How to Use Resistive Sensors in Raspberry pi

You need to connect a variable resistor to a Raspberry Pi and measure its resistance so that you simply can utilize the position of the variable resistor’s knob in your Python program. Today I show you How to How to Use Resistive Sensors in Raspberry pi

Components Required :

6. Resistors

This book will help you to gain more knowledge of Raspberry pi Software and Hardware Problems and Solutions

Raspberry Pi Cookbook

Circuit diagram:

shows the arrangement of components on the breadboard

Circuit diagram

You'll be able to measure the resistance on a Raspberry Pi using nothing more than a capacitor, a few resistors, and two GPIO pins. In this case, you'll be able to gauge the position of the handle on a small variable resistor by measuring its resistance from its slider contact to one end of the pot.

Code Resistive Sensors Raspberry pi:

import RPi.GPIO as GPIO
import time, math

C = 0.36 # uF
R1 = 1000 # Ohms

GPIO.setmode(GPIO.BCM)

# Pin a charges the capacitor through a fixed 1k resistor 
# and the pot in series
# Pin b discharges the capacitor through a fixed 1k resistor 
a_pin = 18
b_pin = 23

# Discharge the capacitor, leaving it ready to start filling up
def discharge():
    GPIO.setup(a_pin, GPIO.IN)
    GPIO.setup(b_pin, GPIO.OUT)
    GPIO.output(b_pin, False)
    time.sleep(0.1)

# Return the time taken (uS) for the voltage on the capacitor 
# to count as a digital input HIGH
# which is 1.65V or higher
def charge_time():
    GPIO.setup(b_pin, GPIO.IN)
    GPIO.setup(a_pin, GPIO.OUT)
    GPIO.output(a_pin, True)
    t1 = time.time()
    while not GPIO.input(b_pin):
        pass
    t2 = time.time()
    return (t2 - t1) * 1000000

# Take an analog reading as the time taken to charge after 
# first discharging the capacitor
def analog_read():
    discharge()
    t = charge_time()
    discharge()
    return t

# Convert the time taken to charge the cpacitor into a value of resistance
# To reduce errors, do it 100 times and take the average
def read_resistance():
    n = 10
    total = 0;
    for i in range(1, n):
        total = total + analog_read()
    t = total / float(n)
    T = t * 0.632 * 3.3
    r = (T / C) - R1
    return r
try:
    while True:
        print(read_resistance())
        time.sleep(0.5)
finally:
    GPIO.cleanup()

When you run the program, you should see some output like this:

10232.2157936
10155.1559448

The reading will shift as you turn the knob of the trimpot. the reading of resistance would change between and 10,000Ω but in practice, there will be a few mistakes. You might like to change the value of the constant C at the top of the program in the event that you need more exact readings, but remember the value of C will probably have to be changed in the event that you put another capacitor in, indeed in case the capacitor was nominal of the same value