Gadget Lab Lecture 5: Sensors and Interfacing… Components… More Project Time Dr. Cindy Harnett ECE Dept., U of Louisville Spring 2008

Sensor projects are abundant Especially 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+ R B /R A ).

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 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.

Computer Interfacing  LabView PCI or PCMCIA cards and connector board very common in research labs  Lower cost: “Phidgets” have analog inputs and USB to PC  Even cheaper, dedicated circuits such as the 1-Wire DS2450 converter 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) 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 transistor B. A voltage regulator C. Could be either A or B

Component Identification Quiz Question 5: what are these? A. Crystal oscillators B. Light emitting diodes C. Capacitors

Component Identification Quiz Question 6: what is this? A. A potentiometer B. An inductor C. A photocell

Component Identification Quiz Question 7: what is this? A. An integrated circuit B. A 555 timer C. Both A and B

Component Identification Quiz Question 8: what are these? A. Inductors B. Crystal oscillators C. Capacitors

Component Identification Quiz Question 9: what are these? A.Resistors B. Diodes C. Capacitors

Component Identification Quiz Question 10: what is this? A.A crystal oscillator B.A bouillon cube C. 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