IBEW / DC lab guides

Week 7 · DC Theory Chapter 7

DC circuit analysis

Predict the tap voltage. Build one divider, measure it, and use your readings to explain the circuit.

Make Circuits · Optional builds ↓

Night 2 · Core build

Compare the tap with and without a load

Parts · A two-AA holder, three 1 kΩ resistors rated ¼ W or higher, a breadboard, and a DC voltmeter.

1Predict

For a 3.00 V pack, predict the tap voltage with two equal series resistors. Then predict it with another 1 kΩ resistor in parallel with the lower one.

Show the divider math

Unloaded, the lower leg is 1 kΩ of the 2 kΩ total, so X–N is 1.50 V. With the load, two equal 1 kΩ resistors in parallel give 500 Ω. Then 3.00 V × 500 Ω ÷ 1500 Ω = 1.00 V.

2Build

Keep both battery leads disconnected while wiring. Have the instructor check the board before you apply power.

Reveal the wiring schematic
PXNR1 · 1 kΩR2 · 1 kΩ3 V+−Load · 1 kΩMeasure X–N before and after connecting the dashed load branch.
Reveal the breadboard and hole list
One side of the center gap · selected strips5 · P10 · X15 · NabcdeR1 · 1 kΩR2 · 1 kΩLoad · 1 kΩPack +Pack −
  1. Use one side of the center gap. Verify which five holes share a strip. Place R1 from b5 to b10 and R2 from c10 to c15.
  2. Leave the load out for your first reading. Disconnect power and place it from d10 to d15 when instructed.
  3. After the wiring check, connect pack positive to a5 (P) and pack negative to e15 (N).

3Measure and check

Use DC volts, black in COM and red in V/Ω. Place red on the first named point and black on the second. Readings use volts.

Record P–N and X–N with the load out. Disconnect the pack, add the load, and get the wiring checked before reconnecting. Then record all three loaded readings.

Show the probe points

P–N spans the battery. X–N spans the lower leg. P–X spans R1. Use the exposed component leads or spare holes on those same strips.

4Explain

Why did the load reduce the tap voltage? Show the signed loop sum and compare current entering and leaving X.

Check with Thévenin and Norton

At 3.00 V the unloaded tap is 1.50 V. With the ideal source suppressed on paper, the two 1 kΩ divider resistors look like 500 Ω in parallel as seen from X–N. A 1 kΩ load connected to the 1.50 V Thévenin equivalent therefore gets 1.00 V.

The Norton model is 3.00 mA in parallel with 500 Ω. The 1 kΩ load takes 1.00 mA, so its drop is also 1.00 V. Calculate the equivalent on paper and keep the powered battery terminals apart.

Disconnect the pack. Show your measurements and explanation to the instructor.

Week 7 · Optional extra builds

Make circuits

Finish tonight’s core build first. If there’s time and the parts are available, choose an optional circuit with your instructor.

Before taking the lab apart: Save the core build readings in your notebook. Disconnect both pack leads before rebuilding.

Bonus-build safety and parts check
  • All builds use one instructor-approved nominal 3 V two-AAA pack, a checked breadboard, jumpers and a DC-voltage/Ω meter. No mains, extra supply voltage, parallel packs, motors or relay loads.
  • Keep each LED's fixed 220 Ω series resistor in place. Check component power/current ratings and LED polarity; use red LEDs suitable for this low-voltage build.
  • Disconnect both source leads for wiring changes and Ω/continuity tests. Have every new build checked before power. The bonus measurements use DC volts across resistors to calculate current, not an ammeter across the pack.
  • Identify the exact transistor maker, marking and package, then verify base B, collector C and emitter E with the instructor. A 2N2222-family name or flat face does not establish a universal pin order. Keep the specified 10 kΩ base-feed resistor effective.
  • Operate only the light/cover, insulated knob or switch actuator while powered. If a part warms unexpectedly, disconnect. Skip a build if parts or ratings are unavailable; do not improvise a substitute without approval.
Recommended first · LDR + 10 kΩ + 2N22221 · Automatic night-light

Build a red LED that generally gets brighter when you shade the LDR. This is a healthy light-control build, not the Chapter 4 mystery-fault test.

