Push buttons and selector switches

Push buttons and selector switches are the operator's side of every control panel. They look simple, but choosing NO or NC contacts, the right colours and the right wiring decides whether a machine is safe and easy to use.

How a push button is built

An industrial push button has two parts: the operator (the button head you see on the door) and one or more contact blocks clipped on behind it. Contact blocks are either:

• NO (normally open), terminals 13-14 (or x3-x4): closed only while pressed.
• NC (normally closed), terminals 11-12 (or x1-x2): open only while pressed.

Why STOP buttons are NC

A stop button is wired normally closed so the PLC input is 1 while nobody presses it. If the wire breaks or the contact block falls off, the input goes to 0, which looks exactly like a pressed stop: the machine stops. With an NO stop button, a broken wire would make the stop button silently useless. In the ladder program you therefore use a normally open contact for the NC stop button (see ladder logic basics).

Momentary and maintained

Momentary (spring return) buttons are on only while pressed; they are used for start, stop, reset and jog. Maintained (push-on/push-off) buttons stay in position; the E-stop is a special latching type.

Button colours (IEC 60204-1)

Illuminated push buttons

An illuminated button has an LED in the head with its own terminals (X1-X2). The PLC drives the lamp from an output, so one device acts as both input and output: for example a flashing blue RESET button that lights steady when reset is possible.

Selector switches

A selector switch is turned rather than pressed. A 2-position selector (e.g. MANUAL / AUTO) has a contact block that changes over between positions; a 3-position selector has two NO blocks, one for each side, with the centre position having both open. Spring-return selectors go back to the centre when released.

HAND-OFF-AUTO

A HOA switch is a 3-position selector common on pumps and fans:

• HAND: the motor runs now, regardless of the PLC (often hard-wired, so it works even if the PLC is down).
• OFF: the motor is stopped.
• AUTO: the PLC decides, e.g. from a level or pressure signal.

Wire both the HAND and AUTO contacts to PLC inputs as well, so the PLC and HMI can show which mode is selected.

Wiring to a PLC

A button is just a contact: +24 V → contact → PLC input, with the input COM at 0 V (sinking input). Each button gets its own input. Label every wire with the input address (see PLC addressing).