop → %I0.1 goes FALSE → the rung breaks → Output turns OFF

This logic is widely used in industrial control panels, especially for motor starters and manual mode operations.

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🚀 Bonus Enhancements (Optional)

• Add an indicator light on the HMI showing “Motor Running” 🟢
• Add a TON delay before turning the motor on
• Add interlocks for safety (e.g., Emergency Stop or Overload trips)
• Replace physical buttons with HMI buttons using %M bits

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✅ Conclusion

The Start/Stop with two buttons logic is a must-know for anyone working with PLCs. It introduces:

• Memory logic
• Safety-conscious design
• Real-world industrial control basics

This simple circuit lays the foundation for more complex automation systems. 💪

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🔜 Coming Next in the Series:

• TON Timer – Delay ON ⏱️
• TOF Timer – Delay OFF ⌛
• Blinking light (TP timer) 💡💡
• Counters (CTU / CTD) 🔢