Review: 555 Timer. ◇ Used to create timing circuits like. • Audible alarms. •
Flashers. • Windshield wiper delays. ◇ Two modes of operation. • Monostable-
single ...
Electronic Instrumentation Project 3 Build an Astable Multivibrator Experiment 8 Diodes www.rpi.edu/~sawyes/courses
Agenda
Review Experiment 7: Flip Flops Review: Steps to understanding the 555 Timer • • • • •
RC circuit charge and discharge Voltage Divider Comparators Role of Flip Flop Discharge transistor
Project 3: Building the 555 Timer Experiment 8: Diodes • Ideal vs. Real • Rectifiers and Limiters • Light Emitting Diodes and Photodetectors
What you will know
How the pieces of the 555 Timer fit together What to expect from Project 3 What a diode does and how it works The difference between real and ideal diode What a I-V characteristic curve is What diodes can be used for
Simple Flip Flop Example: The RS Flip-Flop
Q=0 Q=1
Note that the output depends on three things: the two inputs and the previous state of the output.
Inside the R-S Flip Flop
Note that the enable signal is the clock, which regularly pulses. This flip flop changes on the rising edge of the clock. It looks at the two inputs when the clock goes up and sets the outputs according to the truth table for the device.
Inside the J-K Flip Flop
Note this flip flop, although structurally more complicated, behaves almost identically to the R-S flip flop, where J(ump) is like S(et) and K(ill) is like R(eset). The major difference is that the J-K flip flop allows both inputs to be high. In this case, the output switches state or “toggles”.
By-Pass Capacitors V+ GND
In a sequential logic device, a noisy signal can generate erroneous results. By-pass capacitors are placed between 5V and 0V to filter out high frequency noise. A by-pass capacitor should be used in any circuit involving a sequential logic device to avoid accidental triggering.
Review: 555 Timer
Used to create timing circuits like • Audible alarms • Flashers • Windshield wiper delays
Two modes of operation • Monostable-single pulse output (the clapper) • Astable-oscillating output (counter circuit)
Block Diagram
Circuits are often represented by block diagrams that show the flow of the signal between different functional blocks. Above is a block diagram of the astable multivibrator. Your circuit won’t include the Reset feature
Components in each Block R-R-C Combination
Threshold Comparator
Discharge Transistor
Trigger Flip Flop Comparator Voltage Divider
How does the Astable Multivibrator work? What makes this circuit generate a string of pulses? This is discussed in detail in the experiment 7 notes. http://www.academy.rpi.edu/5.downloads/modules.html
Review Comparators: Threshold Non-Inverting 12V-Reference (12V offset) Input voltage above offset go HIGH Input voltage below offset go LOW
Voltage from RC circuit
12V (voltage divider)
vout Vref=12V
vin= Voltage from RC circuit
Review Comparators: Trigger Inverting 6V-Reference Comparator (6V offset) Input voltage below offset go HIGH Input voltage above offset go LOW 6V (voltage divider) Voltage from RC circuit
Vref=6V
How does the Astable Multivibrator work?
Ton 0.693( R1 R 2)C1 Toff 0.693( R 2)C1
These equations determine the characteristics of your output pulses based on the values you choose for R1, R2 and C1.
How does the Astable Multivibrator work?
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 of
ON = (R1 + R2)C1.
The capacitor discharges through R2 with a time constant of OFF = R2C1
Where do the equations come from? The equations that determine the on and off time of the output pulses are based on the charge and discharge time of the capacitor. The capacitor equations are: charging t VC V0 1 e
discharging t VC V0 e
Relating Charge equations and time
t VC 2 V0 V0 1 e 3
1 2 e 3
Time to charge up to 2/3V0 is?
t
In class problem: Charge equations and time
a) Find the time it takes to charge from 0 to 2/3 V0 (keep τ in answer) b) Find the time it takes to charge from 0 to 1/3 V0 (keep τ in answer)
c) How much time should it take to charge between 1/3 and 2/3 of V0 d) Where have you seen this value before? What is τ? e) With what part of the 555 timer circuit can you manipulate this charge time?
The Animation
Animation applet
=0 =1
Your initial design will be a PSpice simulation and working circuit based on this animation.
