PHY C19/20: Electronic Sensors

2 chapters in 1 post? Let’s go!

  • Electronic sensors
  • LDR
  • NTC Thermistors
  • piezo-electric transducer
  • metal-wire strain gauge
  • Potential dividers

What is an electronic sensor?
A device which senses input & provides a signal based on that input.

Generally, electronic sensors consist of:

  • a sensing device
  • a circuit that provides an output that can be registered as a voltage

A common representation of an electronic sensor is as this block diagram:

Now we will dive into the workings of specific sensors.


Light-dependent resistor (LDR)

LDR Symbol
(there are versions without the circle, & those with a zig-zag instead of a rectangle)
Design

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2 intersecting metal grids

Space between grids contains a high-resistance semiconductor (ex: copper-doped cadmium sulfide)
How does it work?High-resistance semiconductor contains electrons which are bound to atoms (not free to move)

When light is incident on the semiconductor, some electrons gain energy & are released – they are free to conduct

As light intensity increases, number of conducting electrons increases

Thus, as light intensity increases, conductivity increases

Thus, as light intensity increases, resistance decreases
CharacteristicsGraph of Resistance against Light Intensity is Logarithmic:

Thermistor

Thermistor Symbol

There are 2 main types of thermistor:

  • Negative temperature coefficient (NTC): as temperature increases, resistance decreases
  • Positive temperature coefficient (PTC): as temperature increases, resistance increases

Here, we only consider negative temperature coefficient thermistors.

Design
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Semiconductor material in various shapes
How does it work?High-resistance semiconductor contains electrons which are bound to atoms (not free to move)

When temperature increases, some electrons gain heat energy (microscopic kinetic energy) & are released – they are free to conduct

As temperature increases, number of conducting electrons increases

Thus, as temperature increases, conductivity increases

Thus, as temperature increases, resistance decreases
CharacteristicsGraph of Resistance against Temperature is non-linear.
Across small temperature ranges, it is approximately an exponential decrease:

Metal wire strain gauge

Design
File:StrainGaugeVisualization.svg
A length of very thin metal wire in a zig-zag pattern sealed between thin sheets of plastic.

Plastic can be stuck onto the surface being measured for strain.
How does it work?When surface is stretched, metal wire becomes narrower & longer

Longer wire & smaller cross-sectional area = higher resistance

When stretched, resistance increasesWhen surface is compressed, metal wire becomes wider & shorter

Shorter wire & larger cross-sectional area = lower resistance

When compressed, resistance decreases

Strain is proportional to the change in resistance
CharacteristicsChange in R is Linear to change in L

Derivation:
R = ρL/A

When stretched by a small length δL, the cross-sectional area does change, but in the calculation it is by a negligible amount. Thus, we assume A remains constant.

The change in R is given by:
R + δR = ρ(L + δL)/A
δR = ρ(L + δL)/A – R
δR = ρδL/A + ρL/A – ρL/A
δR = ρδL/A

Thus, δR ∝ δL

Piezoelectric transducer

Design
Piezoelectric crystal (ex: quartz) with opposite faces coated with metal films acting as electrodes
How does it work?When stress is applied to an uncharged quartz crystal, the positions of positive & negative ions will change

This creates a potential difference across the crystal

If the crystal is made to vibrate by an ultrasound wave, an alternating potential difference is produced across the crystal

This alternating potential difference can be used to generate an alternating current to be sent to a processing unit

See more about ultrasound here.
CharacteristicsGenerates alternating current

How do we use sensors in simple circuits?
By utilising potential dividers.
I won’t explain much about potential dividers on this post, but you can read up about them here.

In general,

  • Potential dividers enable a change in resistance of a sensor to be converted to a change in potential difference
  • In a simple series potential divider:
    ratio of output voltage over total voltage = ratio of output component’s resistance over total resistance

V1/(V1 + V2) = R1/(R1 + R2)

Here are 2 general potential divider circuits:

Output leads are across the sensor

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Output leads are across the fixed resistor  

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One thought on “PHY C19/20: Electronic Sensors

  1. Pingback: PHY C21: Outputs of Op-Amp Circuits – ProDuckThieves

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