Teach lesson
When does an LED light up?
Change voltage, observe a real green LED and use resistor measurements to investigate current.
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Learning Outcomes
Relate supply changes to calculated LED current using six readings.
Distinguish a camera observation from an electrical measurement and an estimate.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
A light with a hidden story
5 min
A charger’s tiny indicator looks simple: it is either lit or dark. But what happens while its supply voltage rises? You will investigate one green LED, keeping its 1 kΩ resistor in place, and compare what the camera shows with an electrical measurement.
Before opening Hive, predict how the current and visible light will change between 1 and 5 V. Give a reason; your prediction does not have to be correct.
Investigate the light
18 min
Read the circuit
The lab button opens a wired circuit on real equipment. A branch is one path containing a resistor and an LED. The LED has an anode (A) and cathode (K); their orientation is already set. Select the DC Power tab to read or change the positive supply setting; select Multimeter to read the voltage. Click Perform Measurement after every change and wait for the result and rack photographs to refresh. Changing a setting alone does not produce a new result. To adjust voltage with the keyboard, focus the voltage knob: Left/Right changes the step; Up/Down changes the value. Check the displayed voltage before measuring.
The multimeter reads the voltage across the resistor, called $V_R$. Record VDC in volts; divide an mV reading by 1,000. For example, 240 mV = 0.240 V. If the display shows uV (microvolts, µV), divide by 1,000,000: 300 uV = 0.000300 V. For a 1 kΩ resistor, $I\,\text{(mA)}=V_R\,\text{(V)}$. This calculates current from measured voltage and the stated resistance.
Use the camera photographs for observations of the real lights. The drawing’s Estimated glow is an illustration of the last result, not a light measurement; turn it off during camera comparisons. A light hidden by glare is unclear, not necessarily off. The rack also contains status lights. If the circuit LED is unclear, keep the same circuit and voltage and press Perform measurement again. Wait for the result and camera photographs to refresh, then inspect the LED again. Briefly note the first and repeat readings and any change in visibility in your written answers; use the repeat result in the table. If the LED is still unclear, choose Unclear; do not judge it from another light or keep repeating until you obtain the answer you expected.
Read the equipment name shown under each result: successive measurements can use different physical Hives. Small differences may therefore reflect different components. A result labelled as a previous recording is not an independent new measurement.
Keep the circuit protected
Keep the supplied wiring and resistors in place. Use only the positive 0–5 V supply, between 1 and 5 V in this activity. Do not connect an LED directly to the supply. Leave the other supplies and function generator unchanged. The meter is already connected across a resistor: leave it in DC voltage mode, never current or resistance mode.
Read the voltage across the resistor to calculate the current through that branch. A and K identify the LED terminals.
Before you measure
Before you measure. The meter is connected across the resistor. In your own words, what does it measure, and how can you use that reading to calculate the current through the LED? Use the explanation above to check your reasoning before opening the lab.
Open the green LED
Open the green LED
Open the green-LED circuit below. It starts at 1 V with the meter across its 1 kΩ resistor.
Follow the six voltage settings in the Results table below, changing only the positive supply. Click Perform Measurement at each setting.
For the final reading, set the positive supply back to 3 V and leave all other settings unchanged. Record the new result without expecting an exact match.
Finish the Hive session when the table is complete.
Available in a class session
Results
Complete six rows in order: 1, 2, 3, 4, 5 V, then return to 3 V. After each measurement, enter V_R in volts, calculate I in mA, and choose one camera observation. The last row repeats 3 V; leave any extra rows blank.
| Pass | Supply setting V | Resistor voltage V | Calculated current mA | Camera observation |
|---|---|---|---|---|
Explain the evidence
12 min
Use your first 3 V row. Give the current in mA and show how you calculated it from V_R and 1 kΩ.
What happened to the current as the supply rose? Support your explanation with two rows, and say whether equal 1 V steps produced equal current changes.
Compare your two 3 V rows. What do they let you check, and why can neither a dark camera image nor the drawing alone prove that the current is exactly zero?
An indicator is hard to see in a photograph. Which electrical quantity would you check before deciding it is not conducting, and how would you obtain it here?
Submit
After all measurements, leave Hive and return here. Mark the lab practice as done, check every results row and required answer, then submit the activity.
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