Lab 4: Using the Digital Multimeter (DMM)

Resistor Color CodesResistor values can be determined by “reading” the bands of color printed on the resistor.Resistors of different wattages showing color code bands. Bottom = 0.25W. Color bands give resistanceand tolerance. Gold tolerance band is +/‐ 5%, silver is +/‐ 10%. Read values by taking first two digits andmultiply by 10 d3 where d3 is the number represented by the third color band. Chart is below. For examplethe resistor in the middle is a 1W, 47 x 10 3 Ω or 47kΩ resistor. The one above it is 2W, 27 x 10 2 Ω or 2700Ωor 2.7kΩ.

Experiment 1: Resistor tolerances and DMM accuracyEquipment:DMM, five x 1kΩ resistors.Procedure:• Label resistors with your lab station number and a letter, a,b,c, d or e• Lab partner 1 measure the resistance of each resistor and record• Lab partner 2 measure the resistance of each resistor and record• Repeat, so that each person measures each resistor three times.• Trade multimeters with one of the other groups at your bench and repeat, noting the new DMM inyour records. Compare the results by analyzing the differences between the measurements to seeif you can detect the influence of the DMM, the influence of the operator, and the tolerance of theresistors.Writeup:Write up this experiment as instructed in the Lab Report Guide, posted on eCampus under the “Labs”folder. Pay special attention to the analysis section.Experiment 2: LED Current and output intensityEquipment:DMM, LED, 4 x 1kΩ resistor, breadboard, power supply, wiresProcedure:• Construct a circuit as shown in Figure 1. The LED has two leads and a flat spot on one side of thecase. The Anode is usually the longer of the two leads, and the cathode has the flat spot. TheAnode goes toward the “+” side of the circuit and the cathode toward the “–“ side. Measure theresistance of the resistor you use, or use one with known resistance from Experiment 1, but recordthe value.• Turn on the power supply and switch the DMM to measure DC Volts. Measure the voltage acrossthe LED. Record the reading. Measure the supply voltage and record the reading. Note that thesetwo readings, plus the knowledge of the resistor value allow you to compute the current throughthe circuit. Do that now and record it.• Measure the resistance of a second 1kΩ resistor and add it in parallel with the first one. Note whathappens to the brightness of the LED. Compute the equivalent resistance of the two parallelresistors and record this value.• Switch the DMM back to DC Volts function. Measure and record the voltage across the LED.Compute the current through the circuit.

• Measure the resistance of a third 1kΩ resistorand add it in parallel to the other two. Againrecord the voltage across the LED. Don’t forgetto switch back to DC Volts before trying tomeasure voltage!! Compute the equivalentresistance of the three resistors in parallel andrecord the value. Compute the current throughthe circuit.• Now set up the DMM to measure current. The 0‐20mA scale should be adequate. Insert the DMMinto the circuit as shown in the lab introduction,and measure the current through the LED with allthree resistors in parallel. Record the current forthis arrangement. Is it the same as the currentyou computed?• Remove the third resistor from the parallelarrangement and again measure and record thecurrent. Compare to the computed current.Figure 1: Diode testing circuit• Remove the second resistor from the parallel arrangement and again measure and record thecurrent. Compare to the computed current.MeasuredR1R2R3Equiv. RParallel V(LED) I (calc) I(meas) BrightnessWriteup• Write up this experiment as instructed in the Lab Report Guide. Plot diode current versus diodevoltage, diode current versus series resistance and diode current versus 1/(series resistance). Ineach plot, the current is the dependent variable (vertical axis). Explain the results. Discuss anydiscrepancies you find between the measured and calculated values for the circuit current.

Experiment 3: PhotoresistorEquipmentBreadboard, photoresistor, DMM, assorted fixed resistors, potentiometer, 5V power supply, wires.Procedure• Insert the photoresistor into the breadboard so it faces “up” toward the ceiling.• Measure and record the resistance of the photoresistor with no “shade” on it.• Cover the photoresistor completely and again measure and record its resistance.• Experiment with partially covering the photoresistor and determine how the resistance changeswith different degrees of coverage.ProjectDesign a circuit that uses a 5V power supply, one or more fixed resistors and a photoresistor (PR) to make aday/night detector. Depending on the range of your photoresistor you may or may not have the fixedresistors you need in your lab kit to construct your design. That’s OK, create the design and we’ll find youthe resistors you need. Your circuit should be set up so that if it is “daylight” the voltage at the output ofthe circuit is less than 2 Volts, and if it is “night” the voltage is greater than 3.5 volts. To do this, you shouldhave a photoresistor in which the ratio of Rdark to Rlight is at least 4:1, preferably more. If your PR is notthat sensitive, try shining a flashlight onto it during the “daylight” phase and re‐measure its resistance tosee if you get a 4:1 or better ratio. If not, speak to a T/A about getting a different PR.Document your design , including your calculations to choose a fixed resistor value, and build your circuit.Show the T/A your calculations and demonstrate your circuit to a T/A by showing the output on your DMMfor the dark and daylight situations.Experiment 4: PotentiometersEquipmentPotentiometer, DMM, 5V power supply, wires.ProcedureConnect the power supply to your potentiometer as shown in thediagram at right.1. Measure the voltage from ground to the center terminal of thepotentiometer. Turn the shaft of the pot all the way counterclockwiseand slowly turn it clockwise and watch as the voltage changes on theDMM.2. Disconnect the power supply and set the potentiometer somewhere inthe middle of its range. Measure the resistance between the “left”terminal and the wiper and between the “right” terminal and the wiper.