ME456: Mechatronics Systems Design Lecture 3 Chapter 2: Lights On –Lights Off Prof. Clark J. Radcliffe Mechanical Engineering Michigan State University

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 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)

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 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/10x10 6 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 on when P14 is High

Turning LED’s On/Off Active Low (Often Recommended) LED goes on when P14 is Low Many microcontroller pins work best as sinks Vdd (5v) P14 (0v) + - Current

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

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 2 mA “ON” gives 0-30 A I/O pin +5v Vdd Load AC or DC Power