ds18b20.py

# ds18b20.py
# written by Roger Woollett

import os
import glob
import time

class DS18B20:
	# much of this code is lifted from Adafruit web site
	# This class can be used to access one or more DS18B20 temperature sensors
	# It uses OS supplied drivers and one wire support must be enabled
	# To do this add the line
	# dtoverlay=w1-gpio
	# to the end of /boot/config.txt
	#
	# The DS18B20 has three pins, looking at the flat side with the pins pointing
	# down pin 1 is on the left
	# connect pin 1 to GPIO ground
	# connect pin 2 to GPIO 4 *and* GPIO 3.3V via a 4k8 (4800 ohm) pullup resistor
	# connect pin 3 to GPIO 3.3V
	# You can connect more than one sensor to the same set of pins
	# Only one pullup resistor is required
	
	def __init__(self):
		# load required kernel modules
		os.system('modprobe w1-gpio')
		os.system('modprobe w1-therm')
		
		# Find file names for the sensor(s)
		base_dir = '/sys/bus/w1/devices/'
		self.device_folder = glob.glob(base_dir + '28*')
		self._num_devices = len(self.device_folder)
		self._device_file = list()
		i = 0
		while i < self._num_devices:
			self._device_file.append(self.device_folder[i] + '/w1_slave')
			i += 1
		
	def _read_temp(self,index):
		# Issue one read to one sensor
		# you should not call this directly
		f = open(self._device_file[index],'r')
		lines = f.readlines()
		f.close()
		return lines
		
	def tempC(self,index = 0):
		# call this to get the temperature in degrees C
		# detected by a sensor
		lines = self._read_temp(index)
		retries = 5
		while (lines[0].strip()[-3:] != 'YES') and (retries > 0):
			# read failed so try again
			time.sleep(0.1)
			#print('Read Failed', retries)
			lines = self._read_temp(index)
			retries -= 1
			
		if retries == 0:
			return 998
			
		equals_pos = lines[1].find('t=')
		if equals_pos != -1:
			temp = lines[1][equals_pos + 2:]			
			return float(temp)/1000
		else:
			# error
			return 999

	def tempC_byDeviceName(self,name):
		return self.tempC(self.device_folder.index('/sys/bus/w1/devices/'+name))
			
	def device_count(self):
		# call this to see how many sensors have been detected
		return self._num_devices

	def device_name(self,index = 0):
		return self.device_folder[index].replace('/sys/bus/w1/devices/','')