Presentation on theme: "ECE101 Section 5 “Gadget Lab” Lecture 2: Light and Motion Dr. Cindy Harnett ECE Dept."— Presentation transcript:
ECE101 Section 5 “Gadget Lab” Lecture 2: Light and Motion Dr. Cindy Harnett ECE Dept.
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.phphttp://www.lomont.org/Projects /LEDCube/LEDCube.php Kits available at
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/
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
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)
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 degree cone (typical)
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, lumens for ordinary LED
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.
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.
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 R Green LED Battery Cap
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 Magnet Energy Density
GES101 Motor had one coil
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
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) Motor Type 1 Motor Type 2 Motor Type 3 Voltage
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.
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 V = 0.8V onto the motor. Motor Circuit 6V battery pack 4 identical resistors ( 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
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)