CMSC838. Tangible Interactive Assistant Professor Computer Science

Transcription

1 CMSC838 Tangible Interactive Computing Week 04 Lecture 05 Feb 17, 2014 Electronic Components Sensing and Sensors Human Computer Interaction Assistant Professor Computer Science

2 Inspiration

3 [source:

4 [source: Nathan Brunstein, This one is concept only

5 [source: Nathan Brunstein, This one is concept only

6 oday s lass

7 Today 1. MPA01 Chat / Pitches 2. Intro to Electricity Concepts 3. Wires 4. Switches and Voltage Dividers 5. Resistive Sensing

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10 You have to make a new type of physical input for a computer using an Arduino. You do not have to make a custom application to demonstrate said input but this is highly recommended.

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14 ommon components: ires

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16 WIRE TYPES This wire is useful for wiring breadboards; the solidcore ends slip easily into breadboard sockets and will not fray in the process. However, these wires have the tendency to snap after a number of flexes Comprised of a number of individual strands of copper. Better conductor than solid-core wire because the individual wires together comprise a greater surface area. Also, stranded wire will not break easily when flexed. Made up of a number of individual strands of wire braided together. Like stranded wires, better conductors than solid-core wires, and will not break easily when flexed. Often used as an electromagnetic shield in noisereduction Cables. [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

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18 22 AWG solid core wire

19 American wire gauge (AWG) is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire. The cross-sectional area of each gauge is an important factor for determining its current-carrying capacity.

20 AWG Gauge AWG Gauge vs. Wire Diameter (mm) Wire Diameter (mm) [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

21 AWG Gauge AWG Gauge vs. Wire Diameter (mm) Yes, AWG gauge number is counter intuitive. As the wire diameter gets big, the wire gauge gets small! Wire Diameter (mm) [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

22 AWG Gauge AWG Gauge vs. Wire Diameter (mm) AWG Wire AWG Wire AWG Wire Wire Diameter (mm) [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

23 RESISTANCE R Resistance is the opposition to the flow of electric current. It is measured in the SI derived unit ohm (symbol: Ω) Resistance: R = ρ A A piece of resistive material with electrical contacts on both ends. ρ is the electrical resistivity of the material measured in ohms-meters is the length of the piece of material (measured in meters, m) A is the cross-sectional area of the material (measured in square meters, m 2 ). [source:

24 Ohms/km The Relationship Between Wire Diameter (mm) and Resistance (Ohms/km) Wire Diameter (mm) [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

25 Ohms/km The Relationship Between Wire Diameter (mm) and Resistance (Ohms/km) AWG Wire Wire Diameter (mm) [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

26 Ohms/km Current Carrying Capacity (A) The Relationship Between Wire Diameter (mm) and Resistance and Current Carrying Capacity (A) Ohms per km Current Carrying Capacity (A) Wire Diameter (mm) 0 [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

27 Ohms/km Current Carrying Capacity (A) The Relationship Between Wire Diameter (mm) and Resistance and Current Carrying Capacity (A) If too much current is sent through a large-gauge (smalldiameter) wire, the wire could become hot enough to melt! Ohms per km Current Carrying Capacity (A) Wire Diameter (mm) 0 [source: Chapter 3 of Scherz & Monk, Practical Electronics, 3 rd Edition, 2013]

28 ommon components: witches

29 SWITCHES There is an enormous variety of switches from toggle to rotary to DIP to push-button to DIP to rocker. Switches differ in how they are actuated and how many circuits they can control

30 WHAT IS A SWITCH? A circuit diagram with an LED, resistor, and a switch. When the switch is closed, current flows and the LED can illuminate. Otherwise no current flows, and the LED receives no power. [source:

31 SWITCHES Momentary vs. Maintained Momentary: remain active only as long as they are actuated (e.g., keys on a keyboard, buttons) Maintained: Stay in one state until actuated into a new one (e.g., light switches, DIP switches) [source:

32 SWITCHES Arduino Pro 328 The Arduino Pro has two SMD switches: a slide switch for power control, and a pushbutton for reset control. [source:

