How to Setup a Raspberry Pi Pressure Pad (FSR)?

A pressure pad, also known as a force-sensitive resistor (FSR), is a great way to add physical interactivity to your Raspberry Pi projects. With a pressure pad connected, you can detect physical pressure, squeezes, and weight. This allows you to build projects that can respond to human touch, like home automation controls, robotic sensors, musical instruments, and more.

How to Setup a Raspberry Pi Pressure Pad (FSR)?

In this comprehensive guide, you’ll learn how FSR pressure pads work, how to connect one to a Raspberry Pi, and how to write Python code to detect touches and measure pressure data. By the end, you’ll have the skills to integrate these affordable sensing components into your own Pi builds.

How Pressure Pads Work

A force-sensitive resistor is a thin flexible pad that changes resistance when you apply pressure to it. They are made of polymer thick film that conducts electricity.

Inside the pad, there is an ink layer printed in a special pattern. At rest, the particles in the ink are only slightly conductive to allow high resistance. But when you press on the pad, the particles connect more, allowing more current to flow through (lowering resistance).

When pressure is removed, the ink particles separate again back to an open circuit. This allows an FSR to act like a variable resistor, giving analog values depending on squeezing force.

Diagram showing how force-sensitive resistor material works [Source: SparkFun]

By continuously monitoring the changing resistance of the sensor, your Raspberry Pi can detect presses, patterns, and intensity.

Raspberry Pi FSR Wiring Guide

To use a force-sensitive resistor with a Raspberry Pi, you’ll need to connect it as part of a simple voltage divider circuit. This converts the variable resistance of the FSR into a measurable voltage level that can be monitored via the Pi’s GPIO pins.

Components

You’ll need the following parts:

  • Raspberry Pi
  • Jumper wires
  • 1x Force sensitive resistor
  • 1x 10kΩ resistor

Circuit Diagram

Here is the circuit diagram showing how to connect the FSR to the Raspberry Pi:

To build this circuit:

  • Connect the FSR pad between 3.3v power and GPIO pin. I used BCM GPIO 4 for the code examples.
  • Connect a 10kΩ pull down resistor from the GPIO pin to ground. This forms the voltage divider.
  • When pressure is applied to the pad, resistance will decrease and voltage on GPIO will rise.

No soldering is required since you can build this circuit using female-to-female jumper wires.

With this complete, you are ready to start measuring varying voltage data on the GPIO pin as you squeeze and release pressure on the sensor.

Reading Raspberry Pi Pressure Pad Values with Python

To handle the analog input and convert voltages from the force sensitive resistor, it’s best to write a Python script.

This handles initializing GPIO pin input, sensing resistance changes in a loop, and printing pressure measurements.

Here is basic Python code to continuously monitor an FSR pressure pad on Raspberry Pi:

python

import RPi.GPIO as GPIO

import time

# Setup GPIO pin as input

GPIO.setmode(GPIO.BCM) 

GPIO.setup(4, GPIO.IN)

# Infinite loop for continuous sensing  

while True:

    # Read GPIO as analog   

    fsr_value = GPIO.input(4)

    # Print pressure value  

    print(fsr_value)

    time.sleep(0.5)

This script imports the GPIO library to interface with Raspberry Pi pins. It sets pin 4 as an input.

Inside the loop, it reads the GPIO pin repeatedly as analog input. This returns a value between 0 – 1 based on voltage from force pad pressure.

Finally prints out the pressure value and delays 0.5 seconds before repeating.

You can now run this Python code before/after connecting the FSR to see values change from idle to pressed states.

Calibrating Sensitivity Threshold

The default script produces values from 0 (no pressure) up to 1 (max pressure), but the top end may differ based on your exact resistor setup.

To better understand the range of values for your sensor, you can add a calibration routine:

python

# Define min and max pressures  

fsr_min = 0              

fsr_max = 1

# Calibrate for 5 seconds

while time.monotonic() < 5:

    fsr_value = GPIO.input(4)

    # Update min and max

    fsr_min = min(fsr_value, fsr_min)  

    fsr_max = max(fsr_value, fsr_max)

# Calculate threshold as average

threshold = (fsr_min + fsr_max) / 2

This calibration occurs when the script first starts. It monitors values for 5 seconds looking for extremes when idle and fully pressed. This gives custom threshold for your sensor component.

From there, you can detect real presses and touches in the main program logic:

python

# Main loop

while True:

    fsr_value = GPIO.input(4)

    # Check if pressure exceeds threshold

    if fsr_value > threshold:

       print(“Pressure Detected!”)

    else:

       print(“No Pressure”)

Now you have optimized control over what constitutes a pressure trigger with your Raspberry Pi.

Touch Detection with Intervals

Rather than constant sensing, you may also want to implement touch detection using timed intervals.

