- Raspberry Pi
- Used in article: Model B Revision 1.0 with Raspbian (Debian GNU/Linux 7.6 (wheezy))
- I2C temperature sensor
- Used in article: TMP102
- Method to connect Raspberry Pi to internet
- Use in article: Raspberry Pi wired directly to router
- Google account to create and access sheets
- NMOS or NPN transistor capable of handling the voltage and current requirements of the fan
- Schottky diode
- Small DC fan
- Used in article: 12V/600mA fan
Set Up the Hardware
Follow the instructions in this article to connect a temperature sensor to the Raspberry Pi.
Follow the wiring diagram below to connect the Raspberry Pi to the temp sensor and to the fan through the transistor. The diode is there to prevent the fan from damaging the transistor due to potential voltage flyback when turning the fan off. A diode in this configuration is sometimes called a snubber. You have to cut the ground wire feeding the fan in order to put the transistor in series. The transistor is acting as a low-side switch in this configuration.
The maximum current of the Pi GPIO output is 16mA. This means the transistor must have a high enough hFE to conduct the current needed to run the fan. Size the base resistor to limit the amount of current the Pi delivers to the transistor to avoid damage. I used 180 ohms which would give a base current of approximately (3.3-0.7)/180 = 14.4mA. The transistor I selected has an hFE of 150 when conducting 2A, so switching a 600mA load is no problem.
Follow the article here to setup I2C to communicate to the temp sensor.
Installing GPIO capability
Type the following in the Pi terminal to install GPIO capability: sudo apt-get install python-rpi.gpio
Upload the following Python program to the Raspberry Pi in order to test the fan control from the GPIO. Run the script "turn_fan_on.py" or "turn_fan_off.py" in order to verify the connections.
import RPi.GPIO as GPIO GPIO.setwarnings(False) GPIO.setmode(GPIO.BCM) FAN_PIN = 23 GPIO.setup(FAN_PIN, GPIO.OUT) GPIO.output(FAN_PIN, True)
import RPi.GPIO as GPIO GPIO.setwarnings(False) GPIO.setmode(GPIO.BCM) FAN_PIN = 23 GPIO.setup(FAN_PIN, GPIO.OUT) GPIO.output(FAN_PIN, False)
Controlling the fan based on temperature
The following script implements some logic that turns on the fan when the temperature has risen above TEMP_THRESHOLD. The fan will stay on until the temperature drops below the threshold - TEMP_HYST. This way the fan doesn't rapidly turn on and off when the room is around the temperature threshold.
import RPi.GPIO as GPIO import smbus import time #0 = /dev/i2c-0 #1 = /dev/i2c-1 I2C_BUS = 0 bus = smbus.SMBus(I2C_BUS) #7 bit address (will be left shifted to add the read write bit) DEVICE_ADDRESS = 0x48 TEMP_THRESHOLD = 78 TEMP_HYST = 2 GPIO.setwarnings(False) GPIO.setmode(GPIO.BCM) FAN_PIN = 23 GPIO.setup(FAN_PIN, GPIO.OUT) while True: time.sleep(1) #Read the temp register temp_reg_12bit = bus.read_word_data(DEVICE_ADDRESS , 0 ) temp_low = (temp_reg_12bit & 0xff00) >> 8 temp_high = (temp_reg_12bit & 0x00ff) #convert to temp from page 6 of datasheet temp = ((( temp_high * 256 ) + temp_low) >> 4 ) #handle negative temps if temp > 0x7FF: temp = temp-4096; temp_C = float(temp) * 0.0625 temp_F = temp_C * 9/5+32 print "Temp = %3.1f C -- %3.1f F" % (temp_C,temp_F) #control the fan based on the temp if(temp_F > TEMP_THRESHOLD): GPIO.output(FAN_PIN, True) if(temp_F < (TEMP_THRESHOLD - TEMP_HYST)): GPIO.output(FAN_PIN, False)
In the video below, I'm using ice cubes in a plastic bag to simulate the room cooling down. When the ice cubes are applied to the temp sensor, the fan turns off. When I remove them, the temperature rises and the fan turns on.