Ohm's Law Activity for Students: 2-Lesson Plan with LEDs

A good Ohm's law activity for students does not start by writing the formula on the board and asking them to memorise it. It starts with a prediction that students then test in their own circuit. The lesson plan below runs over two lessons (2 × 40 minutes) and has four steps: predict (which resistor makes the LED brighter?), calculate (how many milliamps flow, using V = I × R?), observe and measure (how bright is the LED, what voltage does the Arduino read?) and compare (why are the prediction, the calculation and the measurement close but not identical?). Once students have used the formula with their own hands, they remember what V, I and R mean from experience rather than from a list.

We are not repeating the theory here: the formula itself, the units and the power calculation are in what is Ohm's law, and how a resistor and an LED work is covered in what is a resistor and what is an LED. This post focuses on what you take into the classroom: learning objectives, a materials table for a class of 24, a minute-by-minute plan for both lessons, a short Arduino measuring sketch that compiles, a measurement table template, a 6-question worksheet with answer key, common misconceptions, and ideas for differentiating between fast and slow groups.

How the activity works: predict, calculate, measure, compare

The point of this order is to turn the formula from an answer machine into a checking tool. If a student makes a wrong prediction in the first minute (many will say “the bigger resistor is stronger, so it makes the LED brighter”), that is not a problem, it is the material of the lesson. Seeing their own prediction fail with their own eyes in the second lesson sticks far better than reading the same fact from the board. Working with a single LED color (red) throughout is a deliberate choice too: when the resistor is the only variable, students can clearly see what changes what. The other colors are kept as extension tasks for fast groups.

Learning objectives and prior knowledge

• Reads the value of a four-band resistor from its color code and checks it with an app.
• Calculates the third quantity in V = I × R when two are known, and converts milliamps and kilohms correctly.
• Explains that an LED needs a certain forward voltage to light up, and that the voltage across the resistor is the supply voltage minus that value.
• Shows by experiment that a larger resistor means less current and a dimmer LED, and gives at least two reasons for the gap between calculated and measured values.
• Concludes from the measurement table that an LED does not behave like a resistor: its voltage changes very little while the current changes several times over.

As prior knowledge, it is enough for students to have heard the words voltage, current and resistance. The plan also works with a class that has never seen the formula; you just stretch the triangle section in the first lesson by a few minutes. Knowing how to connect two parts in series on a breadboard makes things much easier. If they do not, show the internal connection diagram from what is a breadboard as a five-minute warm-up in the lesson before. No Arduino experience is needed: you can upload the sketch for the second lesson in advance, and students only open the Serial Monitor and read the numbers. The plan was written with roughly 12 to 15 year olds in mind, but the same flow works in vocational courses and introductory electronics workshops.

Materials list: per group and for a class of 24

Option without a multimeter: the Arduino Uno's analog input turns 0 to 5 V into a number between 0 and 1023, so the board is a 5 V supply and a simple voltmeter at the same time. The second lesson of this plan is built exactly that way; a multimeter is only a bonus for groups who want to answer “how many volts does the 5 V pin really give?” or measure current directly. If you have a few multimeters, you can do the current measurement as a demonstration at your desk; the right range and where to plug the probes are explained in how to use a multimeter. If you have neither Arduinos nor multimeters, the first lesson runs unchanged, and in the second lesson you only do the brightness experiment and leave the measurement columns empty.

Lesson 1 (40 minutes): prediction and calculation

Do not give away the answer to the warm-up question; collect the prediction cards and keep them for the second lesson. Some students will say “1 kΩ is bigger, so it gives more energy”, others will predict correctly and write “a resistor holds back current” as the reason. When both groups' reasons are read out again at the end of lesson 2, the discussion starts on its own. In the color code section, students read the bands and then check the result on the phone with the Color code tool; the tool is not there to do the reading but to confirm the student's own reading. So set the rule from the start: write it on paper first, then check.

In the triangle section, do not make them memorise the formula in three separate forms; one triangle is enough. When you cover the unknown with a finger, the two remaining letters are either side by side (multiply) or one above the other (divide). Three examples we suggest: 5 V and 1 kΩ, how many mA (5 mA)? 2 V and 10 mA, how many ohms (200 Ω)? 20 mA and 150 Ω, how many volts (3 V)? Students check the result by entering two values in the Ohm's law tool. Units are the most common mistake: a student who writes 10 instead of 10 mA gets an absurd result like 0.2 Ω. To catch this, ask “does the result make sense?” out loud for every example.