Parts beyond the pack, board, jumpers and meter

  • Verified 2N2222-family NPN transistor (identify the exact maker/package and B, C, E pins)
  • LDR / photoresistor
  • Red LED
  • 220 Ω LED resistor, ¼ W or verified suitable rating
  • One 10 kΩ resistor

Predict first: Will more light make the LDR resistance rise or fall? Predict the direction of the LED-current change before testing.

Show the electrical wiring schematic
Disconnect both pack leads. Measure the isolated LDR in bright light and under an opaque cover. Identify the 10 kΩ and 220 Ω resistors, LED polarity and the actual transistor's B/C/E pins. Never infer pin order from a flat face or a similar part number. Positive → 10 kΩ → base node B. LDR from B → negative. The 10 kΩ must remain in the positive base-feed path. Make the separate LED path: positive → 220 Ω → LED anode A; LED cathode K → collector C. Emitter E → negative. Keep the 220 Ω in series with the LED at all times. Have the instructor check it before power. With the approved nominal 3 V pack, compare a flashlight on the LDR with shading only the LDR. Do not move wiring while powered. Actual sensitivity depends on the LDR, transistor and room light. Both states may still glow; measure the change rather than promising a crisp switch. If there is no clear response, disconnect and check pins, polarity, strips and the isolated LDR readings with the instructor. Do not raise the supply voltage. Junction dots connect wires. Functional terminal labels are not a physical package pinout.Automatic night-lightOne two-AAA pack · dots = junctions · functional terminals, not a physical pinout+−2×AAA+3 V0 V / −10 kΩLDR220 ΩAKRed LEDBCE2N2222 NPNVerify actual pinsLED: A = anode, K = cathode. Disconnect both pack leads before changing connections.

Use the same named connections on your breadboard. Verify which holes share a strip and any power-rail breaks. The transistor drawing labels electrical functions, not left-to-right lead order; the actual manufacturer's package diagram controls placement. Crossing wires without a dot are not connections.

Walk me through the build
  1. Disconnect both pack leads. Measure the isolated LDR in bright light and under an opaque cover. Identify the 10 kΩ and 220 Ω resistors, LED polarity and the actual transistor's B/C/E pins. Never infer pin order from a flat face or a similar part number.
  2. Positive → 10 kΩ → base node B. LDR from B → negative. The 10 kΩ must remain in the positive base-feed path.
  3. Make the separate LED path: positive → 220 Ω → LED anode A; LED cathode K → collector C. Emitter E → negative. Keep the 220 Ω in series with the LED at all times.
  4. Have the instructor check it before power. With the approved nominal 3 V pack, compare a flashlight on the LDR with shading only the LDR. Do not move wiring while powered.
  5. Actual sensitivity depends on the LDR, transistor and room light. Both states may still glow; measure the change rather than promising a crisp switch. If there is no clear response, disconnect and check pins, polarity, strips and the isolated LDR readings with the instructor. Do not raise the supply voltage.
What should I measure?
  1. Use DC volts with leads in V/Ω and COM. Record the loaded source voltage, B-to-E voltage and the 220 Ω resistor drop in light and shade.
  2. Use the same lamp distance and opaque cover for repeatable tests. Record actual signs/polarity in your notebook; use the resistor-drop magnitude for current.
  3. Keep the LED from illuminating its own LDR, which can change the response. Disconnect both pack leads before any Ω test or wiring correction.
Help with the calculation and a sample
  1. LED current in mA = measured voltage across its 220 Ω resistor ÷ that resistor's measured isolated Ω × 1000. Do not use the whole battery voltage: the LED and other devices also have voltage drops.
  2. Compare the calculated LED current in light and shade. Explain the separate sensor/base and collector/LED current paths.
  3. The transistor loads the sensor circuit. Do not use an unloaded-divider formula as an exact base-voltage prediction, or assume a fixed 0.7 V turn-on voltage, fixed gain or exact LED drop.

