Survey of Electronics ET 100B
• Overview
• Circuits and Time Constants
• RC and R/L Circuits
• Photo Sensors used with the Boe Bot
• Basic Stamp 2 Input and Output Characteristics
• Testing a simple photo resister Circuit w/ the Boe Bot
• Analog to Digital Converters (ADCs)
• Build a more complex ADC for light levels using a
circuit with a RC time constant and a Boe Bot
Capacitor
• Characteristics
• Two parallel plates separated
by an insulator
• In the neutral state, both plates have
an equal number of free electrons
• When a voltage source
is connected to the
capacitor, electrons
are removed from
Q
one plated and
V
deposited on the other
C
Coulombs
VoltageCap 
Farads
Capacitor
• Characteristics
• No electrons flow through the dielectric
• When the supply is removed
from the capacitor, the
capacitor retains the charge
• The amount of charge that a capacitor can store per volt
across the plates is its capacitance (C).
• The unit of capacitance is the farad (F).
• One farad is the amount of capacitance when one coulomb of
charge is stored with one volt across the plates.
• Most capacitors in electronics work have capacitance values
of F (10 -6 F) or F (10-12 F)
• A capacitor stores energy in the form of an electric field that is
established by the opposite charges on the two plates
Capacitor
• Characteristics
kQ1Q2
F 2
d
W 1 / 2 CV
F =force in Newtons
Q’s are charge in Columbs
D= distance in meters
2
W = Energy in Joules
Capacitance in Farads
Voltage in volts
• A capacitor obeys Coulomb’s Law:
A force exists between two point-source charges that is
directly proportional to the product of the two charges and
inversely proportional to the square of the distance between
the charges.
• Capacitor Ratings
• The voltage rating specifies the
• Maximum dc voltage that can be applied without risk of damage
• a.k.a. breakdown or working voltage
• Determined by the dielectric strength
Capacitor
• Capacitor Ratings
• The voltage rating specifies the
• Maximum dc voltage that can be applied without risk of
damage
• Temperature coefficient indicates the amount and
direction of a change of capacitance with temp
• Positive coefficient means that capacitance increases with
increasing temp, Negative coefficient means capacitance
decreases with increasing temp
• Capacitance
• Directly proportional
to physical size of the
plates
• Specifically plate area
• Inversely proportional to
the distance between the plates
Capacitor
• Capacitance
• The measure of a materials’ ability to establish an
electric field is called the dielectric constant (
)
• Capacitance is directly proportional to the dielectric
constant
2
Ar 0
C
d
A = Area in m
gr = dielectic constent (relative
permittivity)
g0 = Permittivity of a vacuum
D = distance in meters
• Fixed Capacitors
• Stacked-foil mica capacitors are made of alternate
layers of metal foil and thin sheets of mica.
• Silver mica are formed by
stacking mica sheets with silver
electrode material screened
on them
Fixed Capacitors
• Ceramic dielectrics provide very high
dielectric constants, and relatively large
capacitance in a small physical size.
• Capacitance ranges from 1pF to 2.2F.
Electrolytic Capacitors
• Electrolytic capacitors are polarized so
that one plate is positive, and the other
negative.
• They come in capacitance values from
1F to 200,000 F, with voltage ratings to
350 V.
Capacitors in DC Circuits
• A capacitor will charge up when it is
connected to a dc voltage source.
• When a capacitor is fully charged, there
is no current.
• There is no current through the dielectric
of the capacitor because the dielectric is
an insulating material.
• A capacitor blocks constant dc.
RC Time Constant
• The buildup of charge across the plates
occurs in a predictable manner that is
dependent on the capacitance and the
resistance in a circuit.
• The time constant of a series RC circuit is
a time interval that equals the product of
the resistance and the capacitance.
= RC
Charging and Discharging
• The charging curve is an increasing
exponential.
• The discharging curve is an decreasing
exponential.
Transient time
• It takes 5 time constants to change the voltage by 99%
(charging or discharging), this is called the transient
time.