The LED section is the key part of the activity. Tell students that an LED does not glow a little at any voltage like a bulb: a red LED passes almost no current below 1.6–1.7 V, and around 2 V the current rises very quickly. So about 2 V of the 5 V supply stays across the LED, the remaining 3 V drops across the resistor, and it is this 3 V that sets the current. Have them look up typical values per color in the LED voltages tool; it shows about 1.85 V for red, and in class we round it to 2 V to keep the arithmetic easy. That small gap makes a good question too: with 220 Ω, is the current 13.6 mA or 14.3 mA? Both mean “about 14 mA”. Then each group calculates the current for its four resistors, enters that current in the LED resistor tool and sees that the resistor it suggests is close to their own. The table below is the teacher's answer key; values are calculated with a 5 V supply.

• Exit question 1: Which passes more current, 220 Ω or 330 Ω? Justify it with a number.
• Exit question 2: In a 5 V circuit, if there are 2 V across the red LED, how many volts are across the resistor?
• Exit question 3: A blue LED has a higher forward voltage than a red one. With the same 220 Ω, does the blue LED's current go up or down?

Lesson 2 (40 minutes): brightness experiment and Arduino measurement

Starting the brightness experiment at 1 kΩ matters: going from the dimmest circuit to the brightest, students see the difference more clearly and their eyes do not tire. Give a shared brightness scale: 1 “barely visible”, 3 “easy to see in room light”, 5 “uncomfortable to look at”. Comparing different colors by eye is misleading, because the eye is more sensitive to green than to red or blue; at the same current a green LED can look brighter. So always compare brightness within one color. These two lessons also fit as an extra day in the 5-day Arduino workshop program; they sit well right after the workshop's first LED day.

Build the circuit and turn the Arduino into a voltmeter

1. With the USB cable unplugged, connect the Arduino's 5V pin to a row on the breadboard.
2. Plug one leg of the resistor into the same row and the other leg into an empty row.
3. Plug the red LED's long leg (anode) into the same row as the resistor's second leg, and its short leg (cathode) into another row.
4. Connect the LED's cathode row to the Arduino's GND pin.
5. Run a wire from the row where the resistor meets the LED's anode to the Arduino's A0 pin.
6. Plug in USB, open the Serial Monitor at 9600 baud and write the two values that arrive every second into the measurement table.
7. Unplug USB, swap the resistor for the next one and repeat step 6. You do not need to upload the sketch again.

1// Ohm kanunu etkinliği: LED ve direnç gerilimini ölç / Ohm's law activity: measure LED and resistor voltage
2// Devre / Circuit: 5V -> direnç / resistor -> A0 + LED anot / anode -> LED katot / cathode -> GND
3const float VCC = 5.00;      // 5V pinindeki gerilim, multimetre varsa ölçüp yaz / voltage on the 5V pin, measure it if you have a multimeter
4const int SENSE_PIN = A0;    // direnç ile LED'in birleştiği nokta / node where the resistor meets the LED
5const int SAMPLES = 20;      // ortalaması alınacak okuma sayısı / number of readings to average
6
7void setup() {
8  Serial.begin(9600);
9  Serial.println(F("V_LED (V)   V_R (V)"));
10}
11
12void loop() {
13  long sum = 0;
14  for (int i = 0; i < SAMPLES; i++) {
15    sum += analogRead(SENSE_PIN);         // 0-1023
16    delay(5);
17  }
18  float raw = sum / (float)SAMPLES;       // ortalama okuma / average reading
19  float vLed = raw * VCC / 1023.0;        // LED üzerindeki gerilim / voltage across the LED
20  float vRes = VCC - vLed;                // direnç üzerindeki gerilim / voltage across the resistor
21
22  Serial.print(vLed, 2);
23  Serial.print(F("        "));
24  Serial.println(vRes, 2);                // akımı kağıtta bul: I = V_R / R / find the current on paper: I = V_R / R
25  delay(1000);
26}

The sketch deliberately does not calculate the current: the Serial Monitor only prints the voltage across the LED and across the resistor, and students work out the current on paper with I = V_R / R. That way Ohm's law is used once more at the end of the lesson, this time with a real measurement, and the sketch never needs re-uploading when the resistor changes. analogRead() turns the voltage at A0 into a number from 0 to 1023; with a 5 V reference one step is about 4.9 mV. For a class that has not seen how this number is read, the analog reading example in what is a potentiometer is good preparation. Because the sketch averages 20 readings, the numbers do not jump around on screen. The 5 V coming from USB is usually between 4.7 and 5.1 V; if you have a multimeter, measure between 5V and GND once and enter it in the VCC line, which makes both columns a few percent more accurate.