Sample numbers only: Illustration only: 0.22 V across a measured 220 Ω resistor gives 1.00 mA; 0.88 V gives 4.00 mA. These are not required sensor readings or guaranteed switching points.

Explain in your notebook: Why does a lower LDR resistance pull this base node toward negative, and why can the LED branch carry a different current than the sensor branch?

Record the build, conditions, readings and units. This page does not automatically check, grade, submit or save bonus answers. A diagram or calculated value cannot certify the physical wiring.

Finish: disconnect the pack, leave the station safe, and keep your core build readings.

Link to this optional build
Reverse the response · One extra 10 kΩ resistor2 · A light-activated indicator

Make the LED generally get brighter in light instead of shade, while keeping a protected base feed.

Parts beyond the pack, board, jumpers and meter

  • Verified 2N2222-family NPN transistor (identify the exact maker/package and B, C, E pins)
  • LDR / photoresistor
  • Red LED
  • 220 Ω LED resistor, ¼ W or verified suitable rating
  • Two 10 kΩ resistors: one divider resistor and one separate base-series resistor

Predict first: What happens to the sensor-junction voltage when the upper LDR resistance falls? Why must the extra base resistor remain?

Show the electrical wiring schematic
Disconnect and take apart the night-light control branch. Keep its LED/220 Ω/collector branch only if it has been checked. Verify the actual B/C/E pins again when repositioning the transistor. Positive → LDR → sensor junction T. Connect one 10 kΩ resistor from T → negative. Connect the SECOND 10 kΩ resistor from T → base B. Do not connect the LDR directly to B: the extra resistor limits the base feed even when the LDR becomes very low resistance. Make the separate LED path: positive → 220 Ω → LED anode A; LED cathode K → collector C. Emitter E → negative. Keep the 220 Ω in series with the LED at all times. Obtain an instructor check, then compare bright light and shade at nominal 3 V. Use only the flashlight/cover while powered. The actual response can be gradual; disconnect for every wiring change. Junction dots connect wires. Functional terminal labels are not a physical package pinout.A light-activated indicatorOne two-AAA pack · dots = junctions · functional terminals, not a physical pinout+−2×AAA+3 V0 V / −LDR10 kΩ220 ΩAKRed LEDBCE2N2222 NPNVerify actual pins10 kΩ baseTLED: A = anode, K = cathode. Disconnect both pack leads before changing connections.

Use the same named connections on your breadboard. Verify which holes share a strip and any power-rail breaks. The transistor drawing labels electrical functions, not left-to-right lead order; the actual manufacturer's package diagram controls placement. Crossing wires without a dot are not connections.

Walk me through the build
  1. Disconnect and take apart the night-light control branch. Keep its LED/220 Ω/collector branch only if it has been checked. Verify the actual B/C/E pins again when repositioning the transistor.
  2. Positive → LDR → sensor junction T. Connect one 10 kΩ resistor from T → negative.
  3. Connect the SECOND 10 kΩ resistor from T → base B. Do not connect the LDR directly to B: the extra resistor limits the base feed even when the LDR becomes very low resistance.
  4. Make the separate LED path: positive → 220 Ω → LED anode A; LED cathode K → collector C. Emitter E → negative. Keep the 220 Ω in series with the LED at all times.
  5. Obtain an instructor check, then compare bright light and shade at nominal 3 V. Use only the flashlight/cover while powered. The actual response can be gradual; disconnect for every wiring change.
What should I measure?
  1. With power disconnected, record isolated LDR resistance in bright light and shade.
  2. With the checked build powered, measure T-to-negative, B-to-E and the 220 Ω drop for both light conditions. T and B are different nodes separated by the base-series resistor.
  3. Disconnect before correcting anything. If the indicator stays on or off, check the light range, both 10 kΩ placements, transistor pins and LED polarity rather than increasing voltage.
Help with the calculation and a sample
  1. LED current in mA = measured voltage across its 220 Ω resistor ÷ that resistor's measured isolated Ω × 1000. Do not use the whole battery voltage: the LED and other devices also have voltage drops.
  2. Calculate the base-series current from its own measured 10 kΩ drop divided by its measured resistance. It is not the whole LDR/divider current.
  3. The transistor loads the sensor circuit. Do not use an unloaded-divider formula as an exact base-voltage prediction, or assume a fixed 0.7 V turn-on voltage, fixed gain or exact LED drop.