This involves sleeping for a longer period, taking a pressure sample, comparing to the previous state, and then going back to sleep.

Here is Python interval code to detect touch events on the Raspberry Pi pressure pad:

python

import RPi.GPIO as GPIO

import time

# Initial state

touch_state = False

while True:

  # Take pressure reading

  fsr_value = GPIO.input(4)  

  # Compare to previous 

  if fsr_value > 0.5 and touch_state is False:

      print(“Pressure applied!”)

      touch_state = True

  if fsr_value < 0.5 and touch_state is True:

      print(“Pressure removed!”)  

      touch_state = False

  # Sleep for 250 ms 

  time.sleep(0.25)

This compares the latest analog value to the previous touch state on every iteration. When the state changes from low (no touch) to high (touch applied) it prints and updates the detection flag.

The opposite case occurs when pressure is removed, toggling the state back.

Using this technique, you can detect distinct touch press and release events while sleeping the majority of time.

Measuring Exact Force Values

Beyond just touch detection, it can also be useful to measure the exact pressure force being applied to the FSR pad.

Force can be derived using the resistance value of the sensor compared to its unpressed state. But since resistance has an inverse exponential relationship with pressure, it requires a bit of math to convert.

Here is some Python code that calculates the compression force from analog values off the Raspberry Pi GPIO pin:

python

import RPi.GPIO as GPIO

import math

# Static parameters   

R_MIN = 5000.0 # Ohms

R_MAX = 1000000.0 # Ohms

A = 0.00054 # m^2

N = 1.588 

while True:

    # Read FSR analog  

    fsr_value= GPIO.input(4)  

    # Convert to resistance

    R_FSR = (1fsr_value)/fsr_value * R_MAX   

    # Calculate force using constants

    f = (R_FSR R_MIN)/R_MIN

    f = A/(N*f+1)

    print(“Force: {} N”.format(round(f,3)))

    time.sleep(0.1)

Don’t worry about the constants too much. But this uses the Steinhart–Hart thermistor equation to calculate absolute compression force based on changing resistance as the pad is pressed.

The output when squeezing the FSR increments from 0 Newtons up as pressure increases. Giving precise real-time measurements.

Graphing Force Over Time

In addition to printing force values, you may want to graph changes over time. This helps visualize pressure trends.

Here is sample code to graph force readings from the Raspberry Pi pressure pad using the Python Matplotlib library:

python

import matplotlib.pyplot as plt

import RPi.GPIO as GPIO

force_values =[]

time_values =[] 

# Plot styling  

plt.xkcd()  

plt.title(“Pressure Over Time”)

plt.ylabel(‘Force’)

plt.xlabel(‘Time’)

while True:

    # Read force     

    force = calculate_force() 

    # Store value

    force_values.append(force) 

    time_values.append(time.monotonic())

    # Plot every 50 values

    if len(force_values) % 50 == 0:

        plt.plot(time_values, force_values)

        plt.pause(0.001)

This captures force and time data points each loop iteration, appending to arrays.

After 50 values collected, it plots the live graph. By calling plt.pause() quickly, the graph refreshes over time.

You’ll see a line plot that responds to changing pressure on the sensor. Spiking under fast squeezes and dropping back down when releasing.

Now save your plots after tests complete to review response characteristics.

Building a Raspberry Pi Push Button

A great project using a pressure pad is converting it into an interactive push button.

Here is a simple Python script for Raspberry Pi that prints a message whenever the FSR is squeezed:

python

import RPi.GPIO as GPIO

# Flags for sensing state

button_pressed = False  

try:

    while True:

        # Read pressure value

        fsr_value = GPIO.input(4)

        # Check for button press threshold

        if fsr_value > 0.5 and button_pressed is False:

            print(“**Button Pressed!**”)

            button_pressed = True

        # Check for release threshold

        if fsr_value < 0.1 and button_pressed is True:

            button_pressed = False 

finally:  

    GPIO.cleanup()

This watches for pressure spikes over a calibrated threshold to detect presses. And checks for stops below a minimum value to catch releases.

When a new button press begins, it prints the message, allowing any action code to be run.

To complete this into an actual button, mount the pressure pad PCB into an enclosure, along with the Pi. Some 3D printed cases have slots perfect for integrating FSR pads.

Now you can interface with scripts and programs by squeezing this DIY push button!

Conclusion

Force sensitive resistors provide affordable but powerful sensing for Raspberry Pi home automation and robotics experiments.

With proper GPIO voltage divider wiring and Python scripting, pressure pads enable touch interaction and physical controls.

The analog inputs translate into all kinds of great event detection, from recognizing taps and swipes to measuring exact compression force over time. Raspberry Pi’s flexibility helps map these sensors to any actions imaginable.

Now integrate FSR pads into your upcoming builds for adding tactile interactivity!

