ELECTRONICS PRIMER

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ELECTRONICS PRIMER
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Assignment WEB-based Electronics
Tutorial Basic definitions  Components  O
hm's Law  LEDs and Transistors  Additional
electronics tutorials
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Basic Electronics
Current (I) Amount of charge passing a given
point per unit time Voltage (V) Electrical
pressure or force. If we compare current to
water flowing through a pipe then voltage is the
the water pressure. Resistance (R) Conductors
are not perfect. They resist the flow of
current.
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DC
An electrical current can flow in either of two
directions. If it flows in only one direction
(whether steadily of in pulses), it is called
direct current (DC). A battery is an example of
a device that can supply DC current!
Electrical engineers also use the term DC to
refer to an average voltage or current.
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AC
A current which alternates in direction or
polarity is called an alternating current (AC).
The current flowing from a wall outlet is an
example of an AC current!
DC voltage, RMS Voltage, Frequency, Period
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Resistors
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Ohms Law
V I R !!!!! V I Z !!!!!
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Kirchoffs Voltage Law
Summation of voltages around any closed loop is 0.
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Kirchoffs Current Law
Summation of currents into a node must equal 0.
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Voltage Divider
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Resistor Color Code
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Capacitors
There are many kinds of capacitors but they all
do the same thing store electrons. The simplest
kind of capacitor is two conductors separated by
an insulating material.
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Difference Between R and C
Like resistors, capacitors can impede the flow
of current. Unlike resistors, which resist the
flow of both DC and AC currents in exactly the
same way, capacitors can be used to COMPLETELY
BLOCK the flow of DC currents. As the
frequency of the alternations associated with the
flow of AC currents, capacitors impede the flow
of current to a lesser degree!
Low Frequency
High Frequency
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Inductors (Coils)
Inductors are formed by taking a wire and
wrapping it as a coil. Like resistors, inductors
can impede the flow of current. Inductors,
however, resist rapid changes in the current
flowing through them while freely passing DC
currents. When current is passed through the
coil, an electromagnetic field encircles it. The
coil can act like a magnet!
High Frequency
Low Frequency
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Diodes
A diode is like and electronic one-way valve.
It will allow current to flow in only one
direction!
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Transistors
Transistors are three terminal devices. A very
small current or voltage at one terminal can
control a much larger current flowing between the
other two leads.
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Operational Amplfier
Operational Amplifiers take small voltages and
make them MUCH larger.

• Golden Rules
• No-current flows into either () or (-) inputs.
• The () and (-) inputs are at the sam potential.

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Signal Conditioning
Electrical engineers use operational amplifiers
(Op Amps), resistors, capacitors, diodes,
transistors, etc. to perform mathematical
operations like Addition/Subtraction Multiplicat
ion/Division Absolute Value Natural
Log Filters
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Amplifier Example
Inverting gain amplifier with a gain of 250 (48
dB)!
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Filter
Bandpass filter.
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Sensor Fundamentals

• How do sensors function?
• Common and useful robotic sensors
• Touch Sensor
• Resistive Position Sensor
• Photocell Light Sensor
• Phototransistor Light Sensor
• Shaft Encoder