Sample numbers only: Sample only: a 0.80 V drop across a measured 10,000 Ω base-series resistor gives 0.080 mA. A separate 0.66 V across 220 Ω gives 3.00 mA in the LED path; these are different branch currents.

Explain in your notebook: How does moving the LDR to the positive side reverse the response? What does the added base-series resistor protect against?

Record the build, conditions, readings and units. This page does not automatically check, grade, submit or save bonus answers. A diagram or calculated value cannot certify the physical wiring.

Finish: disconnect the pack, leave the station safe, and keep your core build readings.

Link to this optional build
A familiar potentiometer becomes a rheostat3 · Turn-the-knob LED dimmer

Adjust the LED brightness without removing its fixed current-limiting resistor.

Parts beyond the pack, board, jumpers and meter

  • One suitable approximately 5 kΩ potentiometer from Chapter 4
  • Red LED
  • 220 Ω fixed resistor, ¼ W or verified suitable rating

Predict first: Will adding resistance in this series path increase or decrease LED current? Is knob position likely to give a linear brightness change?

Show the electrical wiring schematic
Disconnect both pack leads. Identify the pot ends A/B and wiper X with an isolated Ω test. Check the actual pot's track, wiper-current and power ratings with the instructor; skip this build if suitability is unknown. Tie wiper X to the unused end B. Use A and the joined X/B node as the two rheostat terminals. Start at the maximum measured resistance between those terminals. Wire positive → 220 Ω → LED anode A; LED cathode K → pot end A; joined pot X/B → negative. LED A and pot A are terminal names on different parts, not a shared connection. Have the instructor check the circuit. Power from the approved nominal 3 V pack and turn only the insulated knob slowly. The fixed 220 Ω stays in series even at the pot's minimum-resistance setting. Junction dots connect wires. Functional terminal labels are not a physical package pinout.Turn-the-knob LED dimmerOne two-AAA pack · dots = junctions · functional terminals, not a physical pinout+−2×AAA+3 V0 V / −220 ΩAKRed LEDABX5 kΩ potX tied to BLED: A = anode, K = cathode. Disconnect both pack leads before changing connections.

Use the same named connections on your breadboard. Verify which holes share a strip and any power-rail breaks. The transistor drawing labels electrical functions, not left-to-right lead order; the actual manufacturer's package diagram controls placement. Crossing wires without a dot are not connections.

Walk me through the build
  1. Disconnect both pack leads. Identify the pot ends A/B and wiper X with an isolated Ω test. Check the actual pot's track, wiper-current and power ratings with the instructor; skip this build if suitability is unknown.
  2. Tie wiper X to the unused end B. Use A and the joined X/B node as the two rheostat terminals. Start at the maximum measured resistance between those terminals.
  3. Wire positive → 220 Ω → LED anode A; LED cathode K → pot end A; joined pot X/B → negative. LED A and pot A are terminal names on different parts, not a shared connection.
  4. Have the instructor check the circuit. Power from the approved nominal 3 V pack and turn only the insulated knob slowly. The fixed 220 Ω stays in series even at the pot's minimum-resistance setting.
What should I measure?
  1. At three knob positions, record LED brightness and the DC-voltage drop across the fixed 220 Ω resistor.
  2. Disconnect both pack leads before measuring the rheostat resistance; isolate it from the rest of the circuit for the Ω reading, then recheck wiring before repowering.
  3. A large 5 kΩ range may leave much of the travel dim. That is useful evidence, not permission to remove the fixed resistor.
Help with the calculation and a sample
  1. LED current in mA = measured voltage across its 220 Ω resistor ÷ that resistor's measured isolated Ω × 1000. Do not use the whole battery voltage: the LED and other devices also have voltage drops.
  2. Use the measured pot voltage drop × the calculated series current in amperes to estimate power in the active pot segment. This alone does not verify its wiper/track rating.
  3. Do not apply source V ÷ (220 + pot Ω) as an exact LED-current result: the LED has a nonlinear voltage drop.

