ME456: Mechatronics Systems Design Lecture 3 Chapter 2: Lights On

1
ME456 Mechatronics Systems DesignLecture
3Chapter 2Lights On Lights Off

• Prof. Clark J. Radcliffe
• Mechanical Engineering
• Michigan State University

http//www.egr.msu.edu/classes/me456/radcliff
2
Indicator Lights

• First external device
• An Output to human operator
• Indicates binary condition
• On/Off, True/False, OK/Fault, etc.
• Your Indicator Light the LED
• Light Emitting Diode
• Makes other outputs possible

3
LED

• Electrical Properties
• Constant diode drop of 0.7-1.4 volts
• Depends on color (semiconductor) used
• Current Max about 15 ma, Min about 5 ma
• Get light anywhere in that range
• Too much of either
• SMOKE!!!

4
Current Limiting Resistor

• Assume 5 volt supply
• Resistance, Ohm Potential/current
• But there is no 500 Ohm Resistor
• Use 470 Ohm (Closest to 500)

5
Wrong Current Limit?

• With 5 volt supply and 1 volt diode drop
• R 220 I V/R (5-1)/220 18.2 mA
• R 470 I V/R (5-1)/470 8.5 mA
• R 690 I V/R (5-1)/690 5.8 mA
• Either 220 or 470 are commonly used

6
The Board of Education

• Has connections directly to BS2 pins
• No current protection for I/O pins
• Too much current (over 20-25 mA) burns out I/O
  pin
• Be Careful
• The BS2 you save may be yours

7
The Homework Board

• Built-in 220 Ohm current limiters
• Why???
• To limit max current (short) on any pin
• I V/R 5 volt/220 Ohm 23 mA
• This protects the BS2 components from an
  inadvertent short
• When can this happen?
• Lets talk I/O pins

8
The Homework Board

• 220 Ohm resistors protect I/O pins

Serial Connector
9 volt Battery Connector
Breadboard
Reset Switch
220 Ohm Resistors(16)
I/O pin current limited to 5 volt/220 Ohm 23 mA
9
Basic Stamp I/O Pins

• Two Operating Conditions
• Input or High Impedance
• Used to sense level
• Near 5 volts (above 3.5 volts) True, 1
• Near 0 volts (below 1.5 volts) False, 0
• Input impedance is about 10M Ohm (very high)
• I 5 volts/10x106 Ohm 5 10x10-7 Amp (very
  small)
• Output or Low Impedance
• Used to set level 1 5 volts, 0 0 volts
• No change in potential with current
• UNTIL you overload pin

10
BS2 I/O pin Overload

• Set I/O pin to output, then
• Pin 0 connected to 5 volts gt OVERLOAD
• Pin 1 connected to 0 volts gt OVERLOAD
• In either case, I/O pin is competing with power
  supply and one will lose
• Usually the I/O pin
• Power supply 2 A, pin 25 mA max

11
Turning LEDs On/Off

• Two Methods
• Active High (what the book does)

LED goes onwhen P14 is High
12
Turning LEDs On/Off

• Active Low (Often Recommended)

LED goes onwhen P14 is Low
Many microcontroller pins work best as sinks
13
Bi-Color LED

• A Red and Green LED packaged as one
• Pin 1 , Pin 2 - yields Red
• Pin 1 -, Pin 2 yields Green

Look for the clear LED in your package
14
Basic Stamp Commands

• HIGH pin
• sets a pin to output and 5v
• LOW pin
• sets a pin to output and 0v
• PAUSE count
• pauses count milliseconds
• FOR NEXT
• Allows for a fixed number of repeats
• DO LOOP (new in PBASIC 2.5)
• Allows for an infinite number of repeats

15
Solid State Relays

• Your ticket to real POWER
• 3.5volt _at_ 2 mA ON gives 0-30 volts _at_40 A