Using LEDs

LEDs are amazing

LEDs are fussy

700 mV drop

3.6 V drop

Incandescent lights vs. LEDs

Characteristic Incandescent bulbs LEDs

Lifespan Hundreds of hours Thousands of hours

Efficiency 90% of power goes to heat Much more efficient (4X)

Heat Hot Cooler (but smaller)

Dimming Simple Complex

Circuit complexity Simple Complex

Omnidirectional Yes No

Easily diffused Yes No

Colors With filters Inherent color

Color

Red Introduced in 1960sVery efficient

Green Introduced in 1980sEyes most sensitive to green

Blue Introduced in 1990sVery efficientBasis for green and white

White Based on blue LEDUses phosphorsCool white to warm white

Measuring Light

Unit Description

Watt Radiant flux/power Real power

Lumen Luminous flux As seen by humans

Candela Luminous intensity Directional

Lumen / watt Luminous efficacy Overall efficiency

Driving LEDs

Polarity matters

+ -

LEDs have a “reverse breakdown voltage” of ~5V

Ohm’s Law

I = V/RCurrent is voltage divided by resistance

Ohm’s law works for resistors

Current is voltage divided by resistance

For a 5V supply with a 100 ohm resistor:

I = 5 / 100I = 0.05 amps

Ohm’s law works for resistors

Current is voltage divided by resistance

For a 9V supply with a 100 ohm resistor:

I = 9 / 100I = 0.09 amps

Ohm’s law doesn’t model LEDs

Small changes in voltage result in large changes in current

Powering an LED with 2.8 V requires 0.085 A of current

Ohm’s law doesn’t model LEDs

Small changes in voltage result in large changes in current

Powering an LED with 3.05 V requires 0.212 A of current

LED is destroyed

“Forward” voltage

Color VF typical

VF max

Red 1.9 2.2

Green 3.8 4.5

Blue 2.7 4.2

White 2.7 4.2

OK, how to control current?

Inline resistor

Constant-current driver

Inline resistor

Resistor regulates the current (I = V/R)

Picking the right resistance

R = (VS – VL) / IR = Resistor voltage in ohmsVS = Supply voltageVL = Forward voltage of LEDI = Rated current for LED

5V power driving red LED

R = (5 – 1.9) / 0.025

Round up to nearest resistance – 150 ohms

VS = 5 VVL = 1.9 VI = 0.025 A

R = 124 ohms

Dimming LEDs

Can’t vary voltage

Changing current has some effect

Switch on and off really fast - PWM

Pulse-width modulation

Constant-current PWM

Three channels for RGBhttp://www.sparkfun.com/datasheets/Components/LED/

WS2801.pdf

SMD 5050 RGB LEDs used on strips

WS 2801 based individually addressable LED strip - http://www.sparkfun.com/products/

10312Or on eBay

WS2803 18 Channels! - can be used for PCB-based arrays

http://www.ebay.com/sch/i.html?_trksid=p5197.m570.l1313&_nkw=ws2803&_sacat=0

• Sparkfun – All kinds of LEDs, strips, support boards. Good documentation and commentary. Great for new discoveries.

• Evil Mad Science – Good set of individual LEDs, connectors, multiplexing kits. Owners are very knowledgeable and helpful.

• Adafruit – Whole LED section. Good documentation and support.

• Macetech – Specializes in LEDs.

• Jameco – Good place for high-current power supplies

Resources

robertcook@gmail.com

Suite 213 in this building

Thanks