Sample numbers only: Sample: 0.44 V across a measured 220 Ω fixed resistor gives 2.00 mA. Increasing the rheostat resistance should reduce that current in this circuit.

Explain in your notebook: Why is the fixed resistor still needed when the knob reaches minimum resistance? Why does a three-terminal pot act as a two-node rheostat here?

Record the build, conditions, readings and units. This page does not automatically check, grade, submit or save bonus answers. A diagram or calculated value cannot certify the physical wiring.

Finish: disconnect the pack, leave the station safe, and keep your core build readings.

Link to this optional build
Two parallel control paths · Simple OR logic4 · Either button turns it on

Let either of two buttons turn on the same LED. Pressing both is allowed; it must not short the pack.

Parts beyond the pack, board, jumpers and meter

  • Two normally-open low-voltage pushbuttons or switches, if available
  • Red LED
  • 220 Ω resistor, ¼ W or verified suitable rating

Predict first: Predict the LED state for neither button, button 1 only, button 2 only and both buttons.

Show the electrical wiring schematic
Disconnect the pack. Use continuity to identify each switch's two electrical terminals. A four-leg tactile switch often has permanently paired legs: do not assume all four are separate contacts. Skip this build if no suitable switches are available. Wire positive → 220 Ω → LED anode A. Connect LED cathode K to a new node S. Wire switch 1 between S and negative, and switch 2 between the SAME S and negative. The two switches are parallel with each other, not across the pack. Get the instructor's check. Power at nominal 3 V and operate only the buttons. Trace the protected path through the resistor and LED for every closed-button combination. Junction dots connect wires. Functional terminal labels are not a physical package pinout.Either button turns it onOne two-AAA pack · dots = junctions · functional terminals, not a physical pinout+−2×AAA+3 V0 V / −220 ΩAKRed LEDS1S2SLED: A = anode, K = cathode. Disconnect both pack leads before changing connections.

Use the same named connections on your breadboard. Verify which holes share a strip and any power-rail breaks. The transistor drawing labels electrical functions, not left-to-right lead order; the actual manufacturer's package diagram controls placement. Crossing wires without a dot are not connections.

Walk me through the build
  1. Disconnect the pack. Use continuity to identify each switch's two electrical terminals. A four-leg tactile switch often has permanently paired legs: do not assume all four are separate contacts. Skip this build if no suitable switches are available.
  2. Wire positive → 220 Ω → LED anode A. Connect LED cathode K to a new node S.
  3. Wire switch 1 between S and negative, and switch 2 between the SAME S and negative. The two switches are parallel with each other, not across the pack.
  4. Get the instructor's check. Power at nominal 3 V and operate only the buttons. Trace the protected path through the resistor and LED for every closed-button combination.
What should I measure?
  1. Record the LED state and voltage across the 220 Ω resistor in all four button combinations.
  2. Compare one-button and both-button resistor drops under similar battery conditions. Only measure in DC volts while powered; disconnect for continuity or wiring changes.
Help with the calculation and a sample
  1. LED current in mA = measured voltage across its 220 Ω resistor ÷ that resistor's measured isolated Ω × 1000. Do not use the whole battery voltage: the LED and other devices also have voltage drops.
  2. Calculate LED current for each state. The two buttons provide alternate returns for one LED path, so two pressed buttons do not create two separate LED loads.

Sample numbers only: Expected logic: 00 = off, 10 = on, 01 = on, 11 = on. Sample resistor drop 0.88 V gives 4.00 mA whether one or both return paths are closed, if the other operating conditions stay the same.

Explain in your notebook: Why does either button complete the circuit? Why is putting these buttons directly across the pack a dangerous wiring error instead of OR logic?

Record the build, conditions, readings and units. This page does not automatically check, grade, submit or save bonus answers. A diagram or calculated value cannot certify the physical wiring.

Finish: disconnect the pack, leave the station safe, and keep your core build readings.

Link to this optional build
Turn the three-resistor lesson into a tiny lighting system5 · Two lights, two independent paths

Build two independently switched LED branches across one pack. Open one branch and keep the other light operating.

