UMHB ENGR 1310 - Lab 8 - Basic Circuits and Measurements

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Lab 8 - Basic Circuits and Measurements .docEGR 1301 Lab #8Basic Circuits and MeasurementsTeam 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 leadsFigure 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.htmPart 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.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:1Lab 8 - Basic Circuits and Measurements .docR1(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) = __________ Vc) 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. Then1. for R3 = 1.0 M Ω, (slow flashing)t (ch arge )=0 .693 (R 1+R 2+R 3 )∗C 1______________________sect (discharg e )=0 . 693( R 2+R 3 )∗C 1______________________secT =t (ch arg e)+t (disch arge )______________________secf ( slow)=1T______________________Hz2. for R3 = 0.0 M Ω, (fast flashing)t (ch arge )=0 .693 (R 1+R 2+R 3 )∗C 1______________________sect (discharg e )=0 . 693( R 2+R 3 )∗C 1______________________secT =t (ch arg e)+t (disch arge )______________________secf ( slow)=1T______________________HzPart 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 directionfrom pin 1. For LEDs the common practice is to manufacture the device with the anode lead longer2Lab 8 - Basic Circuits and Measurements .docthan the cathode lead. In a schematic symbol the anode (+ terminal) is represented by a filled arrowand the cathode (- terminal) by the bar at the end of the anode arrow.Figure 2. Location of Pin 1 on Timer Module 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 SchematicHow 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 supplyvoltage (Vcc). It stops discharging the capacitor when the voltage detected by the Trig terminal fallsbelow 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 ofthe 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 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.3Lab 8 - Basic Circuits and Measurements .docPart 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. Disconnect the battery from the circuit, and turn the pot fully CCW.1. Plug the USB cable from the Parallax box into the USB port of the PC.2. Run the program: Start Menu|Parallax|USB Oscilloscope| USB Oscilloscope 4.0.03. Click on the Title Screen to start the program.4. Connect Channel 1 probe to TP2. Connect the ground probe to any lead at ground.5. Connect Channel 2 probe to TP1. Connect the ground probe to any lead at ground.6.


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