Create a system that will turn a fan on when the temperature in a greenhouse reaches a specific threshold temperature and off when it falls back below that threshold.
This project uses a Raspberry Pi 4, a BHT22 (AM2302) temperature and humidity sensor, and and SSR-40 DA solid state Relay to turn a 120V fan.
CAUTION: This project requires knowledge of working with 120V electricity. Do not attempt this without the proper training and experience.
What you Need
Raspberry Pi (3 or 4. 2 might also work)
DHT22 (ASM2302) Temperature and Humidity Sensor
SSR-40 DA solid state relay
Breadboard
40-pin Ribbon Cable (not required if you're not using a cobbler)
If you're in the US, Adafruit https://www.adafruit.com/ is the source for so much maker wonderfulness.
Hardware setup
The video covers the hardware setup but here are the main points.
The Raspberry Pi 4 connected to the breadboard by the ribbon cable and cobbler.
I’ve hooked the 3.3 volt power up to the power bus and the ground to the negative bus.
Then I connected the DHT22 sensor to power (red positive 3.3 V and black to ground) and the yellow wire to GPIO 4.
There is an LED that connects from the ground to a 1k resistor that connects to GPIO 25
The push button connects from the 3.3 V to a resistor and then GPIO 16.
SSR – Solid State Relay. GPIO 26 goes to the positive terminal of the 3.3V side and the negative terminal connects to the ground.
Source Code
''' Fan turns on when temperature reaches threshold temperature or when button is pressed and turns off when button is pressed or interval time ( in seconds) is reached. Fan stays off for set interval if turned off when temperature is above threshold. '''
import Adafruit_DHT import time import RPi.GPIO as GPIO from datetime import datetime
def round_up(n, decs=0): # used for rounding up specified value n import math multiplier = 10 ** decs return math.ceil(n * multiplier) / multiplier
def shut_down(): GPIO.output(led_pin, GPIO.LOW) # Turn off print(datetime.now().strftime("%H:%M:%S")+" Shutting off Fan") GPIO.setup(ssr_pin, GPIO.OUT, initial=GPIO.LOW) # Turn off 120V device print(datetime.now().strftime("%H:%M:%S")+" Fan was on for: " + str(round(now - start_time, 1)) + " seconds")
def start_fan() : GPIO.output(led_pin, GPIO.HIGH) # Turn on GPIO.setup(ssr_pin, GPIO.OUT, initial=GPIO.HIGH) # Turn on 120V device print(datetime.now().strftime("%H:%M:%S")+" Starting Fan at " + "Temp={0:0.1f}*C ".format(temperature))
def my_time(): #rounds up the time to the nearest second and returns an integer #mytime=round_up(time.time()) mytime=round(time.time()) return int(mytime)
DHT_SENSOR = Adafruit_DHT.DHT22 # create a sensor object
##### Most common variables on_interval = 10 # this is the interval (in seconds) the fan will stay on before checking to see if the temperature has dropped. threshold_temp = 25 # the temperature that has to be reached before the fan starts off_interval = 10 # if the fan is manually switched off, it will stay off for this period of seconds ##### Pins DHT_PIN = 4 # specify the GPIO pin that the sensor yellow wire is connected to led_pin = 25 # specify which GPIO the LED is connected to ssr_pin = 26 # pin to trigger the relay button_pin = 16 # pin that looks for the button being pressed ##### GPIO Setup GPIO.setmode(GPIO.BCM) # Use Broadcom GPIO numbering not physical numbering GPIO.setwarnings(False) # Ignore warning for now GPIO.setup(led_pin, GPIO.OUT, initial=GPIO.LOW) # LED output GPIO.setup(ssr_pin, GPIO.OUT, initial=GPIO.LOW) # SSR output GPIO.setup(button_pin, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) # connected to push button
##### Variable initialization start_time = int() # the time when the temperature first went above the threshold or the button was pressed init_time = int() # the time the program was started off_time = int() # the time that the fan was shut off last_check = int() # the time that the temperature was last checked. now = int() # the time right now init_time = my_time() timer = int() # counts seconds that the system is running
humidity, temperature = Adafruit_DHT.read_retry(DHT_SENSOR, DHT_PIN)
if humidity is not None and temperature is not None: print(datetime.now().strftime("%H:%M:%S") + " Temp={0:0.1f}*C Humidity={1:0.1f}%".format(temperature, humidity)) else: print(atetime.now().strftime("%H:%M:%S") + "Failed")
while True: humidity, temperature = Adafruit_DHT.read_retry(DHT_SENSOR, DHT_PIN) now = my_time()
# fan is not running and it was not switched off if GPIO.input(ssr_pin) == 0 and off_time == 0: if (temperature >= threshold_temp) or GPIO.event_detected(16): #threshold temperature reached or button is pressed start_fan() start_time = my_time()
# Fan is running and button pressed elif GPIO.input(ssr_pin) == 1 and GPIO.event_detected(16): now = my_time() off_time = now shut_down() # shut down fan
# Fan is running and time interval exceeded elif GPIO.input(ssr_pin) == 1 and now - start_time > on_interval: if temperature < threshold_temp: # temp falls below threshold now = my_time() shut_down() # shut down fan else: # temp is still above threshold but the on interval has been exceeded start_time = now
# fan is off because it was switched off. Need to keep it off. elif GPIO.input(ssr_pin) == 0 and off_time > 0: if now - off_time > off_interval: print (datetime.now().strftime("%H:%M:%S")+" Fan was off for: " + str(now - off_time) + " seconds") off_time = 0
if (now-init_time) % 3 == 0: # If the current number of seconds since the system started is even ie: a 3-second interval print(datetime.now().strftime("%H:%M:%S ") + "Temp={0:0.1f}*C Humidity={1:0.1f}%".format(temperature, humidity))
