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ECE101 Section 5 “Gadget Lab” Lecture 2: Light and Motion Dr. Cindy Harnett ECE Dept.

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Presentation on theme: "ECE101 Section 5 “Gadget Lab” Lecture 2: Light and Motion Dr. Cindy Harnett ECE Dept."— Presentation transcript:

1 ECE101 Section 5 “Gadget Lab” Lecture 2: Light and Motion Dr. Cindy Harnett ECE Dept.

2 Light as a circuit output Semiconductor LEDs most common light source Typical: 1.5-3V voltage, 20 mA current. Other options: incandescent lamps, electroluminescent lamps, laser diodes, organic LEDs, vacuum fluorescent displays, backlit LCD panels, cold cathode fluorescent lamps, plasma and more Tiny, multicolor surface mount light- emitting diodes (LEDs) at odot---wrist-watch-LED-pattern- timepiece/ Ordinary LEDs are light-sensitive as well as light-emitting! They can be used as proximity sensors that detect reflected light. ouch.mpg Multicolor 3-D LEDCube /LEDCube/LEDCube.php /LEDCube/LEDCube.php Kits available at

3 Light as a circuit output: More examples TV-B-Gone: invisible infrared LEDs combined with a microcontroller to communicate with most TVs Persistence-of-Vision devices create a large display from a line of LEDs Firefly project/ Electroluminescent Lamp project let you experiment with light LED-Bike-Wheel-Images/

4 Motion DC Motors: most common motion source Even tiny “pager motors” require 100 mA. Typically 1-12V. Other options: solenoids, combustion motors, steam engines, “muscle wire,” electroactive polymers, thermal expansion actuators, piezoelectric actuators, capacitively driven micromotors, hydraulic actuators using air or liquid pumps, and more Tiny DC motors vibrate cell-phones and pagers, and focus digital cameras A hard drive motor plus timing circuit drives a “persistence of vision” clock “Stepper” motors offer precise control over rotor angle for robotics and other applications

5 LED/Motor Safety LEDs can be “painfully bright.” Laser diodes amplify the light to “blinding” levels. Don’t stare into ultraviolet LEDs Use a current-limiting resistor (CLR) with LEDs to avoid smoke. Motors: watch out for fingers. Usually not a problem w/tiny motors. More of a problem when gears are involved When motors stall, things can heat up. Remember GES101 wire motors. When large motor coils turn on suddenly, there can be a voltage spike that can damage circuits. Use a protection diode or “snubber” capacitor to protect the circuit (more later)

6 LED Polarity and Construction LEDs have a flat side AND a short leg at the cathode (negative terminal) Inside the LED is a tiny semiconductor chip that emits light. You can see two small wires connected to the chip. The domed plastic housing focuses the light into a 20-30 degree cone (typical)

7 LED Varieties 3mm, 5mm standard sizes Surface Mount sizes Integrated Resistor type Bicolor/Tricolor LEDs Bicolor/Tricolor Flashing, Synchronized LEDs Superbright LEDs 1A, 200 lumens (Luxeon) Vs 20 mA, 0.005 lumens for ordinary LED

8 LED Current and Intensity depend on voltage Red 5, Ultra Red 4, HE Red 6, Orange 7, Bright Red 3, HE Green 9, Yellow 8, Pure Blue C Note sudden turn on at a particular forward voltage for each type of LED. For most LEDs, brightness is proportional to current, up until some point.

9 LED Current-Limiting Resistor Calculation Normal LEDs use ~20 mA current (0.02 A) Datasheets give “diode forward voltage” which depends on color. Typically: IR1.5 V red2.0 Orange 2.0 Yellow 2.1 Green 2.2 true green 3.3 blue3.3 white3.3 UV3.3 blue (430 nm)4.6 Example: (6V-2.2V)/0.02A = 190 ohms; next “standard”value is 220  Resistor value = (V battery -V forward )/0.02 A  Good online calculator at  Or just start w/ large resistor (1K) and reduce value if it’s dim.

10 Build Green LED Circuit, then add a capacitor Different capacitors adjust fade-off speed Watch capacitor polarity (Stripe is on - side) 220 ohm 6V 0V Battery 220 ohm 6V 0V Battery 47 uF- 1 F capacitor + - 220 R Green LED Battery Cap + - - +

11 Motors  Electric motors haven’t changed much lately. Permanent magnets interact with pulsed electromagnetic coils to turn an axle.  Strong, compact rare earth magnets have helped miniaturize motors.  NdFeB magnets discovered in 1983. Magnet Energy Density

12 GES101 Motor had one coil

13 A dissected motor with 3 coils See for schematics Hard to put back together again (sometimes) Magnet wire looks bare but it has a very thin insulation. Housing Brushes Electromagnets on axle: 3 wire coils, 3 pole pieces and 3 contacts Permanent magnets

14 Driving a Small DC Motor Tested three sample motors which produce a few thousand RPM at 1.5V. They whine loudly at 2V and higher voltages. But, we have a 6V battery. Try a voltage divider to generate 1.5 V to feed to the motor. What’s wrong with this circuit? + - R R R R Motor 1.5V 6V 0V Motor Speed vs Voltage Speed (RPM) 0.2 0.5 0.75 1.0 1.25 1.5 2.0 Motor Type 1 Motor Type 2 Motor Type 3 Voltage

15 Driving a Small DC Motor What can we do to drive the motor at 1.5V from a 6V battery? The voltage divider didn’t work because the motor’s resistance is too small. It shorts out R2.  Design new voltage divider based on the motor’s resistance…but note that the resistance changes over a cycle. Probably not a good approach.  Use a 1.5 V battery (but often need 6V elsewhere in the circuit)  Use a 1.5 V voltage regulator - a specialized chip that takes 6v and outputs 1.5V -- this could work OK.  Use “pulse width modulation” -Drive the motor with a pulsed 6V signal that averages out to 1.5 V over time. A very common solution.  Use the voltage regulator with a “buffer chip” that does not pull down the output voltage (An op-amp or transistor). Try this.

16 Build voltage divider on breadboard V out =V in (R2)/(R1+R2) 1.5V=6V(0.25) R1 = 3R2 Connect V out to a transistor: PN2222. The transistor doesn’t draw much current at the center terminal, but it can control a large current. It puts 1.5-0.7V = 0.8V onto the motor. Motor Circuit 6V battery pack 4 identical resistors (100-1000 ohms) Motor PN2222 NPN Transistor: note orientation of flat side + - R R R R b c e Motor 1.5V 6V 0V NPN Transistor 0.8V Transistor gets warm: could use heatsink or bigger transistor

17 Teardowns VCR - Many mechanical parts, a few interesting electromagnetic parts (read/write/erase heads) Solenoid door lock mechanism Next time-electric toothbrush, computer parts Have other teardowns for next time? (no TVs or computer monitors please)

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