whats wrong with line 16 on pot_step.py

[Hackertalkz]

import RPi.GPIO as GPIO
import time, math

C = 0.33 # uF
R1 = 1000 # Ohms

GPIO.setmode(GPIO.BCM)

Pin a charges the capacitor through a fixed 1k resistor and the thermistor in series

pin b discharges the capacitor through a fixed 1k resistor

a_pin = 18
b_pin = 23

empty the capacitor ready to start filling it 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

than means around 1.65V

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()

[Hackertalkz]

Warning (from warnings module):
File "/home/hackertalkz/Desktop/pot_step.py", line 18
GPIO.setup(a_pin, GPIO.IN)
RuntimeWarning: This channel is already in use, continuing anyway. Use GPIO.setwarnings(False) to disable warnings.

[grantwinney]

I've seen that message before when the script doesn't clean up after itself. The last line of the file, GPIO.cleanup(), should have taken care of it though.

It's possible the script died unexpectedly, such as killing it with ctrl+c, and that it didn't get a chance to run that last line. Or perhaps it was already running and you started it again in another window?