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Transcript
Lab 8 - Basic Circuits and Measurements .doc
EGR 1301 Lab #8
Basic Circuits and Measurements
Team Names:_______________________________________________________________________________
Purpose:



To learn the relationship between capacitance, resistance, and charging voltage in a flashing
LED circuit.
To learn the operation of an astable multivibrator using the 555 Timer IC.
To learn fundamental electrical measurement techniques using a VOM and oscilloscope.
Reading Assignment: Prior to beginning the experiment, read the handout on the operation of the
555 Timer IC and the use of the breadboard and components. Pay particular attention to the
description of the Astable Operation section.
Required Equipment: Your team should have the following equipment and parts:





Circuit breadboard with IC 555 chip installed (DO NOT remove the chip from the board)
Assorted resistors, capacitors, LEDs (resistors: 2-150, 1-27K, 1-100K, 1 M
variable; capacitors: 1-1.0 FD, 1-0.01 FD; 1 Red LED, 1 Green LED; assorted jumper
wires)
6 V Battery pack (4-1.5 V “C” batteries)
Parallax USB Oscilloscope with USB cable and red, green and blues probe cables.
Volt-Ohm Meter (VOM) with probe leads
Figure 1: Protoboard, USB Oscilloscope and Multimeter (VOM)
Resistor values (Ohms) and tolerances (%), are marked on the body of the resistor using a number
of colored bands. If you don’t remember the color code there are web resources that you can use as
a guide. You might try the following URL: http://marvin3m.com/resist/index.htm
Part I - Locate the VOM. Insert the black lead into the middle connector and the red lead into the
right-hand connector. Turn the meter’s function switch so that it “points” to the Ω symbol (i.e.
measures Ohms). When measuring resistance, be careful not to make hand contact with both the
meter’s leads, otherwise your measurement may not be accurate.
1
Lab 8 - Basic Circuits and Measurements .doc
a)
First, use the resistor color code to locate the following fixed resistors: 27K, 100K, and
two 150 Ohm. Compare the actual value of the resistors, as measured by the Ohm meter to their
marked color codes:
R1(27K) = __________ Ω, R2(100K) = __________ Ω, R4(150-1) = __________ Ω,
R3(1M-ccw) = _________ Ω, R3(1M-cw) = _______ Ω, R5(150-2) = __________ Ω,
Note: Variable resistors are also potentiometers or “pots”, because they vary the voltage potential
dropped across them.
Caution: Do not short the battery pack’s leads together. Shorting the leads will cause to
batteries to heat and the insulation on the connected wires to melt.
b)
Next, set the VOM to measure the voltage of the battery pack:
V(battery pack) = __________ V
c)
Calculating time constants. For our circuit the charging time constant uses R1+R2+R3.
When discharging, only use R2+R3, because Pin 7 is grounded. Then
1. for R3 = 1.0 M Ω, (slow flashing)
t (ch arg e)  0.693( R1  R 2  R3) * C1
______________________sec
t (disch arg e)  0.693( R 2  R3) * C1
______________________sec
T  t (ch arg e)  t (disch arg e)
______________________sec
f ( slow ) 
1
T
______________________Hz
2. for R3 = 0.0 M Ω, (fast flashing)
t (ch arg e)  0.693( R1  R 2  R3) * C1
______________________sec
t (disch arg e)  0.693( R 2  R3) * C1
______________________sec
T  t (ch arg e)  t (disch arg e)
______________________sec
f ( slow ) 
1
T
______________________Hz
2
Lab 8 - Basic Circuits and Measurements .doc
Part II: Construct the circuit shown. CAUTION: Double check your connections and polarities.
Do not connect the battery until the instructor checks your circuit. Pin number 1 on the 555 is
adjacent to the small dimple on the plastic case. Pins are numbered in a counter-clockwise
direction from pin 1. For LEDs the common practice is to manufacture the device with the anode
lead longer than the cathode lead. In a schematic symbol the anode (+ terminal) is represented by a
filled arrow and the cathode (- terminal) by the bar at the end of the anode arrow.
Figure 2. Location of Pin 1 on Timer Module ifyme
and Anode on LEDs.
Note that there is more then
one way to correctly wire up
this circuit using jumpers and
components. Just be sure to
observe the proper pin
connections on the IC and that
no wires are accidentally
touching.
Figure 3. Circuit Schematic
How does the circuit work: This circuit works on the principle of alternately charging and
discharging the 1.0 MFD capacitor (C1). The 555 begins to discharge the capacitor by grounding
the Disch terminal when the voltage detected by the Thresh terminal exceeds 2/3 the power
supply voltage (Vcc). It stops discharging the capacitor when the voltage detected by the Trig
terminal falls below 1/3 the power supply voltage. Thus, when both Thresh and Trig terminals are
connected to the capacitor's positive terminal, the capacitor voltage will cycle between 1/3 and 2/3
power supply voltage in a "sawtooth" pattern.
