• Day 4 - Input


    • Add a button

      • Pull up/down resistors


        This schematic shows a pull up resistor of 50K Ω.

      • The purpose of the pull up resistor is to ensure that the input is not floating.  If it were left floating then random environmental electrical noise could cause the input to go from 0 to 1, like, randomly.   This is not desirable.  We could wire our switch circuit with a 50K Ω resistor as seen on the right.

        With the Pi, we can also just turn on the internal pull up resistor.  See the diagram at the right.  The software commands to do this are shown in the example code below.

        https://en.wikipedia.org/wiki/Pull-up_resistor.

        The 1K         Ω resistor between the switch and ground is a safety precaution.  If the Pin was accidentally set to output and and the switch closed it would send 3.3 V to ground with nothing limiting the current. The 1K Ω resistor will limit the current.  To ... How much? 



      • Count the button presses

      • Make sure your Pi is shutdown safely and powered off.

      • First add a button and a a pull up resistor and a safety resistor to your prototype board.  Connect switch to GPIO 19.  Board Pin 35.




        Looking at the back of the button to better see what is connected when the button is pressed.



        Note that depending on the cobbler you may not need the ground and power wires shown at the left bottom of the proto board.



        Have your switch circuit checked before you power it up and add the code.


      • Code to count the button presses:


        #! /usr/bin/python

        import RPi.GPIO as GPIO
        GPIO.setmode(GPIO.BCM)
        GPIO.setwarnings(False)

        btn = 19

        # Set up Switch as input with pull up resistor. 
        # If you wired a pull up resistor as well that is OK
        GPIO.setup(btn, GPIO.IN, GPIO.PUD_UP)

        # Set the last input to 1 and the count to 0.
        lastInput = 1
        count = 0

        # Carry on forever
        while True:
            input = GPIO.input(btn)
            # Delay for a bit to allow the button to settle
            time.sleep(.010)

            # If the input has changed count and display the new value
            # and set the lastInput value to the new value.  
            if input <> lastInput:
                print (input)
                count = count + 1
                lastInput = input

      • Debounce:

        You need to debounce the input from a switch with some logic or delays.  As the switch closes there is a period when it goes from open to closed a few times before it closes solidly.  The easiest way to do this is to delay a few milliseconds before using a switch value.  https://en.wikipedia.org/wiki/Switch#Contact_bounce

      • Copy and paste this code into your pi into a program named countSwitch.py

        Let's Build It. 

      • And the same logic using interrupts/callback/events


            #! /usr/bin/python

            import RPi.GPIO as GPIO
            GPIO.setmode(GPIO.BCM)
            GPIO.setwarnings(False)

            btn = 19
            count = 0

            # Set up Switch as input with pull up resistor. 
            # If you wired a pull up resistor as well that is OK
            GPIO.setup(btn, GPIO.IN, GPIO.PUD_UP)

            # Set up event detect
            GPIO.add_event_detect(btn, GPIO.FALLING, callback=buttonDown, bouncetime=delay)

            # Callback  function
            def buttonDown(channel):
                global count
                count = count + 1
                print (count)
             
            # Loop forever - the time sleep is so that this is not a
            # CPU hog. This is probably not required as
            # Linux will lower the process priority.
            while True:
                time.sleep(.1)
        What is debouncing?  Why?


    • Control

      We don't need to tell you to turn your Pi off correctly anymore do we?

      • Measure Temperature, humidity, light or ...
        • Temperature and humidity

        • Wire up the following DHT22 temperature and humidity sensor and have the wiring checked before you power up.


      • Program to measure temperature.
        • https://learn.adafruit.com/dht-humidity-sensing-on-raspberry-pi-with-gdocs-logging/software-install-updated

          Copy the following program and run it.


          #! /usr/bin/python
          # Read and display the temperature and humidity

          import RPi.GPIO as GPIO
          import time
          import signal, os

          import sys

          import Adafruit_DHT

          # Set up parameters
          sensor = 22       # sensor type
          pin = 4           # sensor data pin

          # Loop forever reading humidity and temperature
          while 1:
              #get and display the humidity and temperature.
              humidity, temperature = Adafruit_DHT.read_retry(sensor, pin)
              print (humidity, ' ', temperature)
              time.sleep(2)




      • Your output should look something like:

        $  python testTemp.py
        33.7999992371   22.7000007629
        33.7000007629   22.7000007629
        33.7000007629   22.7000007629
        33.7000007629   22.7000007629
        33.7000007629   22.7000007629
        33.7000007629   22.7000007629
        33.5999984741   22.7000007629
        33.5999984741   22.7000007629
        33.5999984741   22.7000007629

        - clearly your humidity and temperature will be different.

      • Combining the LED circuit and the temperature and humidity circuit.   The switch is still there as well.


        Two circuits combined.



        • Create a new program from part of the temperature, humidity program above, and part of the blink LED program from yesterday.
          • Instead of blinking the LED, turn it on if the temperature goes above a given temperature. 

        • See if you can write the python code do do this.  Start with the code described above.  And as always ask for help if you need it.

        • Let's Build It. 

    • Transistors and Fans - so if we measure the temperature and it is too hot we could turn on a fan. 

    • If Time permits.

      • A fan takes too much power for the Pi to provide on a GPIO pin.  So we add a transistor. 
      • The transistor provides amplification - a small signal controls a larger signal.


         



        Explaining this new circuit

      • Transistors, relays and larger loads. 
        • Transistors allow a small amount of current to control a larger amount of current.    https://en.wikipedia.org/wiki/Transistor
        •  As you probably know, the computer systems we use are built with millions of small transistors that can be on or off representing the 0s and 1s of digital logic.  Transistors can also be used as single components to switch more current than can be switched by the digital logic of a SBC (Single Board Computer) or micro controller.

          Transistors have 3 leads:
          1. The Base which controls the current flow, CBE as flat faces you
          2. The Emitter which goes to ground to complete the circuit. 
          3. For a NPN transistor the Collector which is the positive lead for the controlled current

            When the Base Emitter circuit is conducting, then the Collector Emitter circuit will conduct more.  The result a small current through the Base can control a much larger current through the Collector.  In the 2N2222 this amount more (gain) can be as much as 100 times.  The 2n2222 has about 600 mA max collector current which is lots enough for the fan we want to use.

      • The snubber diode (  https://en.wikipedia.org/wiki/Snubber  ) across the relay coil is there to dissipate the current when the relay is turned off.  The coil is an electromagnet and when it is turned off by switching the transistor off the collapsing magnetic field creates a current in the circuit in same direction as the current was flowing when the transistor was on.  Unfortunately because the transistor is off the current can't flow to ground.  So the voltage across the transistor can rise to well over 200 Volts.  At some point the transistor will fail and conduct current to ground.  Hopefully this failure will not destroy the transistor.  To prevent this voltage rise, we install the diode across the relay coil and allow the current to flow back to the 5Volt supply.  Since the current is flowing through the diode, the dangerously high voltage never builds up.  https://www.youtube.com/watch?v=c6I7Ycbv8B8

      • If Time Permits
        • Build it and turn the fan on and off depending on temperature

    • What are the following used for in our circuit?
      • Transistor
      • Diode
      • 10K resistor

    • apt-get - How to update the Pi

      • update
        • apt-get update

      • install
        • apt-get install <package>

      • Show what is available.
        • apt-cache search <keyword>
        • apt-cache search .