• Pi Camp Day 2

  • Purpose Day 2

    • Understand series circuits - ohms law
    • know about resistors
    • know about LEDs
    • know about GPIO pins
    • navigate about the Linux screen and command line.

  • Some Theory - sorry there always has to be some theory ...  theory is good.
    • Ohms Law
      • I = V/R  Which in English says that the amount of current is equal to the voltage divided by the resistance, or, the more voltage for a given resistance the higher the current.  The higher the resistance for a given voltage the lower the current.
        • I = current in Amps (A)
        • V = Voltage in Volts (V)    (note that sometimes voltage is shown as E for electromotive force)
        • R = Resistance in Ohms (Ω)

      • P = V x I
        • P is power in watts (W).

      • Why do we care.  We need to calculate how to use only the amount of power a particular device will allow before it overheats and breaks.  ( "lets the smoke out". https://en.wikipedia.org/wiki/Magic_smoke)

    • Question:  If we want a current of 1 milliamp (.001A) and we have a voltage of 9 volts, what size does the resistor need to be?

    • Resistors
      • Colour Code



    • What would the first 3 band colours be for a 470 Ohm Resistor?


    • LEDs in detail
      • How it works
        • Current flows in one direction only - Which is why it is called a diode
          • If current is flowing it gives off light.  A characteristic colour for each LED
          • If the LED is wired backwards, no current will flow and no light will happen.
          • Demonstrate this with the 9V battery circuit.

      • Max Current for the LED is  20mA.  We can use less current, and should as the Pi wants less.  The LED will not be as bright.

        Calculate the resistor for safely connecting a LED to a nine volt battery.

    • Introduction to the GPIO Connector
      • Two different numbering systems Pin Placement (BOARD) and Broadcom (BCM) name.
        • Normally we will use BCM (broadcom) as the  broadcom names are printed on the GPIO to prototype board adapter.
        • But for our first experiment we will not be using the prototype board so we will use the BOARD numbering (see code below).
      • The pins can be
        • Input or outputs - selected by software
          • Max current for a Pi pin is 16mA,  Max total through all pins is 51mA.
        • Power
        • Ground
        • Special purpose pins.  Some of these pins can also be normal input or output pins. 



    •                                                     Board Pin Numbering
      BCM GPIO# 2nd func pin#
      		pin# 2nd func BCM GPIO#
      - +3V3 1
      		2 +5V -
      GPIO2 SDA1 (I2C) 3
      		4 +5V -
      GPIO3 SCL1 (I2C) 5
      		6 GND -
      GPIO4 GCLK 7
      		8 TXD0 (UART) GPIO14
      - GND 9
      		10 RXD0 (UART) GPIO15
      GPIO17 GEN0 11
      		12 GEN1 GPIO18
      GPIO27 GEN2 13
      		14 GND -
      GPIO22 GEN3 15
      		16 GEN4 GPIO23
      - +3V3 17
      		18 GEN5 GPIO24
      GPIO10 MOSI (SPI) 19
      		20 GND -
      GPIO9 MISO (SPI) 21
      		22 GEN6 GPIO25
      GPIO11 SCLK (SPI) 23
      		24 CE0_N (SPI) GPIO8
      - GND 25
      		26 CE1_N (SPI) GPIO7
      (Models A and B stop here)
      EEPROM ID_SD 27
      		28 ID_SC EEPROM
      GPIO5 - 29
      		30 GND -
      GPIO6 - 31
      		32 - GPIO12
      GPIO13 - 33
      		34 GND -
      GPIO19 - 35
      		36 - GPIO16
      GPIO26 - 37
      		38 - GPIO20
      - GND 39
      		40 - GPIO21



    • Calculation of what value of resistor we need to safely light a LED with 3.3 volts from the Pi
      • What happens if we use a resistor of a higher value?
      • What happens if we use too low a value of resistor?
      • Let's say 5mA, so R is V/I  is 3.3/.005 ~ 660 Ω

    • Create/(Copy) a Python program to blink the LED (connected it to pin 11) and debug it.
      • Terminal or GUI?
        • Menu, Accessories, gedit
        • Copy and paste the program below
        • Save it as blink.py

      • Editor - nano or idle or gedit???.

        #! /usr/bin/python

        # Blink a LED
        # Import the libraries we need
        import RPi.GPIO as GPIO
        import time

        # Set the pin mode to Broadcom
        GPIO.setmode(GPIO.BOARD)
        pin = 11

        # Set the pin to be an output
        GPIO.setup(pin, GPIO.OUT)

        i=0

        # Loop turning it on and off
        while i < 10:
            GPIO.output(pin, 1)
            time.sleep(1)
            GPIO.output(pin, 0)
            time.sleep(1)
            i = i + 1

        # Set the pins back to default
        GPIO.cleanup()





    • Rebuild the circuit you created in day one without the  9 volt battery.
      • The Pi will be both a switch and the "battery"
      • Shutdown the Pi and power it off.
      • Connect the circuit to the Pi instead of the 9 volt battery using pins 11 for the plus (+) side and pin 9 for the minus (-) side.
      • Have your circuit checked and your program checked.


        What might happen if you do not shut your Pi down properly?

      • Power on your Pi.
      • Run the program
        • Click the Terminal icon to start a terminal session
          • You can run the blink.py program two ways
            • python blink.py
            • ./blink.py
              • First chmod 755 blink.py
              • Then ./blink.py.

        • Does it work?  Yes bonus.
          • Maybe help someone else who has not got theirs working yet.
        •  No.  Debug.  Ask for help.

    • Shutdown and power off your Pi


      Why should you power off your Pi before you modify a circuit?

    • Introduction to Prototype board










      • Have this connection checked before you proceed


        What is the purpose of a breadboard?
            1. Cutting bread
            2. For carving wires to get shorter wires
            3. A way of connecting circuits together
            4. A place to connect your Pi to.
            5. A place for chopping vegatables?

    • Next step - wire up the battery LED circuit that you built with connectors on the prototype board to ensure you understand how the board works.
      •  with a 9 volt battery. 
        • Remember + and -.
      • The LED should turn on when the battery is connected.
    • Control with the Pi

    • Wire up the prototype board LED circuit  for control by the Pi. 
      • Remove the 9 V battery
      • Again remember + and -.  The + lead of the LED connects through the resister to Pin 11  (GPIO 17) on the Pi
      • Connect the minus to pin 9 (GPIO GND).



      • Have the circuit checked before you power on your PI.


    • Run the blink program

  • The Pi Cobbler

    • Shutdown and power off the Pi
    • Disconnect the wires going directly to the Pi GPIO pins.  We will connect the cobbler.





  • Connect the Prototype board to the Pi using the ribbon cable and Pi Cobbler.
    • Make sure you connect the cobbler to the prototype board the correct way.
      • The +- labels on the prototype board should match the +- labels on the cobbler.

  • Connect the GPIO cable and have it checked.
    • Make sure you connect the ribbon cable to the Pi correctly. 
      • The red stripe on the cable should connect to Pin 1 on the Pi GPIO pins
    • Have these connections checked before you proceed

  • Rewire your circuit so that it now connects to to pin 11, GPIO pin 17 and ground through the Pi Cobbler.

    Note, that depending on the type of cobbler we have you may not need the two connections from 3.3Volts and GND to the proto board.


    What is the cobbler for?


    Have your connections checked before you power on.

  • Power on the Pi and run the blink program