Using FSRs with Smoothieboard

FSRs (Force Sensing Resistors) can be used as bed leveling sensors with Smoothieboard to provide automatic bed leveling for 3D printers.

Overview

To use FSRs with the Smoothieboard, an auxiliary board is needed to interface the FSRs to the Smoothieboard’s endstop input.

Smoothieware will accept this input as if it were a simple mechanical switch.

Available FSR Controller Boards

JohnSL FSR Endstop Board

JohnSL on GitHub has created an FSR controller board specifically for this purpose.

Where to buy:

Resources:

This board is powered directly from the endstop pins, making installation straightforward.

Toucher - 4-Point FSR Controller

For Cartesian machines with square heat beds, there is a 4-point FSR controller called “Toucher”.

Resources:

Community Discussion and Resources

The Deltabot group is where most discussion on using FSRs is located.

Topics to look for:

  • Alternative boards like the Arduino Mini
  • WingWong’s Trinket-derived design
  • FSR mounting techniques and best practices

Configuration

Once you have your FSR controller board connected to an endstop input, you need to configure Smoothieware to use it as a Z probe.

# Z probe configuration for FSR
zprobe.enable                        true
zprobe.probe_pin                     1.28!^       # Pin connected to FSR controller output
zprobe.slow_feedrate                 5            # Slow probe speed in mm/sec
zprobe.fast_feedrate                 100          # Fast probe speed in mm/sec
zprobe.return_feedrate               50           # Speed to return after probe
zprobe.probe_height                  5            # Height above bed to start probe
zprobe.debounce_count                10           # Number of samples for debouncing

# Leveling strategy
leveling-strategy.three-point-leveling.enable  true
leveling-strategy.three-point-leveling.point1  10.0,10.0    # First probe point
leveling-strategy.three-point-leveling.point2  190.0,10.0   # Second probe point
leveling-strategy.three-point-leveling.point3  100.0,190.0  # Third probe point

Pin syntax:

  • 1.28!^ means pin 1.28, inverted (!), with pull-up enabled (^)
  • Adjust based on which endstop connector you’re using
 
[zprobe]
enable = true
probe_pin = P1_28!^                  # Pin connected to FSR controller output
slow_feedrate = 5                    # Slow probe speed in mm/sec
fast_feedrate = 100                  # Fast probe speed in mm/sec
return_feedrate = 50                 # Speed to return after probe
probe_height = 5                     # Height above bed to start probe
debounce_count = 10                  # Number of samples for debouncing

[leveling_strategy.three_point]
enable = true
point1 = 10.0, 10.0                  # First probe point
point2 = 190.0, 10.0                 # Second probe point
point3 = 100.0, 190.0                # Third probe point

Pin syntax:

  • P1_28!^ means pin P1.28, inverted (!), with pull-up enabled (^)
  • Adjust based on which endstop connector you’re using
 The probe pin configuration depends on which endstop connector you're using. Refer to the Smoothieboard pinout to identify the correct pin for your setup.

Troubleshooting

Common Issues

FSR Not Triggering:

  • Check wiring: Verify all connections between FSRs, controller board, and Smoothieboard
  • Test controller output: Use a multimeter to verify the controller board outputs a signal when FSRs are pressed
  • Verify pin configuration: Ensure the probe pin matches the physical connection
  • Check inversion: Try removing or adding the ! (inversion) flag in the pin configuration
  • Adjust sensitivity: Most FSR controller boards have a potentiometer to adjust trigger threshold - try increasing sensitivity

False Triggers:

  • Reduce sensitivity: Adjust the controller board’s potentiometer to require more force
  • Add debounce: Increase zprobe.debounce_count to filter electrical noise
  • Check mechanical stability: Ensure FSRs are mounted firmly and the bed doesn’t wobble
  • Shield wiring: Use shielded cable if electrical noise is suspected
  • Verify weight distribution: Make sure the nozzle contacts all FSRs evenly during probing

Intermittent Detection:

  • FSR placement: Ensure FSRs are positioned where the nozzle will consistently contact them
  • FSR condition: Check for damaged or worn FSRs - they degrade over time with use
  • Temperature effects: FSRs can be affected by heat - keep them away from heated beds
  • Connector issues: Verify all connectors are secure and making good contact
  • Update debounce: Try different zprobe.debounce_count values (typically 5-20)

Testing Your Setup

To test if your FSR setup is working:

  1. Send M119 to check endstop status
  2. Press down on the bed to trigger the FSRs
  3. Send M119 again - the Z min endstop should show as triggered
  4. If it doesn’t change state, check wiring and configuration

FSRs vs BLTouch

Both FSRs and BLTouch can provide automatic bed leveling, but with different characteristics:

FSRs:

  • Sense actual nozzle contact with the bed
  • More reliable on textured or irregular surfaces
  • Require mounting under the bed or build plate
  • Need auxiliary controller board
  • Measure true “nozzle-to-bed” distance
  • More complex installation

BLTouch:

  • Optical/mechanical probe with deployable pin
  • Easier to mount (attaches to print head)
  • May have issues with certain bed surfaces (glass, mirrors)
  • Self-contained unit (no additional controller needed)
  • Measures “probe-to-bed” distance (requires Z offset)
  • Simpler installation
FSRs are often preferred for delta printers where the entire effector contacts the bed evenly, while BLTouch is popular for Cartesian printers where mounting a probe near the nozzle is straightforward.

For more information about BLTouch, see the BLTouch documentation.

For more information about bed leveling and probing, see:

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