Lecture 3

• how do you choose the right sensor

  1. decide which parameters are important for our application - temperature, humidity, pressure, light
  2. determine what kind of sensor is optimal for measuring that parameter
• what are the different types of sensors

  • analog - continuous output, requires conversion to digital for computer interface
  • digital
  • passive - needs separate power source to obtain the output
  • active - self-generating - produces electrical signal when subjected to senses quality (radioactive)
• what are the two states a digital sensor can be in?

  ON or OFF - often 5V or 0V. Most work with a threshold. if the measurement is below the threshold if will output OFF, if it is above the threshold it will output ON
• which type of sensor is easier to work with, digital or analog?

  digital, since it can only have two different states
• which sensor do you receive more information from, digital or analog?

  analog
• what is often the output of an analog sensor

  a variable resistance that can be used to control a voltage
• what does a temperature sensor do

  it outputs a voltage directly proportional to temperature
• how does a touch sensor work

  it changes value from max to 0, when touched. it works through 1/4 inch of glass
• how does a force sensor work

  when it has no force applied, this sensor will read 0. As force increases on the button, the value increases. It is not accurate enough to be used as a weight measurement device and is not designed to have force applied constantly over time
• what is the Piezoelectric effect

  an effect in which energy is converted between mechanical and electrical forms
• what does piezo mean

  it's greek for pressure
• what are the uses of the Piezoelectric effect?

  • it can measure force, flexure, acceleration, heat, and acoustic vibrations
  • Piezoelectric transducers can be used both as as speakers (voltage to mechanical) and microphones (mechanical to electrical)
• what are the methods to measure force

  • acceleration methods - measure the acceleration of a known mass on which the unknown force operates
  • gravity-balance methods - compare the unknown force with the action of the gravitational force on a known mass
  • pressure-sensing methods - convert the force to a fluid pressure, which is measured using a pressure transducer
• what are the different types of sensors commonly used to measure temperature

  • thermal resistors
  • thermocouples
• what are thermocouples and how do they work?

  they are the most popular temperature sensors and they rely on the Seebeck Effect in a junction of two different types of metals
• what are the main strengths and limitations of thermocouples

  • they are cheap, interchangeable, have standard connectors, and can measure a wide range of temperatures
  • the main limitation is accuracy, difficult to achieve system error of less than 1 degree Celsius
• what is the Seebeck Effect


  the junction between two metals generates a voltage which is a function of temperature.
• what are the two categories of light detectors

  • quantum detectors - convert incoming radiation directly into an electron in a semiconductor device and process the resulting current with electronic circuitry
  • thermal detectors - simply absorb the energy and operate by measuring the change in temperature with a thermometer
• What is a sensor

  A device which converts a physical phenomena into an electrical signal