gadget lab lecture 5: sensors and interfacing… components… more project time dr. cindy harnett...
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Gadget Lab Lecture 5:Sensors and Interfacing…
Components…More Project Time
Dr. Cindy Harnett
ECE Dept., U of Louisville
Spring 2008
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Sensor projects are abundantEspecially low power wireless sensors
Example of wireless sensor deployment: UMASS BOSTON Center for Coastal Environmental Sensing Networks
Example of a wireless sensor computer interface(Crossbow)
Need to get an electrical signal from a sensor
Most common: Resistive sensors. Measurement -> Resistance Change ->Voltage change. Pressure, sound, temperature, acceleration
Capacitive sensors: touchpad, touchscreen. Typically look for a shift in resonant frequency in a “LC resonator” circuit to determine C.
Inductive sensors: A coil experiences a changing magnetic flux and captures some of the energy as an induced current. Encompasses RFID, antennas and even magnetic resonance imaging (MRI).
Optical sensors: remote control receiver, optical mouse. Detect the current when light interacts with electrons in a photosensitive material
More exotic types: high energy particle detectors, field-effect sensors, quantum sensors, scanning tunneling microscope.
We have an electrical signal, now what?
Record the electrical signal on a computer
Or react to the signal in “real time” (Segway or car airbag for example).
Usually must apply “signal conditioning” to the raw signal:
Amplify a current from nA to mA, or voltage from mV to V
Measure the frequency of a periodic signal
Digitize an analog signal
Debounce a flickering signal such as a button press
And similar operations
For common signal conditioning problems, your work is done.
•“Operational Amplifier” multiplies a voltage. Typically ~10x depending on configuration•Some instrumentation amplifier ICs turn current to a proportional voltage.
•A “comparator” sends out a digital pulse when one voltage passes another.
•“Frequency counters” and analog-to-digital converters are usually a combination of a timer, a comparator and a computer memory (more)
•Debouncer: the “Schmitt Trigger” is like a comparator with two different thresholds (more)
Gain = (1+ RB/RA).
The Schmitt Trigger• Available as an IC similar to the inverter in the Firefly
project• There’s an “up” threshold that’s higher than the “down” threshold
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/schmitt.html
• This prevents the circuit from switching rapidly on and off when the input voltage is near a threshold
The analog-to-digital converter• Feed a voltage signal into a
bank of comparators with different reference voltages
Cheaper alternative: compare the analog voltage with a ramped voltage, and count the number of milliseconds until the ramped voltage surpasses the analog voltage.
The millisecond count is a “digital” representation.
http://www.hardwaresecrets.com/article/317/6 http://www.allaboutcircuits.com/vol_4/chpt_13/8.html
Computer Interfacing
LabView PCI or PCMCIA cards and connector board very common in research labshttp://www.ni.com
Lower cost: “Phidgets” have analog inputs and USB to PChttp://www.phidgets.com
Even cheaper, dedicated circuits such as the 1-Wire DS2450 converterhttp://www.maxim-ic.com
And many more…
Demos based on lab projects at U of L: 1-Wire and wireless sensor interfacing
“1-wire” chips allow multiple sensors to be connected to the same wire for weatherstations and similar devices.
T(hermometer Demo)
http://www.maxim-ic.com/products/1-wire/
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Wireless sensor board can poll multiple sensors on the 1-wire bus. (Flow sensor demo)
Component identification quiz
• Low stress (no grade but there’s a prize)
• Circle answers on paper
• In the event of a tie, we will have a drawing.
Component Identification Quiz
Question 1:what is this?
A. A 1 K resistor
B. A 3.7 M resistor
C. A battery
Component Identification Quiz
Question 2:what is this?
A. A ceramic capacitor
B. A transistor
C. An electrolytic capacitor
Component Identification Quiz
Question 3:what is this?
A. An electrolytic capacitor
B. A ceramic capacitor
C. A light-emitting diode
Component Identification Quiz
Question 4:what is this?
A. A transistorB. A voltage regulatorC. Could be either A or B
Component Identification Quiz
Question 5:what are these?
A. Crystal oscillatorsB. Light emitting diodesC. Capacitors
Component Identification Quiz
Question 6:what is this?
A. A potentiometerB. An inductorC. A photocell
Component Identification Quiz
Question 7:what is this?
A. An integrated circuitB. A 555 timerC. Both A and B
Component Identification Quiz
Question 8:what are these?
A. InductorsB. Crystal oscillatorsC. Capacitors
Component Identification Quiz
Question 10:what is this?
A. A crystal oscillatorB. A bouillon cubeC. A DC-DC converter
•Component Identification Quiz done!
Work on the project of your choice
Using solderless breadboard first, then soldering if possible, is a good idea.
“Troubleshooting” tips:
Use resistance meter to check all pins are really connected how you want
Compare to a known-good circuit.
Most parts are symmetrical, but sometimes polarity is important. Is it plugged in backwards/upside down/not at all?
Project Time