Project 3 Purpose
The purpose of this project is to build an Astable multivibrator without the 555-timer chip. This means you will have to assemble your own components to mimic the behavior of the inside of the chip. You will create a PSpice simulation and a working circuit. You will then modify the 555 timer chip model so that it cycles over a different part of the capacitor charge curve. You will modify your PSpice simulation and circuit to demonstrate that your new model works as predicted.
Build this circuit V5 5V
OS2
OUT
R2 5.6k
Rb 10k
-
OS1 V-
4
uA741
U3A 1
2 R3 5.6k
V+
+
C1
uA741
-
220
4
D_LED MLED81
74LS02 OS2
OUT V
3
7
V
3
R11
0 U2
0.1uF
1
OS1 V-
0
5 6 1
U3B 5
2
5 6 V
+
2
U1 3
V+
Ra 10k
7
R1 5.6k
0
0
4 6 74LS02
0
0
Q1
V
R9 1k
Q2N2222
0
This has all the 555 Timer features except for the reset pin. You will build it on the protoboard
Representation of Flip Flop 2
U5A SET 3
1
Qbar
74LS02
6
5
U5B 4
Q
RESET 74LS02
2 NOR gates can be used to create a SR Flip-Flop Convince yourself that this works
But model this circuit V1 5Vdc
Q1
Q2N2222 R1 5.6k
U2 3
Rb 10k
+
OS2
OUT 2 R2 5.6k
V
C1 .1uF
-
OS1 V-
4
uA741
Flip-Flop
1k
0
7
Ra 10k
V+
0
R6
5 R11 R7 100
6 1
1k
R9 1k
R10 1k
D1 D1N4148
Q2
R8 100
Q3
0 Q2N2222
0
0
0
7
U4 3
V+
R3 5.6k
+
OS2
OUT 2 uA741
-
4
Q2N2222
OS1 V-
Vout
5 R12
D2
1k
D1N4148
R13 330
6 1
0
Build circuit using MAX 473 Op-Amps
V
LED_replacement D1N4148
0
The demo version of Capture won’t model the circuit you will build. It can model this one, which uses 2 transistors to model the SR Flip-flop.
Initial Design PSpice
Build the PSpice circuit and look at the signals at the input and output of each block in the diagram. • No reset circuit
Use the cursors to record voltage levels and times • high and low on digital signals • important points on analog signals (like 1/3 and 2/3 of Vcc) • on and off time of the pulses
Initial Design Protoboard
Build the circuit on your protoboard • don’t forget to put power on the digital chip • add a bypass capacitor
Record data using Mobile Studio and the IOBoard • Use voltage and time features of scope • Use the cursors on the scope • Make sure you have labeled the plots with the numerical values recorded
Final Design
Modify the inside of the timer to make it switch at different voltages. What are the new equations for TON and TOFF? What are the new on and off times for the pulses in your circuit? Modify the PSpice and the circuit on your protoboard and show that your results are consistent with those predicted by the equations.
Project Report
Introduction • What is the objective of the project? • At least two relevant topics Theory • Describe the function of the components in the circuit • How does the multivibrator work? Give details. • Where do the equations for TON and TOFF come from? • What should TON and TOFF be for the circuit you are building?