33 SWITCHES & MCU Hooking up a button to Arduino When the button is open, this input pin is in an unknown state ( floating ), that is bad! With this configuration, when the button closes, the MCU would read LOW from the input pin, but what about when the button is open? [source:

34 SWITCHES & MCU Hooking up a button to Arduino Add VCC to remove this float ambiguity and pull the input pin to HIGH when the button is open. Now, we ve added VCC to pull the input pin to HIGH, but what happens to the circuit when the button closes? [source:

35 SWITCHES & MCU Hooking up a button to Arduino Add VCC to remove this float ambiguity and pull the input pin to HIGH when the button is open. When the button closes, this creates a short circuit! All of the current will flow directly from VCC to GND this is bad! [source:

36 SHORT CIRCUIT Short circuits are never good and should always be avoided When the button closes, this creates a short circuit! All of the current will flow directly from VCC to GND this is bad! Recall Ohm s law, I = V/R. In this case, the circuit has no resistance, so the current is infinite (or will try to be). This could cause your wires to burn up, damage the power supply, drain your battery, etc. [source:

37 SWITCHES & MCU Hooking up a button to Arduino This resistor is a pull-up resistor, required to bias the input high. Without it, when the switch closes, the circuit would short to ground! Now, when the switch is open, the MCU pin is connected through the resistor to 5V (HIGH). When the switch is closed, the pin is tied directly to GND (LOW). [source:

38 SWITCHES & MCU Hooking up a button to Arduino If you choose a low resistor for R1, more current flows to GND but the more power is wasted when the button is hit. In contrast, a high R1 value (e.g., 4MΩ) might not work as a pull-up A low resistor value is called a strong pull-up (more current flows), a high resistor value is called a weak pull-up (less current flows). [source:

39 SWITCHES & MCU Hooking up a button to Arduino This resistor is a pull-up resistor, required to bias the input high. Without it, when the switch closes, the circuit would short to ground! The question then becomes, what resistor value should you select? Now, when the switch is open, the MCU pin is connected through the resistor to 5V (HIGH). When the switch is closed, the pin is tied directly to GND (LOW). [source:

40 SELECT A PULL-UP RESISTOR What should R1 be? The value of the pull-up resistor needs to be chosen to satisfy two conditions: 1. When the button is pressed, the input pin is pulled low. The value of resistor R1 controls how much current you want to flow from VCC, through the button, and then to ground. 2. When the button is not pressed, the input pin is pulled high. The value of the pull-up resistor controls the voltage on the input pin. A very small amount of current flows from VCC through R1 and into the input pin. How small? This depends on the value of R1 and the resistance of the input pin in the MCU. [source:

41 To satisfy these conditions, we have to look inside the MCU

42 SWITCHES & MCU Hooking up a button to Arduino Input pins on MCUs tend to have high resistances of 1k-1MΩ In reality, the MCU has an internal resistor. So the pull-up resistor (R1) and the internal MCU resistor (R2) form a voltage divider [source:

43 VOLTAGE DIVIDER Hooking up a button to Arduino V in If R 1 =R 2, then: V out In reality, the MCU has an internal resistor. So the pull-up resistor (R1) and the internal MCU resistor (R2) form a voltage divider [source:

44 VOLTAGE DIVIDER Hooking up a button to Arduino V in If R 1 =R 2, then: V out In reality, the MCU has an internal resistor. So the pull-up resistor (R1) and the internal MCU resistor (R2) form a voltage divider [source:

45 VOLTAGE DIVIDER Hooking up a button to Arduino 6 V out for varying values of R1 with R2 fixed at 1MΩ V in 5 4 V out V out E+10 If R 1 =R 2, then: R1 Value in kohms (Log Scale) [source:

46 VOLTAGE DIVIDER Hooking up a button to Arduino 6 V out for varying values of R1 with R2 fixed at 1MΩ V in 5 4 V out V out E+10 If R 1 =R 2, then: R1 Value in kohms (Log Scale) [source:

47 ARDUINO BUILT-IN PULL-UP Because pull-up resistors are so commonly needed, many MCUs like the ATmega328 on Arduino, have internal pull-ups that can be enabled or disabled

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49 Let s look at some buttons and switches!

