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Transcript
Sensors and Electricity What is a Sensor? A sensor is a device that: 1) 2) Measures a physical quantity Converts this measurement into a readable signal Sensors can be found in many everyday places – for example: home, car, school Some Important Sensor Properties Accuracy – Does the sensor give the right value? Calibration- The process of adjusting a sensor’s output to correct it’s accuracy Sensitivity- How much does the sensor output change as the quantity it is measuring changes? Ideally, a sensor will be linear Sensor Output Linearity Slope = Sensitivity Measured Property More Sensor Properties Repeatability- Is the sensor reading consistent from measurement to measurement? Range- What values can the sensor detect Speed Cost Transducer A transducer converts energy (either a physical quantity or signal) into another physical form. Some examples: An LED (light emitting diode)- Converts electricity into light A microphone- Converts air pressure into electricity A weather vane- Converts wind direction into position Many transducers use electricity What Is Electricity? A form of energy made up of moving electrons that can produce light, heat, or motion Electrons Two types of charges: Positive and Negative In an atom, positive charge is confined to the nucleus, while electrons are located away from the nucleus Sometimes electrons can be transferred from one atom to another Electron (- charge) Nucleus (+ charge) Conductors and Insulators Conductor Semimetal Insulator Any material (semiconductor) that allows Material whose electrons to conduction move through properties it change with Examples: conditions Metals (esp. Examples: copper) Silicon Your body Some ceramics Sea water Materials that do not allow electric charges to flow freely through them Examples: Wood Glass Rubber Voltage Voltage = Electric Potential Difference Involves electrons Has the ability to make things happen Measured between two points -Voltages are measure in Volts (V) -Electronics can read voltages and send them to a computer Battery In a battery, a chemical reaction removes electrons from the positive terminal (anode) and brings them to the negative terminal (cathode) Cathode Circuit Symbol: Electrons A voltage forms between the two terminals Cathode +++ + + - - - - Anode Circuits Circuits are paths along which electricity moves Electrons Voltage creates current, which provides energy for devices + - Current is measured in Amperes (A) Electrons Circuit in a Flashlight Resistance Resistance is a measure of how much a device in a circuit resists current. It is measured in ohms (W) Wires have zero resistance A resistor is a device whose only function is to provide a specific resistance. Ohm’s Law V=I*R Voltage Current Resistance The voltage drop across a device is equal to the current through the device times the resistance of the device Ohm’s Law –Forms V=I*R V R= I V I= R Ohms Law - Units V=I*R V=A*W VOLTAGE Volts milliAmpere: kiloOhm: CURRENT Amperes RESISTANCE Ohms 1 mA = 0.001 A 1 kW = 1000 W Resistance in a Circuit 2W 10 V V 10 V I= = =5A 2W R Resistors in Series When two resistors are connected in series, it can be treated as one resistor, where the resistances are added together. 5 kW 3 kW Total resistance = 8 kW Temperature Sensors Thermometer Volume of a liquid changes with temperature Bimetallic strip Angle changes with temperature Thermistor Thermistor Resistance A thermistor is a resistor with a resistance that is strongly dependent on temperature 0 20 40 60 80 Celsius Temperature 100 Finally, a Question: How can you use a thermistor to build a temperature sensor?