ME456: Mechatronics Systems Design

Lecture 3Chapter 2:

Lights On –Lights Off

Prof. Clark J. Radcliffe

Mechanical Engineering

Michigan State University

http://www.egr.msu.edu/classes/me456/radcliff

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

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!!!

Current Limiting Resistor

• Assume 5 volt supply– Resistance, Ohm = Potential/current

• But there is no 500 Ohm Resistor …– Use 470 Ohm (Closest to 500)

310*105105 milliampvoltIVR

OhmIVR 500

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

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…

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?– Let’s talk I/O pins

The Homework Board

220 Ohm resistors protect I/O pins

220 Ohm Resistors

(16)

Breadboard

Serial Connector

9 volt Battery Connector

Reset Switch

I/O pin current limited to 5 volt/220 Ohm = 23 mA

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

BS2 I/O pin Overload

• Set I/O pin to “output”, then– Pin = 0 connected to 5 volts => OVERLOAD– Pin = 1 connected to 0 volts => 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

Turning LEDs On/Off

• Two Methods…

• Active High (what the book does)

LED goes onwhen P14 is High

Turning LED’s On/Off

• Active Low (Often Recommended)

LED goes onwhen P14 is Low

Many microcontroller pins work best as sinks

470

Vdd (5v)

P14 (0v)

+

-

Current 470

Bi-Color LED

• A Red and Green LED packaged as onePin 1 = “+”, Pin 2 = “-” yields Red

Pin 1 = “-”, Pin 2 = “+” yields Green

Look for the clear LED… in your package

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

Solid State Relays

• Your ticket to real POWER– 3.5volt @ 2 mA “ON” gives 0-30 volts @40 A

I/O pin

+5v Vdd LoadAC or DC

Power