phys225 lecture 18 - texas a&m...
TRANSCRIPT
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PHYS225Lecture 18
Electronic Circuits
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Oscillators and Timers
• Oscillators & Timers
– Produce timing signals to initiate measurement
• Periodic or single pulse
• Periodic output at known (controlled) frequency
• Shape is also known (controllable)– Sinusoid, square, triangle, etc.
– Part of nearly every electronic device
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Relaxation oscillators
• Use a simple RC decay to generate a sustained oscillation
• Example using an op amp as a “comparator”
– Flips between states at a given frequency (1/RC)
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Sine Wave Generator
• Wien Bridge oscillator
– 1/3 gain amplifier with phase shift cancellation
• Other techniques
– RC phase shift
– quadrature
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Resonators
• Use “piezoelectric” effect
– Voltage generates a strain
– Quartz crystal vibrates at a specific frequency
• Depends on thickness of crystal
– Can cat like a very sharply tuned RLC circuit element
• Q is 104-5
– Very stable and useful!
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• The 555 Timer is one of the most popular
and versatile integrated circuits ever produced!
• It is 30 years old and still being used!
• It is a combination of digital and analog circuits.
• It is known as the “time machine” as it performs a wide variety of timing tasks.
• Applications for the 555 Timer include:
– Bounce-free switches and Cascaded timers
– Frequency dividers
– Voltage-controlled oscillators
– Pulse generators and LED flashers
The 555 Timer
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555 Timer
• Each pin has a function
• Note some familiar components inside
NE555
2
5
3
7
6
4 81
TR
CV
Q
DIS
THR
R
VC
CG
ND
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Inside the 555 Timer
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– The voltage divider (blue) has three equal 5K resistors. It divides the input voltage (Vcc) into three equal parts.
– The two comparators (red) are op-amps that compare the voltages at their inputs and saturate depending upon which is greater.
• The Threshold Comparator saturates when the voltage at the Threshold pin (pin 6) is greater than (2/3)Vcc.
• The Trigger Comparator saturates when the voltage at the Trigger pin (pin 2) is less than (1/3)Vcc
Inside the 555 Timer
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10
– The flip-flop (green) is a bi-stable device. It generates two values, a “high” value equal to Vcc and a “low” value equal to 0V.• When the Threshold comparator saturates, the flip flop is
Reset (R) and it outputs a low signal at pin 3.
• When the Trigger comparator saturates, the flip flop is Set (S) and it outputs a high signal at pin 3.
– The transistor (purple) is being used as a switch, it connects pin 7 (discharge) to ground when it is closed. • When Q is low, Qbar is high. This closes the transistor
switch and attaches pin 7 to ground.
• When Q is high, Qbar is low. This open the switch and pin 7 is no longer grounded
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Types of 555-Timer Circuits
• Astable Multivibratorputs out a continuous sequence of pulses
5V
Ra
C
0.01u
F
LED
NE555
2
5
3
7
64 8
1TR
CV
Q
DIS
THRR
VC
CG
ND
Rb
5V
12
1K
0.01u
F
C
R
LED
NE555
2
5
3
7
6
4 81
TR
CV
Q
DIS
THR
R
VC
CG
ND
Monostable Multivibrator(or one-shot) puts out one pulse each time the switch is connected
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Monostable Multivibrator (One Shot)
+V
-V
-
+
+V
-V
-
+R
S
Q
Q
3
4
1
7
2
6
8
R
R
R
Control Flip-FlopTrigger Comparator
Threshold Comparator
Output
ResetVcc
Trigger
Monstable MultivibratorOne-Shot
C
Ra
cc
2V
3
cc
1V
3
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Behavior of the Monostable Multivibrator• The monostable multivibrator is constructed by adding an
external capacitor and resistor to a 555 timer.
• The circuit generates a single pulse of desired duration when it receives a trigger signal, hence it is also called a one-shot.
• The time constant of the
resistor-capacitor
combination determines
the length of the pulse.
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– Used to generate a clean pulse of the correct height and duration for a digital system
– Used to turn circuits or external components on or off for a specific length of time.
– Used to generate delays.
– Can be cascaded to create a variety of sequential timing pulses. These pulses can allow you to time and sequence a number of related operations.