• General Equations
Charging from zero
• For RC circuits J= RC
v VF (Vi VF )e
i I F ( I i I F ) e
t

v VF (1 e
t
Discharging to zero
t

v Vi e
t
RC
RC
)
The Basic Inductor
• When a length of wire is formed onto a coil, it
becomes a basic inductor.
• Magnetic lines of force around each loop in the
winding of the coil effectively add to the lines of
force around the adjoining loops, forming a
strong electromagnetic field within and around
the coil.
• The unit of inductance is the henry (H), defined
as the inductance when one amp per second
through the coil, induces one volt across the
coil.
Physical Characteristics
• Inductance is directly proportional to
• The permeability of the core material
• The cross-sectional area of the core
• The square of # turns of wire
• Inductance is inversely proportional to
• The length of the core material
L = N2A/l
L = Inductance in henries
N = # of wire turns
A = Cross-sectional area in m2
l = core length in meters
μ= pereability in henries/m
Faraday’s and Lenz’s Laws
• Recall Faraday’s law:
• The amount of voltage induced in a coil is directly
proportional to the rate of change of the magnetic
field with respect to the coil.
• Recall Lenz’s law:
• When the current through a coil changes, an
induced voltage is created as a result of the changing
electromagnetic field, and the direction of the
induced voltage is such that it always opposes the
change in current.
Typical Inductors
Inductors in DC Circuits
• When there is constant current in an
inductor, there is no induced voltage.
• There is a voltage drop in the circuit due
to the winding resistance of the coil.
• Inductance itself appears as a short to dc.
RL Time Constant
• Because the inductor’s basic action
opposes a change in its current, it follows
that current cannot change
instantaneously in an inductor.
= L/R
where: is in seconds
L is in henries (H)
R is in ohms
Energizing Current in an
Inductor
• In a series RL circuit, the current will increase
to approximately 63% of its full value in one
time-constant interval after the switch is closed.
• The current reaches its final value in
approximately 5
.
De-energizing Current in an
Inductor
• In a series RL circuit, the current will decrease
to approximately 63% of its fully charged value
one time-constant interval after the switch is
closed.
• See Percentages on the drawing
• The current reaches 1% of its initial value in
approximately 5
.
Considered to be
equal to 0.
Induced Voltage in the Series
RL Circuit
• Walk-Through
• At the instant of switch
closure, the inductor
effectively acts as an open
with all the applied voltage
across it.
• During the first 5 time
constants, the current is
building up exponentially,
and the induced coil voltage
is decreasing.
• The resistor voltage
increases with current.
Induced Voltage in the Series
RL Circuit
• Walk-Through
• After 5 time constants, all of
the applied voltage is
dropped across the resistor
and none across the coil.
• The general formulas for RL circuits are:
v =VF+(Vi - VF)e-Rt/L
i =IF+(Ii - IF)e-Rt/L
Where VF & IF are final values of voltage & current.
Vi and Ii are initial values of voltage and current.
v and i are instantaneous values of voltage and
current
Increasing/Decreasing Current
• The special formula for an RL circuit
charging from zero is:
i =IF (1 - e-Rt/L )
• The special formula for an RL circuit
discharging to zero is:
i =Iie-Rt/L
Basic Stamp 2 Input and
Output Characteristics
• Recommended Maximum current
• From any I/O pin – 20 mA
• Per 8 I/O pins – 40 mA
• Threshold Voltages
• The voltage at which the Basic Stamp sees either a
logic “1” or logic “0”
• Default level per
TTL logic levels
• Input Characteristics
• High input resistance –
• Minimal effect on input
signals
Testing a simple photo resister
Circuit
• Characteristics of
Photoresisters
• Have no PN junction like
phototransistors or photodiodes.
• Uses bulk resistivity which
decreases with increasing illumination,
allowing more photocurrent to flow.
• Signal current from the detector can be varied over
a wide range by adjusting the applied voltage.
• Thin film devices made by depositing a layer of a
photoconductive material on a ceramic substrate.
Testing a simple photo resister
Circuit
• Photoresisters
• Metal contacts with external
connection. These thin films
have a high sheet resistance.
Therefore, the space between
the two contacts is made
narrow for low cell resistance
at moderate light levels.
