What I'm trying to do is to read temperature from TMP36 sensor (over MCP3008 ADC), light from LDR, and humidity from DHT22 sensor. Firstly, I had one script for temperature and light readings, and it worked ok. And also I had another script for humidity readings, that I downloaded from github, by joan2937 - pigpio site, for DHT22 sensor, and its also working ok. So the circuit is correctly connected.

Now I have everything in one script, here is the full code:


print "Content-type: python\n"

import spidev
import os
import time
import math
import sqlite3
import pigpio
import cgitb
import sys
import DHT22
import atexit


#Open SPI bus
#spi = spidev.SpiDev()
h = pi.spi_open(0,1000000)

conn = sqlite3.connect('sensordb2.db')
curs = conn.cursor()

#Read SPI data from ADC
#Channel is integer 0-7
def ReadChannel(channel):
    #adc = spi.xfer2([1,(8+channel)<<4,0])
    count,adc = pi.spi_xfer(h,[1,(8+channel)<<4,0])
    data = ((adc[1]&3)<<8) + adc[2]
    return data

#Convert data to voltage level
#rounded to specified number of decimal places
def ConvertVolts(data,places):
    volts = (data*3.3)/float(1023)
    #volts = (data*5)/float(1023)
    volts = round(volts,places)
    return volts

#Calculate temperature from thermistor
#rounded to specified number of decimal places
def ConvertTemp(data,places):
    temp = ((data*330)/float(1023))-50
    #temp = ((data*500)/float(1023))-50
    #temp = ((data*1000)-500)/10
    temp = round(temp,places)
    return temp

#Define sensor channels
light_channel = 0
temp_channel = 7

#Define delay between readings
delay = 5

class sensor:
   A class to read relative humidity and temperature from the
   DHT22 sensor.  The sensor is also known as the AM2302.

   The sensor can be powered from the Pi 3V3 or the Pi 5V rail.

   Powering from the 3V3 rail is simpler and safer.  You may need
   to power from 5V if the sensor is connected via a long cable.

   For 3V3 operation connect pin 1 to 3V3 and pin 4 to ground.

   Connect pin 2 to a gpio.

   For 5V operation connect pin 1 to 5V and pin 4 to ground.

   The following pin 2 connection works for me.  Use at YOUR OWN RISK.

   DHT22 pin 2 -----+
   gpio ------------+

   def __init__(self, pi, gpio, LED=None, power=None):
      Instantiate with the Pi and gpio to which the DHT22 output
      pin is connected.

      Optionally a LED may be specified.  This will be blinked for
      each successful reading.

      Optionally a gpio used to power the sensor may be specified.
      This gpio will be set high to power the sensor.  If the sensor
      locks it will be power cycled to restart the readings.

      Taking readings more often than about once every two seconds will
      eventually cause the DHT22 to hang.  A 3 second interval seems OK.

      self.pi = pi
      self.gpio = gpio
      self.LED = LED
      self.power = power

      if power is not None:
         pi.write(power, 1) # Switch sensor on.

      self.powered = True

      self.cb = None


      self.bad_CS = 0 # Bad checksum count.
      self.bad_SM = 0 # Short message count.
      self.bad_MM = 0 # Missing message count.
      self.bad_SR = 0 # Sensor reset count.

      # Power cycle if timeout > MAX_TIMEOUTS.
      self.no_response = 0
      self.MAX_NO_RESPONSE = 2

      self.rhum = -999
      self.temp = -999

      self.tov = None

      self.high_tick = 0
      self.bit = 40

      pi.set_pull_up_down(gpio, pigpio.PUD_OFF)

      pi.set_watchdog(gpio, 0) # Kill any watchdogs.

      self.cb = pi.callback(gpio, pigpio.EITHER_EDGE, self._cb)

   def _cb(self, gpio, level, tick):
      Accumulate the 40 data bits.  Format into 5 bytes, humidity high,
      humidity low, temperature high, temperature low, checksum.
      diff = pigpio.tickDiff(self.high_tick, tick)

      if level == 0:

         # Edge length determines if bit is 1 or 0.

         if diff >= 50:
            val = 1
            if diff >= 200: # Bad bit?
               self.CS = 256 # Force bad checksum.
            val = 0

         if self.bit >= 40: # Message complete.
            self.bit = 40

         elif self.bit >= 32: # In checksum byte.
            self.CS  = (self.CS<<1)  + val

            if self.bit == 39:

               # 40th bit received.

               self.pi.set_watchdog(self.gpio, 0)

               self.no_response = 0

               total = self.hH + self.hL + self.tH + self.tL

               if (total & 255) == self.CS: # Is checksum ok?