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Transducer
A transducer is a device that transforms a
physical quantity into an electrical one or a
device that transforms an electrical quantity
into a physical one. For example A microphone
transforms changes in sound pressure level into
changes in voltage. A condenser microphone is
one in which a moving diaphragm alters the
distance between two metal plates. This results
in a proportional change in the capacitance of
the plates.
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Another Transducer Example
A speaker transforms changes in voltage into
sound pressure waves.
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Sensor
We will use the term sensor in this class to
denote any device used to sense the robots
environment. A senor is the transducer and any
associated electronics needed to interface the
transducer to the Handy Board. For example, even
though a microphone converts changes in sound
pressure level into changes in voltage, we can
not directly connect a microphone to the Handy
Board. The voltage levels are TOO SMALL. The
microphone output must first be amplified and
perhaps filtered!
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Digital Signals
A digital signal can take on only one of two
voltages 0 Volts and 5 Volts. The Handy Board
treats 0 Volts as logical TRUE and the 5 Volt
signal as logical FALSE.
5 Volts
0 Volts
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A Simple Touch Sensor (Digital)
Mechanical switches permit or interrrupt the flow
of current.
WARNING Mechanical switches BOUNCE!!!!!
A few milliseconds.
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Simple Position Sensor (Analog)
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Analog Signals
An analog voltage can take on any value between 0
and 5 Volts. An Analog-to-Digital Converter
(ADC) within the Handy Board will, however,
quantize the analog signal. The HandyBoard ADC
is 8 bits wide.
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Quantization
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Sampling Theorm
In order to avoid a non-linear phenomenon known
as aliasing, an electrical signal must be sampled
at a rate of at least TWICE the highest frequency
component present in the signal.
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Bandlimiting
Once a sampling rate has been determined the
input must be bandlimited. This means that the
incoming electrical signal is filtered so that
all frequency components above one-half the
sampling frequency are removed! Filtering not
only prevents aliasing but also can be used to
remove unwanted noise.
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Noise
Filtering not only prevents aliasing but also can
be used to remove noise. All electronics circuits
generate small, random electrical currents or
voltages. Noise can also enter electronic
circuits by means of electromagnetic waves
generated by things such as electric motors,
radio stations, electric outlets. The HandyBoard
digital circuits also serve as a noise source
which may corrupt your sensor signals.
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Handy Board Reference
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DC Motors
Electrically, a DC motor is modeled by an
inductor. When current flows through the motor
coils mounted on a rotating shaft (armature) , a
magnetic field is created. This field reacts
with permanent magnets positioned around the
coils. The fields push against one another and
the armature turns.
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Electronic Control

• H-Bridge Motor Driver Circuit
• Four transistors form the vertical legs of the
  H, while the motor forms the crossbar
• In order to operate the motor, a diagonally
  opposite pair of transistors must be enabled
• Transistors Q1 and Q4 enabled
• Starting with the positive power terminal,
  current flows down through Q1, through the motor
  from left to right, down Q4, and to the negative
  power terminal
• Results in motor rotating in a clockwise
  direction
• Transistors Q2 and Q3 enabled
• Results in current flowing through the motor
  from right to left

Q1 and Q4 enabled
Q2 and Q3 enabled
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Electronic Control

• Enable and Direction Logic
• Critical that transistors in either vertical leg
  of H are never turned on at same time
• If Q1 and Q2 were turned on together, current
  would flow straight down through the two
  transistors
• There would be no load in this circuit other
  than the transistors themselves, so the maximal
  amount of current possible for the circuit would
  flow, limited only by the power supply itself or
  when the transistors self-destructed
• Actual circuit has hardware to facilitate
  control of transistor switches
• Add four AND gates and two inverters
• AND gates accept enable signal that allows one
  signal to turn whole circuit on/off
• Inverters ensure that only one transistor in
  each vertical leg of the H is enabled at any one
  time

DIRL 0, DIRR1, enable signal 1 Q1 and Q4
turn on, and current flows through the motor from
left to right DIRL 1, DIRR0, enable signal
1 Q2 and Q3 turn on, and current flows through
the motor from in the reverse direction
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Electronic Control

• Active Braking
• What happens if both direction bits are the same
  state, and the enable bit is turned on?
• Effectively, both terminals of motor are
  connected together
• Motor acts as a generator, creating electricity
• If there is a load connected to the motor, then
  the motor resists being turned proportional to
  the amount of the load
• When the motor terminals are grounded through
  the transistors, it is as if the motor were
  driving an infinite load
• Transistors in the H-bridge act as a wire
  connecting the motor terminals the infinite load
• Final result circuit acts to actively brake the
  motors spin transistors absorb the energy
  generated by the motor and cause it to stop. If,
  on the other hand, none of the transistors is
  active, then the motor is allowed to spin freely
  i.e., to coast

