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#!/usr/bin/python
# Reading an analogue sensor with a single GPIO pin
# Basic script from:
# /www.raspberrypi-spy.co.uk/2012/08/reading-analogue-sensors-with-one-gpio-pin/
# Author : Matt Hawkins -- Stephen Pratt (SJP)
# Distribution : Raspbian -- SJP Arch-arm
# Python : 2.7 -- SJP converted to v3.5
# GPIO : RPi.GPIO v3.1.0a
# PIN assignments:
# pin Name Connection
# 1 +3.3V power source
# 11 GPIO17 read sensor
# 9 Ground Ground
# circuit characteristics
# series resistor = 220 Ohm
# LDR resistance (full sun) = 360 Ohm
# 'dusk' = 3 M Ohm
# capactitor = 10 uF (quite large)
# max current = 3.3/580 = 5.7 mA (well within spec)
import RPi.GPIO as GPIO, time
# set GPIO17 as read pin
sensorPin = 17
# set maximum time to wait (in which case it is dark)
maxWait = 5e5
obsCount = 6 # number of observations
# Tell the GPIO library to use Broadcom GPIO references
GPIO.setmode(GPIO.BCM)
# Define function to measure charge time
def RCtime ():
# Discharge capacitor
GPIO.setup(sensorPin, GPIO.OUT)
GPIO.output(sensorPin, GPIO.LOW)
time.sleep(0.1) # 100 ms
GPIO.setup(sensorPin, GPIO.IN)
# Count loops until voltage across
# capacitor reads high on GPIO
timer = 0
while (GPIO.input(sensorPin) == GPIO.LOW):
timer += 1
if timer > maxWait : return -1
return timer
# Main program loop
obs=list() # an empty list
for num in range(0, obsCount):
obs.append(RCtime())
print(obs)
print("sum=", sum(obs))
print("Ave time: ", sum(obs)/obsCount) # Measure timing using GPIO4
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