There are two solutions:

LED Tactile Button:

solder Led Tactile Button on the smoothieboard
led_tactile_button.png

Simple Button:

Wire a simple button on two pin
simple_button.png

The button may be configured as a kill switch, this will halt all operations and turn off heaters and pause the host. It can be cleared with a M999 or reset. to enable a kill button the following is added to the config…

kill_button_enable                        true             # set to true to enable a kill button
kill_button_pin                           2.12             # kill button pin. default is same as pause button 2.12 (Add ^ for external buttons)

When in the halted state (after kill button is pressed) the play led will flash rapidly. Kill can be cleared by holding down the kill button for 2 seconds or more.

Stopping Smoothie

These are the different ways of stopping Smoothie :

Command G-code Movement Heaters File playing Recoverable Documentation Explanation
abort M26 Stops an SDCARD print immediately Not affected Aborts Position maintained, but file must be restarted Player Stops the execution of a file being played from SDCARD, it will complete the current gcode, but stop immediately after that, the rest of the queued commands are discarded. It attempts to maintain the correct position after the abort.
suspend M600 Stops once queue is empty Turned off if option enabled (default) Paused, can be resumed Yes, with resume or M601, position maintained Player Suspends the execution of a file being played from SDCARD or being streamed from a host (upstream support required currently pronterface and octoprint support it, otherwise host needs to be manually paused), all state is saved and jogging and extruding is allowed. Mainly used for mid print filament change, or filament out detection. M601 resumes the print or the resume command
No command, but there is a configurable «kill» button M112 Stops instantly if kill button pressed, if issued from host has to wait for the receive buffer to have room. Turned off aborted No, position is lost, home will be required supported-g-codes Instantly stops all operations, printer fully halts until M999 is sent. Position is lost.
Sending Control-X to smoothie over the serial port or USB serial port should work at any time even when streaming, does the same as the kill button Turned off aborted No, position is lost, home will be required Instantly stops all operations, printer fully halts until M999 is sent (or $X). Position is lost.

If the kill button is pressed (or there is a temperature fault, M112 is issued, a limit switch is hit or other error) the system enters the Halt state, in this state the play led flashes, and the state can be cleared by issuing M999 or holding the flashing kill button for 2 seconds (it can also be cleared from the LCD panel). While in the Halt state any command issued from the host will get a !! response (with a few exceptions). The PSU may be turned off when Halt is entered if there is a psu Switch defined.

All commands can be triggered by a button or a sensor if a Switch module is configured to do so.