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ECE 101
An Introduction to Information
Technology
Digital Sensors
Unit Prefixes
tera
giga
mega
kilo
deci
centi
milli
micro
nano
pico
T
G
M
k
d
c
m

n
p
1012
109
106
103
10-1
10-2
10-3
10-6
10-9
10-12
Information Path
Source of
Information
Information
Display
Digital
Sensor
Information
Processor
& Transmitter
Transmission
Medium
Information
Receiver and
Processor
Electric Circuit Concepts
• Charge, Q or q
• Current, I or i – motion of charge (“through”)
– Direct current
– Alternating current
• Voltage or electric potential, V or v (“across”)
– Motion of charge occurs due to a force pushing on it
– Work per unit charge is voltage
– If current through an element results in an expenditure
of energy, than a voltage or potential drop occurs
– Battery
• Power, P = V*I, or p=v*i
Electric Circuit Laws
• Multiple loads
– Series – same current
– Parallel – same voltage
• Resistance and Ohm’s law, vR=iRR
• Kirchhoff’s Laws
– Voltage law: v1 + v2 + v3 + …. = 0 (sum of
voltages around a loop is zero)
– Current law: i1 + i2 + i3 + …. = 0 (sum of
currents at a node is zero)
Mechanical Switches
• Break the flow of electrical current
• May be activated by the outside
environment, such as, change in switch
position, temperature, acceleration
• May be activated in series
– open: no alarm
– closed: alarm activated
• May be activated in parallel
– open: alarm activated
– closed: no alarm
Mechanical Switches
• Note the matrix arrangement for the
keyboard and the clever scheme to
minimize the number of wires
– Each key is a switch
– Reduce the number of wires by using a matrix
of switches formed by rows and columns of
keys
– Number of wires = number of columns +
number of rows: NW = NC+ NR
– Number of switches, NS = NC* NR
Sensors or Transducers
• Front end of information (electrical)
systems
• Converts physical energy into an electrical
signal
• Produce the data to be transmitted,
processed and/or stored
• Analog (continuous) or digital (example, a
binary switch)
• Mechanical, optical, thermal, electrical
Optical sensors
• Visible [UPC (universal product code) using
lasers)], IR (remote controls)
• Beam interrupt – presence of light may
actuate a switch (often mechanical)
• Digital Data transmission – use of threshold
– Noise and weather (attenuation) not as
significant
– Beam intensity not a factor (good for cellular
telephone)
Optical Proximity Sensors
• Unlike beam interrupt, the the transmitter
and receiver in the same device.
• Reading of Bar Codes, facsimile machines
• Note the clever way of reading the UPC
code at an angle and the operation of an
auto-focus camera. Trigonometry is key
here!
Infrared
(IR)
Range
Sensors
Infrared (IR) Range Sensors
• Uses variation of proximity sensor to
determine the range of an object for setting
the focus of an auto focus camera
transmitter
R
S
x
R/S = f/x
f
receiver
S and f fixed by camera design
Digital IR Range Sensors
Digital IR Range Sensors
• Uses an array of small detector elements no determinations or calculations needed
transmitter
R
S
R/S = f/x
f
receiver
S and f fixed by camera design
Inverse Square Law
• Key law in physics, gravity, light intensity,
Coulomb’s law in E&M…
• I=P/A where A=r2, or I~1/ r2
– Determines the spacing of antennas in cellular
telephone system
– Can be used to locate a transmitting signal
Inverse Square Law
R = Ro
R = Ro
r = ro
r =  ro
I (r = ro) = P/(2r2) = I (r = ro) /2