• Photoresister used with the Boe Bot
• VT935G Group B
• Data Sheet Links on the Learning Module
Testing a simple photo resister
Circuit
• Photoresisters
• Metal contacts with external connection. These thin
films have a high sheet resistance. Therefore, the
space between the two contacts is made narrow for
low cell resistance at moderate light levels.
• Build the Circuits Shown
Testing a simple photo resister
Circuit
• How the Circuit works
• Stamp Inputs
• Above 1.4 V yields the input register for I/O pin with a 1.
• Below 1.4 V yields the input register for I/O pin with a 0.
• When a BASIC Stamp I/O pin is an input, the circuit
behaves as if neither the I/O pin nor 220 Ωresistor is present
• As the photoresistor’s resistance changes with light
exposure, so does the voltage at Input Pins
• As R gets larger, Vo gets smaller,
• As R gets smaller, Vo gets larger.
•
Testing a simple photo resister
Circuit
Testing
• Load the program on page 198 and test the sensors
• When shaded the sensor should yield a logic “0”
• When under enough light the sensor will yield a logic “1”
• Circuit & program converts the analog input voltage
levels to digital information stored in the Basic Stamp
• Questions
• What happens if the ambient level of light in the
room is lower or higher?
• Will the voltage divider network consisting of the 2K ohm
resister and the photo resister still work?
• Or will it be necessary to have different resistors for each
different level of ambient light?
Analog to Digital Converters
• Analog and Digital
• Real world processes produce analog signals
• Voices and music
• Pictures
• Letters and Decimal numbers
• Digital systems use binary signals
• ASCII code for “a” =>> 1100001
• Relative Strengths of Digital Data
• Simplified Storage, retrieval of information stored in digital
form
• Analog Systems - Cumbersome & Expensive
• Devices
• Digital to Analog Converters DACs
• Analog to Digital Converters ADCs
• The last circuit & program functioned as a very simple one
Analog to Digital Converters
• Types of ADCs
• Integrating
• Usually for slowly changing Analog Inputs
• Usually needs approximately 300 ms
• Successive Approximation
• Converges in a few microseconds
• Flash Converters
• More costly
• Much faster - can be used to digitize video signals
• Implementations using Micro-controllers/Microprocessors
• The last circuit was a very simple one bit ADC
• Only provides whether the input voltage is less than/greater
than approximately 1.4V DC
• Can become much more sophisticated and use many I/O ports
Analog to Digital Converters
• Basic Stamp RC-time constant based ADC
• Key aspects
• I/O characteristics of the Basic Stamp
• Discharge time of a charged parallel RC circuit
• PBASIC command for measuring the RC decay time on a
connected circuit
• I/O characteristics of the Basic Stamp
•
•
•
•
Same pins used for both Input and Output
Mode can be quickly switched
Can supply 20 mA/pin when in the Output mode
Very high input impedance when in
the input mode
• Usually an unnoticeable
effect on the input circuit
• Discharge time of a charged
parallel RC circuit
Analog to Digital Converters
• Basic Stamp RC-time constant based ADC
• PBASIC command for measuring the RC decay time
on a connected circuit
• RCTIME command is
designed to measure RC decay time on a
circuit like the one below. Here is the syntax for the RCTIME
command:
• RCTIME Pin, State, Duration
» Pin argument is the number of
the I/O pin that you want to measure
» State argument - 1 if the voltage across the capacitor
starts above 1.4 V and decays downward. 0 if the
voltage across the capacitor starts below 1.4 V and grows
upward
» Duration argument has to be a variable that stores the
time measurement, which is in 2 μs units
Build a complex ADC for light
levels using a circuit with a
RC time constant and a Boe
Bot
• Build the circuit below
Complex ADC for light levels
using a a Boe Bot
• Test the Sensor circuit
• Load the program on page 212 and 213
• Test the Sensors in the ambient light levels
• timeRight ________________
• timeLeft _______________
• Turn the Boe Bot by 1800 and retest the light levels
• timeRight ________________
• timeLeft _______________
• Now test with a Flash light directly on the sensors –
maintain 3” distance
• timeRight ________________
• timeLeft _______________
• Conclusions ??
.
Note: save this sheet
for next week