During the charging cycle, the capacitor receives charging current through the series combination
of the 1 MΩ variable and the 100 kΩ and 27 kΩ resistors. As soon as the Disch terminal on the
555 timer goes to ground potential (a transistor inside the 555 connected between that terminal and
3
Lab 8 - Basic Circuits and Measurements .doc
ground turns on), the capacitor's discharging current only has to go through the 1 MΩ variable and
the 100 kΩ resistors.
The 555's Out terminal produces a square-wave voltage signal that is "high" (nearly Vcc) when the
capacitor is charging, and "low" (nearly 0 volts) when the capacitor is discharging. This alternating
high/low voltage signal drives the two LEDs in opposite modes: when one is on, the other will be
off.
Part III - Operating the Circuit: Turn the shaft on the pot completely counter-clockwise (CCW).
Now connect the positive (red) lead from the battery holder to the positive bus of the breadboard.
If you have wired it correctly, you will see the LEDs flash alternately. If you vary the setting of the
pot, you can change the flash rate of the LEDs.
Part IV - Using the Oscilloscope:
0.
1.
2.
3.
4.
5.
6.
7.
Disconnect the battery from the circuit, and turn the pot fully CCW.
Plug the USB cable from the Parallax box into the USB port of the PC.
Run the program: Start Menu|Parallax|USB Oscilloscope| USB Oscilloscope 4.0.0
Click on the Title Screen to start the program.
Connect Channel 1 probe to TP2. Connect the ground probe to any lead at ground.
Connect Channel 2 probe to TP1. Connect the ground probe to any lead at ground.
In the Vertical Voltage Box, click on CH1 and Set the Vertical Voltage Scale to 1V.
In the Vertical Voltage Box, click on CH2 and Set the Vertical Voltage Scale to 1V.
Note: green indicates the channel is active for display and selected for adjustment,
yellow indicates the trace is active for display only.
8. Set the Horizontal Sweep Scale to 50 ms.
9. Click on the RUN/STOP button. Note the appearance of the two horizontal traces, one color
for each channel.
10. Now activate your circuit by reconnecting the positive battery lead.
a) What is happening? You are observing the time based trace of the voltage changes in your
circuit. Channel 2 shows the rising and falling charge on the capacitor. Observe on channel 1 that
when the capacitor is charging (rising), that the output voltage is high, and when the capacitor is
discharging (falling).
b) Using the O-scope for measurements: Right click anywhere in the display area and select
“Cursors|Paired”. You can click and drag the cursors and they will measure the value at their
position on the screen and show the difference between them. The readout is in the Cursors area
on the upper right hand side of the display. You can click STOP at any time to freeze the display.
1) Using the horizontal cursors:
V(low of the capacitor)_______V. V(high of the capacitor)______V. V(high)-V(low)______V.
What peak-peak voltage did you expect?______________V?
4
Lab 8 - Basic Circuits and Measurements .doc
2) Connect the CH 2 probe to Pin 3 (TP 4) and connect the CH 2 probe ground to the negative
(ground) side of the battery pack.
Using the horizontal cursors: V(p-p of the Output - Pin 3)_______V.
What voltage did you expect?______________V?
3) Connect the CH 2 probe to TP 3.
Using the horizontal cursors: V(at TP 2)_______V,
V(at TP 3)_______V
What voltage relationship did you expect to find between the two LEDs?
Why are the square wave signals on both LEDs in sync? In other words, why are the voltages on
both LEDs high at the same time?
4) Using the vertical cursors, measure the time of one period of a capacitor cycle, and calculate the
frequency:
T =____________________sec
f = ______________________Hz
How does this frequency compare to your predicted frequency?
5) Turn your pot to slow down the flash rate to very slow. You will have to change the Horizontal
Sweep rate to a higher time. Using the vertical cursors, measure the time of one period of a
capacitor cycle, and calculate the new frequency:
T =____________________sec
f = ______________________Hz
Note that the O-scope also automatically calculates time period and frequency for you. How does
this automatic frequency compare to your measured time and calculated frequency?
6) Calculate the Duty Cycle at fast and slow flashing. Why does the Red LED remain On longer
than the Green LED when the flash rate is slowed down? D.C  R 2 /[ R1  2( R 2  R3)]
5
Lab 8 - Basic Circuits and Measurements .doc
7) In general, there are numerous reasons why predicted or calculated values are not exactly the
same as measured values. For our equipment and experiment in this lab, state at least two reasons
why this is so.
6