Hardware Reference
In-Depth Information
Physical
Type of
parameter
sensor
Notes
Linear variable
Miniature transformer with split secondary windings and moving core
differential
attached to a plunger. Requires AC excitation and phase-sensitive
transformer
detector. Analogue input port required.
(LVDT)
Magnetic linear
Magnetic pick-up responds to movement of a toothed ferrous track.
position sensor
Pulses are counted as the sensor moves along the track (typically
using an operational amplifier and Schmitt input logic gates) but
some sensors contain circuitry in order to produce TTL-compatible
outputs. The pulse repetition frequency of the output is proportional
to the linear velocity. Digital input port required.
Light
Photocell
Voltage-generating device. The analogue output voltage produced
level
is proportional to light level. Analogue input port required.
Light
An analogue output voltage results from a change of resistance
dependent
within a cadmium sulphide (CdS) sensing element. Usually connected
resistor
as part of a potential divider or bridge. Analogue input port required
(LDR)*
(alternatively a comparator arrangement can be used for threshold
switching). Maximum sensitivity falls within the visible spectrum.
Photodiode*
Two-terminal device connected as a current source. An analogue output
voltage is developed across a series resistor of appropriate value.
Analogue input port required (alternatively a comparator arrangement
can be used for threshold switching). Maximum sensitivity usually
occurs within the infra-red range (i.e. outside the visible spectrum).
Phototransistor*
Three-terminal device connected as a current source. An analogue
output voltage is developed across a series resistor of appropriate
value. Analogue input port required. Phototransistors are also
available in the form of light-activated switches which provide
TTL-compatible outputs (in which case a digital input port must be
used). Maximum sensitivity usually falls within the infra-red spectrum.
Liquid
Float switch*
Simple switch element that operates when a particular level
level
is detected. Digital input port required.
Capacitive
Switching device that operates when a particular level is detected.
proximity
Ineffective with some liquids. Digital input port required.
switch*
Diffuse scan
Switching device that operates when a particular level is detected.
proximity
Ineffective with some liquids. Digital input port required.
switch*
Pressure
Microswitch
Microswitch fitted with actuator mechanism and range setting springs.
pressure sensor
Suitable for high-pressure applications. Digital input port required.
Differential
Microswitch with actuator driven by a diaphragm. May be used to
pressure
sense differential pressure. Alternatively, one chamber may be
vacuum
evacuated and the sensed pressure applied to a second input.
switch
Digital input port required.
(
continued
)
