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Smart Lighting

Smart Lighting. Lab Module 3: Driving LEDs Instructor: Sara Grier. What is an LED?. Light Emitting Diode Semiconductor light source Diode – conducts current in only one direction; forward biased Light emitted at characteristic wavelength/color Color determined by chemistry of diode

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Smart Lighting

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  1. Smart Lighting Lab Module 3: Driving LEDs Instructor: Sara Grier

  2. What is an LED? • Light Emitting Diode • Semiconductor light source • Diode – conducts current in only one direction; forward biased • Light emitted at characteristic wavelength/color • Color determined by chemistry of diode • Come in various packages • Can be assembled into arrays

  3. Schematic of an LED 1 = Cathode 2 = Anode

  4. Structure of an LED

  5. Diode Junction – “I-V curve”

  6. Intensity vs. Wavelength

  7. Schematics vs. Pictures

  8. Useful Circuit Equations • V=IR • V  Voltage • I  Current • R  Resistance • P = IV • P = V2/R • P = I2R • P  Power

  9. Duty Cycle and Power • Duty cycle – On and Off ; Low duty cycle  Low Power • D = t/T • t duration function is high • T  period of the function • D  duty cycle • Pavg/Po = t/T • Po  Peak Power • Pavg  Average Power

  10. Lab Procedure • Team Exercise 1 • Build circuit to display a current-voltage characteristic for one of our LEDs • Setup the circuit as shown in schematic, connecting wires to the proper pins on the Rhett Board • Open the ‘Function Generator’ tab • Set channel 1 to a 1kHz sine wave with 7V p-p and 0V DC offset (the LED should come on) • Open the ‘Oscilloscope’ tab • Channel X Settings: • Volts/Div: 2V • Coupling: DC • Input: A1 SE • Channel Y Settings: • Volts/Div: 2V • Coupling: DC • Input: A2 SE • Horizontal Settings • Time/Div: 1ms • Mode: X-Y • What is Vf? • Record a screenshot • What are the characteristics of the LED we are using? • How about the white LED?

  11. Team Exercise 2 • Design circuit to power LED with DC input from fixed voltage (resistor in series) • Identify the anode and cathode of the LED. • What voltage and resistance do we need to achieve 30mA? • For the specs of the LEDs we use and a target power level of < 85mW . • What resistor will yield this LED power level.? • Build it. • How much power is wasted in this scheme?

  12. Team Exercise 3 • Design a circuit to power a LED with PWM • The Rhett Board current limits at 50mA. Power can be limited using a duty cycle. • What duty cycle can yield a power level of 17mW? • Implement your scheme using the PMW1 control.

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