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53 TACTILE BUTTON ASSORTMENT Perhaps surprisingly, this button has four connections, you would probably only expect two how come? [source:

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56 TACTILE BUTTON ASSORTMENT This button has four connections! You would probably only expect two. You can see that connections (1) and (2) are connected together and connections (3) and (4) are connected together [source:

57 TACTILE BUTTON ASSORTMENT If you pick up a button and you re confused or you can t find the datasheet, use continuity testing on your multimeter first without the button pressed and then with the switch pressed [source:

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60 MICROSWITCH Microswitches are not designed to be pressed directly but are often used for things like a microwave door to detect that the door is closed or as an anti-tamper switch. [source: Chapter 3, Simon Monk, Hacking Electronics, 2013]

61 MICROSWITCH A microswitch is a double throw or change-over switch. There is one common connection C that switches between B and C depending on the actuation state. C B (N.O.) C (N.C.) [source:

62 TOGGLE SWITCHES If you look in a component catalog (e.g., Digikey), you will find a bewildering number of toggle switches with names like DPDT, SPDT, SPST, SPST, etc. Single-Pole, Single-Throw Single-Pole, Double-Throw Double-Pole, Double-Throw D = Double S = Single P = Pole T = Throw [source: Chapter 3, Simon Monk, Hacking Electronics, 2013]

63 REED SWITCH An electrical switch operated by an applied magnetic field; it consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts close (or open) in the presence of a magnetic field [source:

64 REED SWITCH [source:

65 REED SWITCH Uninsulated Reed Switch: $1.50 Insulated Reed Switch: $ [source:

66 How are reed switches used?

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72 HALL EFFECT SENSOR A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Unlike the reed switch, no moving parts! Can be used for tracking position, speed, and proximity. Hall Effect Sensor: $ Three types: Hall Effect Switch, Hall Effect Latch, Hall Effect Ratiometric; [source:

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74 SENSING THE PHYSICAL WORLD Touch Light Sound Distance Force Movement Temperature Magnetic Fields Vibration

75 READING RESISTIVE SENSORS Many sensors are resistive sensors, which change their resistance based on some stimulus. Thermistor 4.7k; $0.95* Touch Membrane Potentiometer; $12.95 Photocell (or photodetector or photo resistor); $1.50 Thermistor 10k; $0.75 Force Resistive Sensor 0.5 ; $6.95 Flex Sensor 4.5 ; $12.95 *Note: Prices are from Sparkfun.com; parts are cheaper in bulk and often cheaper from Digikey.com [source: Sparkfun.com]

76 Typically, to use these guys, we need our friend:

77 Typically, to use these guys, we need our friend: Because MCUs with analog-to-digital converters can measure voltages easily but not resistances

78 VOLTAGE DIVIDER You ll see it written in a number of ways We used this schematic before [source:

79 VOLTAGE DIVIDER You ll see it written in a number of ways: We used this schematic before There s also these But no matter what, this equation works: If R 1 =R 2, then: [source:

80 VOLTAGE DIVIDER EXAMPLE Recall our friend: We want to use this photoresistor: Schematic now looks like this: [source:

81 VOLTAGE DIVIDER EXAMPLE Why even do a voltage divider? Why can t we just do this? Or this

82 OK, back to our example

83 VOLTAGE DIVIDER EXAMPLE Let s assume the photocell resistance varies from 1kΩ in the light to 10kΩ in the dark. We want to pick R1 that maximizes our V out range. Schematic now looks like this: [source:

84 V out Range VOLTAGE DIVIDER EXAMPLE Let s assume the photocell resistance varies from 1kΩ in the light to 10kΩ in the dark. We want to pick R1 that maximizes our V out range R1 Value (in kω)

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86 Some book sources