Uses of the Monostable Multivibrator
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Astable Pulse-Train Generator (Multivibrator)
+V
-V
-
+
+V
-V
-
+R
S
Q
Q
3
4
1
7
2
6
8
R
R
R
Control Flip-FlopTrigger Comparator
Threshold Comparator
Output
Vcc
Astable Pulse-Train Generator
C
R1
R2
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Behavior of the Astable Multivibrator• The astable multivibrator is simply an oscillator. The astable
multivibrator generates a continuous stream of rectangular off-on pulses that switch between two voltage levels.
• The frequency of the pulses and their duty cycle are dependent upon the RC network values.
• The capacitor C charges through the series resistors R1 and R2
with a time constant
(R1 + R2)C.
• The capacitor discharges
through R2 with a time
constant of R2C
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– Flashing LED’s
– Pulse Width Modulation
– Pulse Position Modulation
– Periodic Timers
Uses of the Astable Multivibrator
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Flashing LED’s
• 40 LED bicycle light with 20 LEDs flashing alternately at 4.7Hz
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Understanding the Astable Mode Circuit
• 555-Timers, like op-amps can be configured in different ways to create different circuits. This one creates a train of equal pulses, as shown at the output.
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• Capacitor C1 is charged up by current flowing through R1
• As the capacitor charges up, its voltage increases and the current charging it decreases
VV V
R1
1k
U2
TOPEN = 01
2C1
1uF
U1
TCLOSE = 0
1 2
0
V1
10V
IV V
R
V
k
CAPACITOR CAPACITOR
1
1
10
1
T i m e
0 s 1 m s 2 m s 3 m s 4 m s 5 m s 6 m s 7 m s 8 m s 9 m s 1 0 m s
V ( U 2 : 1 ) V ( R 1 : 2 ) V ( V 1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
C a p a c i t o r V o l t a g e
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• Capacitor Current
• Capacitor Voltage
• Where the time constant
T i m e
0 s 1 m s 2 m s 3 m s 4 m s 5 m s 6 m s 7 m s 8 m s 9 m s 1 0 m s
I ( R 1 ) I ( C 1 )
0 A
2 m A
4 m A
6 m A
8 m A
1 0 m A
C a p a c i t o r a n d R e s i s t o r C u r r e n t
T i m e
0 s 1 m s 2 m s 3 m s 4 m s 5 m s 6 m s 7 m s 8 m s 9 m s 1 0 m s
V ( U 2 : 1 ) V ( R 1 : 2 ) V ( V 1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
C a p a c i t o r V o l t a g e
I I eo
t
V V eo
t
1
RC R C ms1 1 1
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• Note that the voltage rises to a little above 6V in 1ms.
T i m e
0 s 1 m s 2 m s 3 m s 4 m s 5 m s 6 m s 7 m s 8 m s 9 m s 1 0 m s
V ( U 2 : 1 ) V ( R 1 : 2 ) V ( V 1 : + )
0 V
2 V
4 V
6 V
8 V
1 0 V
C a p a c i t o r V o l t a g e
( ) .1 6321 e
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555 Timer
• At the beginning of the cycle, C1 is charged through resistors R1 and R2. The charging time constant is
• The voltage reaches (2/3)Vcc in a time
1)21(693.01arg CRRTt ech
1)21(arg CRRech
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555 Timer
• When the voltage on the capacitor reaches (2/3)Vcc, a switch (the transistor) is closed (grounded) at pin 7.
• The capacitor is discharged to (1/3)Vcc through R2 to ground, at which time the switch is opened and the cycle starts over.
1)2(arg CRedisch
1)2(693.02arg CRTt edisch
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555 Timer
• The frequency is then given by
fR R C R R C
1
0 693 1 2 2 1
144
1 2 2 1. ( )
.
( )
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PWM: Pulse Width Modulation
• Signal is compared to a sawtooth wave producing a pulse width proportional to amplitude
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20 March 2007 Electronic Instrumentation 27
What Can Be Done With PWM?
• Question: What happens if voltages like the ones above are connected to a light bulb? Answer: The longer the duty cycle, the longer the light bulb is on and the brighter the light.
Low
Duty Cycle
Medium
Duty Cycle
High
Duty Cycle
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20 March 2007 Electronic Instrumentation 28
What Can Be Done With PWM?
• Average power can be controlled• Average flows can also be controlled by fully opening and
closing a valve with some duty cycle