Basic Projects #3 – Start/Stop with One Button – Set/Reset Logic on Siemens PLC

Controlling an output with a single button — press once to turn ON, press again to turn OFF — is a classic and very practical function in PLC programming. In this article, we’ll build exactly that: a Start/Stop toggle using one button, with a Set/Reset memory structure in TIA Portal on a Siemens S7-1200 PLC.

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🔍 Why Use One Button for Start/Stop?

Using a single button to switch a device ON and OFF is:

• ✅ Space-saving
• ✅ Cost-effective (only one input needed)
• ✅ Common in HMI screens, touch panels, and even physical controls

It requires a bit more logic than the basic LED-on-when-pressed example, but it’s a great intro to edge detection and memory bits.

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🧰 What You’ll Need

• Siemens S7-1200 CPU (e.g., 1212C or 1215C)
• 24V pushbutton 🔘
• 24V indicator light 💡 (or just use output LED on PLC)
• 24V power supply ⚡
• TIA Portal (V16 or newer) 💻

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⚡ Wiring Setup

• Button (Input): Connect to %I0.0
• LED or Lamp (Output): Connect to %Q0.0

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🧠 How the Logic Works

We will use:

1. Edge detection → So the PLC only reacts when the button is pressed, not while it’s held down
2. Memory bit (M) → To remember the ON/OFF state
3. Set/Reset output → To turn the output ON or OFF based on memory state

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🧑‍💻 TIA Portal Ladder Logic (LAD)

Step 1: Create a positive edge detection

| %I0.0 (Button) | ——[ ]——+——[R_TRIG]—— M0.0 (Button_Trigger)

• R_TRIG is the rising edge detection (from the “Bit Logic” section).
• Output: %M0.0 → We’ll use this as a trigger.

Step 2: Toggle memory bit with XOR (flip-flop)

| M0.0 (Trigger) | ——[ ]——+
|                |         |
| M0.1 (State)   |——[XOR]——+——( )—— M0.1 (Toggle_State)

• %M0.1 is a memory bit that stores ON/OFF state.

Step 3: Control the output with memory bit

| M0.1 (Toggle_State) | ——[ ]——+——( )—— Q0.0 (LED)

Boom! 🎉 Press once → LED turns ON. Press again → LED turns OFF.

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🧪 How It Works in Action

• First press → Edge detected → memory bit %M0.1 becomes 1 → LED turns ON
• Second press → Edge detected again → %M0.1 becomes 0 → LED turns OFF

This is the basis of many toggle functions in automation — from turning on a conveyor, to switching machine modes.

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💡 Bonus Tips

• You can use this logic for motors, pumps, or any actuator that needs ON/OFF toggle.
• Want to add a reset after power loss? Use a retentive memory (%MR) instead of %M.
• You can also extend this to an HMI by linking a screen button to %M0.0.

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✅ Conclusion

Using one button for Start/Stop toggle is a smart and flexible solution in automation. With edge detection and a simple XOR flip-flop, your Siemens S7-1200 can now handle smart ON/OFF control with ease. 💪

Next time we’ll look at:

• ⏲️ Adding a delay (TON) before output activates
• 🔐 Adding safety interlocks to this logic
• 📱 Controlling it from an HMI or remote panel

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