                  self.rhum = ((self.hH<<8) + self.hL) * 0.1

                  if self.tH & 128: # Negative temperature.
                     mult = -0.1
                     self.tH = self.tH & 127
                     mult = 0.1

                  self.temp = ((self.tH<<8) + self.tL) * mult

                  self.tov = time.time()

                  if self.LED is not None:
                     self.pi.write(self.LED, 0)


                  self.bad_CS += 1

         elif self.bit >=24: # in temp low byte
            self.tL = (self.tL<<1) + val

         elif self.bit >=16: # in temp high byte
            self.tH = (self.tH<<1) + val

         elif self.bit >= 8: # in humidity low byte
            self.hL = (self.hL<<1) + val

         elif self.bit >= 0: # in humidity high byte
            self.hH = (self.hH<<1) + val

         else:               # header bits

         self.bit += 1

      elif level == 1:
         self.high_tick = tick
         if diff > 250000:
            self.bit = -2
            self.hH = 0
            self.hL = 0
            self.tH = 0
            self.tL = 0
            self.CS = 0

      else: # level == pigpio.TIMEOUT:
         self.pi.set_watchdog(self.gpio, 0)
         if self.bit < 8:       # Too few data bits received.
            self.bad_MM += 1    # Bump missing message count.
            self.no_response += 1
            if self.no_response > self.MAX_NO_RESPONSE:
               self.no_response = 0
               self.bad_SR += 1 # Bump sensor reset count.
               if self.power is not None:
                  self.powered = False
                  self.pi.write(self.power, 0)
                  self.pi.write(self.power, 1)
                  self.powered = True
         elif self.bit < 39:    # Short message receieved.
            self.bad_SM += 1    # Bump short message count.
            self.no_response = 0

         else:                  # Full message received.
            self.no_response = 0

   def temperature(self):
      """Return current temperature."""
      return self.temp

   def humidity(self):
      """Return current relative humidity."""
      return self.rhum

   def staleness(self):
      """Return time since measurement made."""
      if self.tov is not None:
         return time.time() - self.tov
         return -999

   def bad_checksum(self):
      """Return count of messages received with bad checksums."""
      return self.bad_CS

   def short_message(self):
      """Return count of short messages."""
      return self.bad_SM

   def missing_message(self):
      """Return count of missing messages."""
      return self.bad_MM

   def sensor_resets(self):
      """Return count of power cycles because of sensor hangs."""
      return self.bad_SR

   def trigger(self):
      """Trigger a new relative humidity and temperature reading."""
      if self.powered:
         if self.LED is not None:
            self.pi.write(self.LED, 1)

         self.pi.write(self.gpio, pigpio.LOW)
         time.sleep(0.017) # 17 ms
         self.pi.set_mode(self.gpio, pigpio.INPUT)
         self.pi.set_watchdog(self.gpio, 200)

   def cancel(self):
      """Cancel the DHT22 sensor."""

      self.pi.set_watchdog(self.gpio, 0)

      if self.cb != None:
         self.cb = None

while True:
    #Read the light sensor data
    light_level = ReadChannel(light_channel)
    light_volts = ConvertVolts(light_level,2)

    #Read the temp sensor data
    temp_level = ReadChannel(temp_channel)
    temp_volts = ConvertVolts(temp_level,2)
    tempc = ConvertTemp(temp_level,2)

    if tempc>24:
        flag = 1
        flag = 0

    # Humidity

    s = DHT22.sensor(pi, 25, LED=21, power=8)   



    #Print out results
    print "-------------------------------"
    print("Humidity (DHT22): {} %".format(s.humidity()))
    print("Light (LDR): {} ({}V)".format(light_level,light_volts))
    print("Temperature (TMP36): {} ({}V) {} deg C flag {}".format(temp_level,temp_volts,tempc,flag))
    curs.execute("INSERT INTO analogPins VALUES(date('now','localtime'),time('now','localtime'),'%d','%f','%d','%f','%f','%d')" %(light_level,light_volts,temp_level,tempc,s.humidity(),flag))
    print "Saved to DB..."

    #Wait before repeating loop


NOTE: All code from class sensor (DHT22.py code) is the same, I didn't changed anything.

I noticed that after execution of DHT22 code part (class sensor), I can't get any readings from another sensors, so can anyone tell me why is this happening and how can I get this work? Thanks in advance.

Here is the output screenshot:

enter image description here

  • How are you reading the MCP3008?
    – joan
    Dec 5, 2015 at 15:16
  • @joan, I edited my question and added the code of MCP3008 reading.
    – hideburn
    Dec 5, 2015 at 15:42
  • Could you comment out the spi = spidev.SpiDev() line if you are using the pigpio SPI functions. It should not make a difference, but it is best to rule it out.
    – joan
    Dec 5, 2015 at 15:52
  • ok joan, I commented the spi = spidev.SpiDev() line, but still same problem.
    – hideburn
    Dec 5, 2015 at 16:41
  • Unless the code is thousands of lines it's probably best to post the full code. The formatting seems to have been lost on the snippets you have posted so far so you may need to find a different method of copy&paste.
    – joan
    Dec 5, 2015 at 16:45

1 Answer 1


Okay, you are using GPIO 8 to power the DHT22.

GPIO 8 is actually the chip select for channel 0 of the main SPI device.

So choose another GPIO, or power the DHT22 from 3V3 and set the power parameter to None (or omit it completely) when starting the DHT22.

Note, if the SPI port was not properly closed it may fail to initialise properly at the next SPI open. From the command line you can use pigs spic h to close SPI handle h.

for ((i=0; i<32; i++)); do pigs spic $i; done

will close all open SPI handles (giving error messages for those not open).

Alternatively kill and restart the daemon (sudo killall pigpiod; sudo pigpiod).

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