Single Push Button Start–Stop Logic in PLC (Ladder Diagram Explained)

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Modern industrial machines often require simple and reliable control using minimal hardware. One common requirement is to start and stop a motor using a single push button instead of separate Start and Stop buttons.

This article explains a Single Push Button Start–Stop PLC logic using edge detection (P_TRIG) and memory bits, as shown in the ladder diagram below.

Why Use Single Push Button Logic?

Using one push button instead of two provides several advantages:

  • Reduced wiring and panel space
  • Lower hardware cost
  • Simple operator interaction
  • Cleaner logic using memory bits

Each press of the button toggles the motor state:

  • First press → Motor ON
  • Second press → Motor OFF

Overview of the Logic

This ladder program uses:

  • Positive edge detection (P_TRIG)
  • Set / Reset (SR) memory logic
  • Internal memory bits (%M)
AddressNameDescription
%M0.0Input_Push_ButtonPhysical push button input
%M0.1P_trigPositive edge trigger
%M0.2Motor_OutMotor output control
%M0.3MemoryToggle memory bit

How the Logic Works (Step by Step)

1. Push Button Edge Detection

The push button is connected to a P_TRIG (Positive Trigger) block.

  • The P_TRIG detects only the rising edge (OFF → ON)
  • Prevents repeated toggling while the button is held
  • Ensures one action per press

2. Toggle Logic Using Memory Bit

When the button is pressed:

  • If the motor is OFF, the memory bit %M0.3 is SET
  • If the motor is ON, the memory bit %M0.3 is RESET

This is achieved using:

  • SET coil (S) when motor is OFF
  • RESET coil (R) when motor is ON

This effectively creates a toggle behavior.

3. Motor Output Control

The final network controls the motor output:

  • The motor output %M0.2 follows the inverse state of the memory bit
  • When memory is ON → Motor ON
  • When memory is OFF → Motor OFF

This separation improves clarity and troubleshooting.

Sequence of Operation

Button PressMemory BitMotor State
InitialOFFOFF
1st PressONON
2nd PressOFFOFF
3rd PressONON

Advantages of This Logic

✅ Prevents button bouncing issues
✅ Safe and predictable behavior
✅ Easy to expand (interlocks, alarms, timers)
✅ Works well in Siemens TIA Portal and similar PLCs

Applications

This logic is commonly used in:

  • Conveyors
  • Pumps
  • Fans
  • Small machines
  • Manual override controls

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