Both direction bits are one and the enable bit is
turned on causing transistors Q2 and Q4 to be
activated. This causes both terminals of the
motor to be tied to the voltage supply less the
voltage drop of the transistor (0.6v).
Contemporary electric car designs incorporate
circuitry to convert the the drive motor into a
generator for recharging the main batteries when
braking. This way, the power stored in the cars
motion is recovered back into electrical energy.
The active braking doesnt apply enough force to
replace conventional brakes, but it can
significantly extend the electrical cars
operating range.
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Electronic Control

• Speed Control
• Pulse Width Modulation (PWM)
• The H-bridge circuit allows control of a motors
  speed simply by turning the drive transistor pair
  on and off rapidly
• Duty cycleproportion between on time and
  off timedetermines fractional amount of full
  power delivered to motor
• Commonly used in practice simpler to build
  circuits that switch transistors on and off than
  to supply varying voltages at the currents
  necessary to drive motors
• Tends to be fairly linear (25 duty cycle yields
  pretty close to one-quarter of full power)
• Reducing the voltage applied to the motor
• Giving a motor 1/4 of its normal operating
  voltage typically would result in much less than
  1/4 of nominal power, since the power increases
  approximately as the square of the voltage

PWM works by rapidly turning the motor drive
power on and off. Waveforms shown would be
connected directly to the enable input. Three
sample duty cycles are shown a 75, a 50, and a
25 rate. The frequency used in PWM control is
generally not critical. Over a fairly wide range,
from between 50 Hz and 1000 Hz, the motor acts to
average the power that is applied to it.
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Electronic Control

• HB Implementation
• HB uses two copies of H-bridge driver, either
  SGS-Thomson L293D or TI SN754410 - chips accept
  digital logic signals as input and drive motors
  directly on their outputs
• Each triangular driver replaces one leg, or
  two transistors, in the H-bridge circuits. Each
  driver may be either driven high (enabled and
  input is high), driven low (enabled and input is
  low), or turned off (disabled and input doesnt
  matter).
• To make the motor spin, the enable input must be
  high, and one driver in-put must be high and the
  other low. If the enable is high, and both driver
  inputs are high or both are low, then the circuit
  actively brakes the motor. If the enable is low,
  then the motor is allowed to coast.
• Rather than individually control IN1 and IN2,
  the Handy Board adds an inverter so that a single
  bit may be used to determine motor direction.
  When the direction input is high, then IN2 is
  high and IN1 is low. When the direction is low,
  IN2 is low and IN1 is high.
• The full Handy Board circuit uses a 8bit latch,
  the 74HC374 chip, which provides the eight bits
  necessary to control four motors.

One-Half of L293D/SN754410 Motor Driver Chip
Handy Board H-Bridge Circuit
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Electronic Control

• Spike-Canceling Diodes
• Also part of the motor driver chips are four
  diodes connecting from each driver output to
  either Vs , the motor voltage supply, or ground.
  These diodes perform the important function of
  trapping and shunting away inductive voltage
  spikes that naturally occur as part of any
  motors operation.
• Diodes allow current to flow in one direction
  only. If there is a higher voltage on the anode
  than on the cathode, then current flows through
  the diode
• The diodes in the motor driver chip may appear
  to be connected backward, but they are drawn
  correctly. When a motor is running, the coil of
  wire in its armature acts as an inductor, and
  when the electricity in this coil changes,
  voltage spikes are generated that might be of
  higher voltage than the Vs power supply or lower
  voltage than ground.

Diode current flows from higher voltages on the
anode to lower voltages on the cathode, in the
direction of the diodes arrowhead.
Example suppose a voltage greater than Vs is
generated by the motor on the OUT1 line. Then
the diode labeled D1 conducts, shunting this
voltage to the Vs power supply. If the diodes
were not present, these inductive voltage spikes
would enter the voltage supply of the rest of the
project circuitry, possibly doing damage to more
sensitive components.
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Motor Driver IC