Project Report
Initial Design • PSpice simulation, plots, and discussion • Protoboard implementation, plots, and discussion • comparison of voltages and times • PSpice • Protoboard • Theory
Project Report
Final Design • • • • •
Determine new threshold and trigger voltages Come up with the new timing equations Modify PSpice Modify Circuit Comparison of voltages and times • voltage levels affected by redesign • new on and off times
Project Report
Conclusion • Is it an astable multivibrator? • Conclusions that can be drawn from your voltage comparisons • Discuss the on and off times of the initial and final design. Are they as expected? • Sources of error • General Conclusions
Appendices
Appendix A: Make you own task list. Appendix B: References and initial design equations. Appendix C: PSpice plots of initial design Appendix D: Plots of data from Mobile Studio for initial design Appendix E: Final design (circuit diagram, calculations, PSpice and Mobile Studio plots)
Electronic Instrumentation Experiment 8: Diodes
* Introduction to Diodes * Part A: Diode i-v Characteristic Curves * Part B: Diode Circuits: Rectifiers and Limiters * Part C: LEDs, Photodiodes and Phototransistors
Introduction to Diodes D1
ANODE
CATHODE DIODE
A diode can be considered to be an electrical one-way valve. They are made from a large variety of materials including silicon, germanium, gallium arsenide, silicon carbide …
Introduction to Diodes
In effect, diodes act like a flapper valve • Note: this is the simplest possible model of a diode
Introduction to Diodes
For the flapper valve, a small positive pressure is required to open. Likewise, for a diode, a small positive voltage is required to turn it on. This voltage is like the voltage required to power some electrical device. It is used up turning the device on so the voltages at the two ends of the diode will differ. • The voltage required to turn on a diode is typically around 0.6 - 0.8 volt for a standard silicon diode and a few volts for a light emitting diode (LED)
Introduction to Diodes 1 0 V
5 V
0 V
- 5 V
- 1 0 V 0 s
0 . 5 m s
1 . 0 m s
1 . 5 m s
2 . 0 m s
2 . 5 m s
3 . 0 m s
V ( D 1 : 1 ) T i m e
10 volt sinusoidal voltage source D1 D1N4002
VAMPL = 10V
V1
R1
FREQ = 1k
1k
0
Connect to a resistive load through a diode
Introduction to Diodes D1
VAMPL = 10V
V
V1
D1N4002
V
R1 FREQ = 1k
1k
Only positive current flows
0 10V
5V
0V
-5V
-10V 0s V(D1:1)
0.5ms V(D1:2)
1.0ms
1.5ms Time
2.0ms
2.5ms
3.0ms
How Diodes Work
At the junction, free electrons from the N-type material fill holes from the Ptype material. This creates an insulating layer in the middle of the diode called the depletion zone.
How Diodes Work
How Diodes Work
When the positive end of the battery is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger and no current flows.
Part A: Diode i-v Characteristic Curves • What is a i-v characteristic curve? • i-v curve of an ideal diode • i-v curve of a real diode
What is an i-v characteristic curve? Recall that the i-v relationship for a resistor is given by Ohm’s Law: i=v/R If we plot the voltage across the resistor vs. the current through the resistor, we obtain i The slope of the straight line is given by 1/R v
What is an i-v characteristic curve? If we change the axis variables in PSpice, we can obtain i-v characteristic curves. R1 500
V1 15V
R2 1k
0 10mA
5mA
V-I Characteristic of a 500 Ohm Resistor
0A
-5mA
-10mA -6.0V I(R1)
-4.0V
-2.0V
0V V(R1:1) - V(R1:2)
2.0V
4.0V
6.0V
i-v characteristic for an ideal diode iD Ideal Diode
When voltage across the diode is negative, the diode looks like an open circuit.
0
vD When voltage across the diode is positive, the diode looks like a short.
i-v characteristic of a real diode
Real diode is close to ideal Ideal Diode
Real diode characteristics
A very large current can flow when the diode is forward biased. For power diodes, currents of a few amps can flow with bias voltages of 0.6 to 1.5V. Note that the textbook generally uses 0.6V as the standard value, but 0.7V is more typical for the devices we will use in class. Reverse breakdown voltages can be as low as 50V and as large as 1000V. Reverse saturation currents Is are typically 1nA or less.
The diode equation
The iD-vD relationship (without breakdown) can be written simply as:
iD
v D nV T I S e 1
vD is the voltage across the diode and iD is the current through the diode. n and Is are constants. VT is a voltage proportional to the temperature, we use 0.0259V. Note that for vD less than zero, the exponential term vanishes and the current iD is roughly equal to minus the saturation current. For vD greater than zero, the current increases exponentially.
R1
Diode equation
V2 1k 5V D1 D1N4148
19m
0
16m
12m
8m
iD 4m
v D nV T I S e 1
iD
0 -16V
-14V
-12V
-10V
-8V
-6V
-4V
-2V
0V
2V
Both the simulated current vs. voltage (green) and the characteristic equation (red) for the diode are plotted.
Diode equation comparison
In this experiment, you are asked to find the parameters for the equation
iD
v D nV T I S e 1
That is, you need to find the constants in this equation so that it matches the data from an actual diode. Note that VT=25.9mV at room temperature, you need to find n and Is
Comparison A good guess for the exact values of IS and n can be determined for a real diode by building the circuit and matching data from it to the diode equation in Excel. Plot two series
ADC2+
ADC2-
R2
ADC1+
1k
• series 1 : vD ( ADC1) ( ADC1) iD
D2 D1N4148 ADC1-
ADC 2 ADC 2 R2
0
• series 2: calculate iD for 0