Key Takeaways

  • Force sensitive resistors (FSRs) change resistance when pressure applied
  • Wire in voltage divider circuit to Raspberry Pi GPIO pin
  • Read analog input in Python script
  • Detect touch presses and patterns over time
  • Calculate compression force based on resistance
  • Map pressure spikes to any actions

Frequently Asked Questions

  1. What’s the typical FSR pressure range?
    Most small through-hole FSR pads can detect applied force between 100 grams and 10 kg. But very thin pads are also available capable of up to 100 kg loads.

  2. Where is the best location to place FSR pads?
    You’ll want to mount the pads in areas expected to receive direct finger pressure when designing your enclosure. For example, underneath push button spots or under finger contact areas if using a bare PCB directly.

  3. What GPIO voltage range can an FSR support?
    The most common force sensitive resistors run at 5V. But many models also work at 3.3V levels compatible with Raspberry Pi GPIO outputs. Just ensure your FSR pad specifies ≥3.3V capability.

  4. Do I need any special Raspberry Pi configuration?
    Nope! FSRs work as basic GPIO inputs. As long as you wire according to the voltage divider circuit covered already, just writing your Python script is all that’s required software-wise.

  5. How weatherproof are these force sensors?
    Most FSR pads feature a durable polymer surface, but the wire traces and connections underneath can deteriorate over time with moisture ingress. For weatherized versions, consider silicone-coated pads.

  6. Can I connect multiple FSRs to the Pi at once?
    Absolutely! With an analog multiplexer such as the TCA9548A I2C module, you can keep adding additional pressure pads, routing each through software. Up to 64 sensors on one Pi.

  7. How durable are FSR pressure pads?
    Industrial grade units made of polyester substrates and carbon-doped inks boast impressive lifespans exceeding 10 million activation cycles. And the polymer materials regain their shape after compressions. Overall extremely durable for projects seeing daily use.

  8. What temperature ranges can FSRs withstand?
    Look for force pads and accompanying electronics all rated down to at least -40 °C when used in cold environments. On the high end, ∼105 °C tends to be the upper limit before internal polymer layers start permanently changing resistance values.

  9. How do I securely mount my FSR circuit inside an enclosure?
    For production use, having a nice finished build is important. Consider designing a custom PCB with the voltage divider circuit traces and connector pads. This can securely mount into laser cut enclosures, with the FSR pad glued or screwed in place atop it through alignment holes.

  10. How fast can values be sampled from the Raspberry Pi?
    Expect to achieve ~100 Hz maximum reading speed when sampling a single analog FSR input using RPi.GPIO python library. For even faster logging if needed, writing lower-level C code against the Broadcom peripherals in Raspberry Pi’s SoC can hit 1 KHz+.

  11. Can pressure pads be used underwater / fully submerged?
    Waterproof FSR variants do exist! Look for silicone coated, fully sealed sensors capable of operation at depths up to 100 meters. Just take care to also enclose any submitting circuitry and connections.

  12. What gauge wire is best for connecting my FSR?
    26-30 AWG jumper cable or ribbon cable works well. Lean towards thicker gauges if long wire runs expected. And ensure flex rated wiring for moving projects.

  13. What screw terminal, pin header, or connector makes attachment easiest?
    JST PH 2-pin connectors are very common and allow snap-in force sensor connectivity. Or try direct soldering to extended pin headers.

  14. How do I protect against ESD and transients when handling FSR pads?
    SparkFun recommends placing 1K resistors in series with both leads coming off the FSR epoxy. This helps limit current surges that could potentially damage the internal ink layers.

  15. What 3D printable material options are food-safe for encapsulating FSRs?
    Consider FDA-approved Nylon materials such as PA12 for consumer product use cases involving edibles. Offers excellent layer adhesion.

  16. Can I waterproof standard FSRs myself using household materials?
    Trying silicone or other sealants could affect accuracy. Instead, opt for pre-made waterproof silicone models if moisture resistance needed.

  17. Are electrically conductive fabrics like Eeonyx interchangeable with FSR pads?
    No – conductive fabrics act more like fixed resistors, not changing resistance under pressure. Only pressure-variable formulations provide analog sensing capabilities.

  18. Which offers greater precision: jagged Velostat strips or smooth FSR pads?
    The consistency of smooth-surfaced FSR pads yields overall better accuracy and force curve linearity during measurements. Velostat experiences slightly more variance based on inconsistent particle dispersions.

  19. How do FSR pressure ranges compare to strain gauges?
    FSR polymers support detection anywhere from grams to tons depending on model and size. By comparison, foil strain gauges typically start reliably measuring in the fractional lbs / kgs and up.

  20. What are some alternatives to FSR pressure sensing for high noise environments?
    Look into fluid pressure or pneumatic based load cells. These pipe pressure into analog transducers, avoiding environmental EMI interference that could affect high impedance FSR circuits.

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