Gadget Lab Lecture 5: Sensors and Interfacing… Components… More Project Time

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…

http://www.maxim-ic.com/

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



A. A ceramic capacitor

B. A transistor

C. An electrolytic capacitor



A. An electrolytic capacitor

B. A ceramic capacitor

C. A light-emitting diode



A. A transistorB. A voltage regulatorC. Could be either A or B


Question 5:what are these?

A. Crystal oscillatorsB. Light emitting diodesC. Capacitors



A. A potentiometerB. An inductorC. A photocell



A. An integrated circuitB. A 555 timerC. Both A and B



A. InductorsB. Crystal oscillatorsC. Capacitors



A. ResistorsB. DiodesC. Capacitors



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