Parts beyond the pack, board, jumpers and meter

  • Two red LEDs
  • Two 220 Ω resistors, each ¼ W or verified suitable rating
  • Two suitable low-voltage switches, if available

Predict first: Predict what happens to the other LED and source current when you switch one complete branch off.

Show the electrical wiring schematic
Disconnect both pack leads. Verify LED polarity, resistor values and the actual switch contacts. Skip this build if suitable switches or the second LED/resistor are unavailable. Branch 1: positive → its own 220 Ω resistor → LED 1 anode; LED 1 cathode → switch 1 → negative. Branch 2: positive → a SECOND 220 Ω resistor → LED 2 anode; LED 2 cathode → switch 2 → negative. Each LED needs its own series resistor: do not share one resistor ahead of both LEDs. Have the instructor check the two source nodes and protected branches. Power at nominal 3 V and test all four switch combinations. Do not add or parallel another battery pack. Junction dots connect wires. Functional terminal labels are not a physical package pinout.Two lights, two independent pathsOne two-AAA pack · dots = junctions · functional terminals, not a physical pinout+−2×AAA+3 V0 V / −220 ΩAKLED 1S1220 ΩAKLED 2S2LED: A = anode, K = cathode. Disconnect both pack leads before changing connections.

Use the same named connections on your breadboard. Verify which holes share a strip and any power-rail breaks. The transistor drawing labels electrical functions, not left-to-right lead order; the actual manufacturer's package diagram controls placement. Crossing wires without a dot are not connections.

Walk me through the build
  1. Disconnect both pack leads. Verify LED polarity, resistor values and the actual switch contacts. Skip this build if suitable switches or the second LED/resistor are unavailable.
  2. Branch 1: positive → its own 220 Ω resistor → LED 1 anode; LED 1 cathode → switch 1 → negative.
  3. Branch 2: positive → a SECOND 220 Ω resistor → LED 2 anode; LED 2 cathode → switch 2 → negative. Each LED needs its own series resistor: do not share one resistor ahead of both LEDs.
  4. Have the instructor check the two source nodes and protected branches. Power at nominal 3 V and test all four switch combinations. Do not add or parallel another battery pack.
What should I measure?
  1. For each switch combination, record the loaded pack voltage, both resistor drops and which LEDs are lit.
  2. One open branch should leave the other branch operating. Small brightness changes can reflect source sag or contacts; record them rather than claiming an ideal battery.
  3. Disconnect before moving parts or using Ω mode. Keep this as a small LED build, not a model for connecting building wiring.
Help with the calculation and a sample
  1. Calculate I1 = measured drop across resistor 1 ÷ its isolated Ω × 1000, and likewise I2. Then calculate the predicted source total I1 + I2 in mA.
  2. Use the drop across each LED's own resistor, not the total source voltage. LEDs are nonlinear loads, so these are not two plain 220 Ω branches.

Sample numbers only: Illustration: 0.88 V / 220 Ω = 4.00 mA in branch 1; 0.66 V / 220 Ω = 3.00 mA in branch 2. Both on: predicted total 7.00 mA. Opening branch 1 leaves branch 2 near its own operating current, subject to source changes.

Explain in your notebook: How do the common source nodes make the branches independent? Why does each LED need its own current-limiting resistor?

Record the build, conditions, readings and units. This page does not automatically check, grade, submit or save bonus answers. A diagram or calculated value cannot certify the physical wiring.

Finish: disconnect the pack, leave the station safe, and keep your core build readings.

Link to this optional build
Transistor and LDR background

An LDR's resistance normally falls in brighter light. An NPN's base-emitter drive controls its collector current. The base-control and LED paths are different.

Identify your actual component before following any pin diagram. Compare the manufacturer's PN2222A drawing with its P2N2222A drawing: similar names need not use the same lead order. Neither diagram automatically identifies an unmarked kit transistor.

Optional sensor reading: SparkFun photocell background. Its microcontroller/5 V example is not the wiring or supply for these nominal 3 V classroom builds.