Lab4: Touch Sensor Control

Introduction

In this lab, you will learn how to use the touch sensors on the Mini Pupper. The touch sensors are the white strips located on the robot’s body. There are four touch areas: Front, Back, Left, and Right. These sensors can be used to trigger different behaviors or commands when touched.

Prerequisites

The RPi.GPIO library should already be installed on your Raspberry Pi. If not:

pip install RPi.GPIO

Running the Touch Test

Navigate to the demos folder and run the test:

cd ~/mini_pupper_bsp/demos/
python touch_test.py

Touch Sensor Layout

The Mini Pupper has 4 touch areas connected to specific GPIO pins:

Touch Area	GPIO Pin (BCM)
Front	GPIO 6
Left	GPIO 3
Right	GPIO 16
Back	GPIO 2

Complete Touch Test Code

#!/usr/bin/env python3
import time
import RPi.GPIO as GPIO

# There are 4 areas for touch actions
# Each GPIO to each touch area
touchPin_Front = 6
touchPin_Left  = 3
touchPin_Right = 16
touchPin_Back  = 2

# Use GPIO number but not PIN number
GPIO.setmode(GPIO.BCM)

# Set up GPIO numbers to input
GPIO.setup(touchPin_Front, GPIO.IN)
GPIO.setup(touchPin_Left,  GPIO.IN)
GPIO.setup(touchPin_Right, GPIO.IN)
GPIO.setup(touchPin_Back,  GPIO.IN)

# Detection Loop
while True:
    touchValue_Front = GPIO.input(touchPin_Front)
    touchValue_Back  = GPIO.input(touchPin_Back)
    touchValue_Left  = GPIO.input(touchPin_Left)
    touchValue_Right = GPIO.input(touchPin_Right)
    display_string = ''
    if not touchValue_Front:
        display_string += ' Front'
    if not touchValue_Back:
        display_string += ' Back'
    if not touchValue_Right:
        display_string += ' Right'
    if not touchValue_Left:
        display_string += ' Left'

    if display_string == '':
        display_string = 'No button touched'
    print(display_string)
    time.sleep(0.5)

Code Breakdown

1. Import Libraries

import time
import RPi.GPIO as GPIO

time: For adding delays between readings
RPi.GPIO: Raspberry Pi GPIO library for reading pin states

2. Define GPIO Pins

touchPin_Front = 6
touchPin_Left  = 3
touchPin_Right = 16
touchPin_Back  = 2

Each touch sensor is connected to a specific GPIO pin using BCM numbering.

3. Configure GPIO Mode

GPIO.setmode(GPIO.BCM)

Sets the pin numbering mode to BCM (Broadcom SOC channel numbers), not physical pin numbers.

4. Set Up Input Pins

GPIO.setup(touchPin_Front, GPIO.IN)
GPIO.setup(touchPin_Left,  GPIO.IN)
GPIO.setup(touchPin_Right, GPIO.IN)
GPIO.setup(touchPin_Back,  GPIO.IN)

Configures each GPIO pin as an input to read the touch sensor state.

5. Detection Loop

while True:
    touchValue_Front = GPIO.input(touchPin_Front)
    # ... read other sensors
    
    if not touchValue_Front:
        display_string += ' Front'
    # ... check other sensors

GPIO.input(): Returns the current state of the pin
Touch sensors are active LOW (returns 0 when touched)
The not operator converts: touched (0) → True, not touched (1) → False

Understanding Active LOW Logic

The touch sensors use active LOW logic:

When NOT touched: GPIO reads HIGH (1)
When touched: GPIO reads LOW (0)

This is why we use if not touchValue_Front: to detect a touch.

Exercises

Exercise 1: Touch Counter

Create a script that counts how many times each sensor has been touched.

touch_counts = {'Front': 0, 'Back': 0, 'Left': 0, 'Right': 0}

# In your loop:
if not touchValue_Front:
    touch_counts['Front'] += 1

Exercise 2: Touch-Triggered Actions

Make the Mini Pupper perform different actions based on which sensor is touched:

Front: Move forward
Back: Move backward
Left: Turn left
Right: Turn right

Exercise 3: Multi-Touch Detection

Detect when multiple sensors are touched simultaneously and trigger special actions.

if not touchValue_Front and not touchValue_Back:
    print("Front and Back touched - Special action!")

Exercise 4: Touch with Debouncing

Implement debouncing to prevent multiple detections from a single touch:

import time

last_touch_time = 0
debounce_delay = 0.3  # 300ms

if not touchValue_Front:
    current_time = time.time()
    if current_time - last_touch_time > debounce_delay:
        print("Front touched!")
        last_touch_time = current_time

Using Event Detection (Advanced)

Instead of polling, you can use GPIO event detection for more efficient touch handling:

def touch_callback(channel):
    if channel == touchPin_Front:
        print("Front touched!")
    elif channel == touchPin_Back:
        print("Back touched!")

# Add event detection
GPIO.add_event_detect(touchPin_Front, GPIO.FALLING, 
                      callback=touch_callback, bouncetime=300)
GPIO.add_event_detect(touchPin_Back, GPIO.FALLING, 
                      callback=touch_callback, bouncetime=300)

# Keep the program running
try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    GPIO.cleanup()

Troubleshooting

Issue	Solution
No touch detected	Check GPIO connections and pin numbers
Multiple detections	Add debouncing or increase sleep time
Permission denied	Run with sudo or add user to gpio group
GPIO already in use	Call `GPIO.cleanup()` before running

Cleanup GPIO

Always clean up GPIO when done:

try:
    # Your code here
    pass
except KeyboardInterrupt:
    pass
finally:
    GPIO.cleanup()

Summary

In this lab, you learned:

How to read touch sensor states using RPi.GPIO
Understanding BCM vs BOARD pin numbering
Active LOW logic for touch sensors
Polling vs event-driven touch detection
Implementing debouncing for reliable touch detection