The table students end up with looks roughly like this: as the resistor drops from 1 kΩ to 220 Ω, the current rises four to five times and the brightness score climbs from 1–2 to 4–5, yet the LED voltage rises by only a few tenths of a volt, typically staying between 1.8 and 2.0 V for a red LED. This is the most valuable moment of the lesson, because students see from their own numbers that an LED does not behave like a resistor: we apply Ohm's law to the resistor and treat the LED as a part that drops a roughly fixed voltage. A gap of a few percent between calculation and measurement is normal; let students find the reasons themselves. Expected answers: the USB voltage is not exactly 5 V, the resistors have ±5% tolerance, the LED's real forward voltage differs from our rounded 2.0 V, and one step of the analog reading is about 5 mV.

Worksheet: 6 questions and answer key

Common misconceptions and how to fix them in class

The first two misconceptions are the most stubborn. For “a resistor uses up current”, the multimeter demonstration is the most convincing route; without a multimeter, show that the two voltages on the Serial Monitor always add up to very nearly 5 V and explain that this is the voltage being shared in a series circuit. This is a natural lead-in to Kirchhoff's laws and can be given as homework to curious students. Disprove “a resistor isn't needed” with calculation rather than by burning an LED: seeing the current grow as R shrinks, and become undefined at zero, is striking enough.

Differentiation: fast and slow groups

• Fast group, series and parallel: Connect two 220 Ω in series (440 Ω, about 6.8 mA) and two 1 kΩ in parallel (500 Ω, about 6.0 mA). Have them predict which will be brighter first; the two come out almost the same, and that surprise sparks a good discussion. They can check the equivalent resistance in the Series / parallel tool. This tool is Premium; free users see it as a Sample calculation showcase, and calculating with their own values needs Premium. The formulas are in series and parallel resistor calculation.
• Fast group, a resistor instead of the LED: Remove the LED and fit a second 1 kΩ in its place; A0 reads about 2.5 V. With 220 Ω on top and 330 Ω below (where the LED was), A0 reads 5 × 330 / 550 = 3.0 V. Let them compare in their own tables how the voltage changes in proportion with the resistor, while across the LED it stayed almost constant.
• Fast group, change color: Have them repeat the measurement with a blue LED, test the calculated 7.7 mA at 220 Ω against the measurement, and explain through the resistor voltage why the blue LED draws less current. They can also tell from the measurement whether their green LED is the older type (about 2.2 V) or a pure green one (about 3.2 V).
• Slow group, ready-made circuit: Hand the group a breadboard with a 330 Ω resistor and a red LED already in place and the A0 wire connected. Students only swap the resistor, write down two voltages and do one operation: I = V_R / R.
• Slow group, step card: Put a card on the desk showing the V = I × R triangle and the line “5 V − LED voltage = resistor voltage”. A student who cannot yet read the color code first enters the colors in the Color code tool to get the value, then does the reading themselves on the second attempt.

Pre-lesson checklist

• The measuring sketch is uploaded to every Arduino and the Serial Monitor has been tested once at 9600 baud.
• USB cables are checked to be data cables (a charge-only cable does not show the board on the computer).
• Resistor bags are ready with no values written on them; the 100 Ω is on the teacher's desk.
• LED legs are not trimmed so the long leg can be recognised as the anode; every group has a spare red LED.
• Prediction card, prediction table, measurement table and worksheet are printed for 24 students.
• BlueGrays is installed on student phones and the Tools screen opens; the four tools used in class have been tried once.
• If a multimeter will be used, its battery and current-range fuse have been checked.

How to use BlueGrays in this activity

• Free tools: Color code, Ohm's law, LED resistor and LED voltages are among the free tools. They open from the Tools tab in the bottom menu. Students do the calculation on paper first, then check it with the tool; the tool is used to confirm their own work, not to hand them the answer.
• LED Blink project: LED Blink is completely free: circuit diagram, parts list, source code, block view with Show as blocks, and Understand the code are all open. In a third lesson, lighting the LED from a digital pin this time and choosing the resistor with their own calculation is the natural follow-up to the activity.
• Daily questions: There are about 400 questions across 6 levels; the topics include Electricity, and the question types include Calculation and Circuit questions. A new set arrives every day and takes about 3 minutes. For the week between the two lessons, a “solve your set every day” task gives useful revision. The app has no class dashboard that shows scores to the teacher; if you want to keep track, ask students to bring their weekly results to class.
• If you need Premium: If you want to use Premium tools such as Series / parallel or Premium projects with the whole class, we explain in Premium codes for a class how handing out codes from an educator package works instead of a subscription per student. The whole of this activity can be done with the free tools.

After two lessons, what students have is not a memorised formula but their own prediction card, a table they calculated and measured themselves, and their own written answer to “why didn't my prediction hold?”. The next time Ohm's law comes up, the numbers in that table are what they will remember. If you install the BlueGrays app on your own phone the day before and try the four free tools and the LED Blink project once, you will know in advance what students will see on their screens.