I want to develop a Python script which can measure the RPM of a wheel

Question

I am working on a project in which I am required to measure the RPM of wheel using a hall effect sensor and a Raspberry Pi. I have developed a script to do so but it does not give me required result. If a magnet comes near the sensor it directly shows the high value or else it shows 0. I want a script that will show me results like 40,39,38,36,9,8,0 every second just like a bike speedometer.

What should I do?

Here is the script that I have made.

import RPi.GPIO as GPIO
import time
from time import sleep
import datetime

j=10000

sensor=12

ts=datetime.time()
w=0
new_time=0
old_time=0
temp=0

GPIO.setmode(GPIO.BOARD)
GPIO.setup(sensor,GPIO.IN,pull_up_down=GPIO.PUD_UP)
while j:
      if (GPIO.input(12)== 0):
            new_time=time.time()
            old_time=temp
            delta_t=new_time-old_time
            temp=new_time
            w=60/delta_t
            v=0.5 * w * 0.10472 * 3.6
            print (v)
            time.sleep(0.1)
      else:
            time.sleep(1)
            print("0")
      j=j-1

These are the results that I am getting

7.73038658487e-09
0
0
5.14198236739
85.7996578022
88.3574855171
88.6053761182
0
9.71547048724
86.4257204462
0
9.57686374353
0
0
0
3.46828868213
86.5939003971
87.7296673227
85.2723314196
87.1933291677
86.5891584273
85.6995234282
86.6861559057
85.5173717138
86.9547003788
87.3698228295
87.2335755975
0
9.6387741631
0
0
0
3.4536179304
82.6103044073
83.5581939399
83.8193788137
82.5174720536
84.0056662004
82.471707599
83.8201193552
86.8997440944
82.6851820147
0

Answer 1

You will get a lot of 0 because you print a 0 in the else. I don't know if python is the best language to do this control, it depends how fast your wheel is and how long stay GPIO.input(12)== 0. If it's too fast you will lose lots of revs. May be you have to do an average of the last N measures. Or, instead of look how long takes the wheel to do a complete rev, measure how many revs has done in the last N seconds.

May be you get enter in the GPIO.input(12) == 0 case for the same rev because GPIO.input(12) stais 0 too long. To count as a new rev has to change GPIO state:

last_state = 1
while j:
      if (GPIO.input(12) == last_state)
            last_state = GPIO.input(12)
            if (GPIO.input(12)== 0):
                  new_time=time.time()
                  old_time=temp
                  delta_t=new_time-old_time
                  temp=new_time
                  w=60/delta_t
                  v=0.5 * w * 0.10472 * 3.6
                  print (v)
                  time.sleep(0.1)
            else:
                  time.sleep(1)
      j=j-1

I'm not a python programmer, you'll have to check time units and grammar:

revs = 0
last_state = 1
end_time = time.time() + 10
while time.time() < end_time:
      # count revs:
      if (GPIO.input(12) == last_state)
            last_state = GPIO.input(12)
            if (GPIO.input(12) == 0):
                  revs = revs + 1;

      # print revs every 0.5 secs:
      if (time.time() - old_time > 0.5)
            delta_t = time.time() - old_time
            old_time = time.time()
            w = 60 / delta_t
            v = revs * 0.5 * w * 0.10472 * 3.6
            print(v)
            revs = 0
      else
            time.sleep(0.01)