#!/usr/bin/env python # 2014-07-11 DHT22.py import time import atexit import pigpio 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. 5V--5K_resistor--+--10K_resistor--Ground | 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. time.sleep(2) self.powered = True self.cb = None atexit.register(self.cancel) 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. else: 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 else: 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) else: 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 pass 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) time.sleep(2) self.pi.write(self.power, 1) time.sleep(2) 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 else: 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.cancel() self.cb = None if __name__ == "__main__": import time import pigpio import DHT22 # Intervals of about 2 seconds or less will eventually hang the DHT22. INTERVAL=3 pi = pigpio.pi() s = DHT22.sensor(pi, 22, LED=16, power=8) r = 0 next_reading = time.time() while True: r += 1 s.trigger() time.sleep(0.2) print("{} {} {} {:3.2f} {} {} {} {}".format( r, s.humidity(), s.temperature(), s.staleness(), s.bad_checksum(), s.short_message(), s.missing_message(), s.sensor_resets())) next_reading += INTERVAL time.sleep(next_reading-time.time()) # Overall INTERVAL second polling. s.cancel() pi.stop()