Lab 5.1: LED Current Control · HW-101 · Virtus Cyber Academy Classroom

~90 min. Pick the correct current-limiting resistor for each LED color at two different supply voltages. Verify by measuring the actual current.

Goal: master LED-resistor sizing; understand forward voltage drop and current limiting

Estimated time: 90 minutes

Prerequisites: lab 1.1 (multimeter); lab 3.1 (breadboard)

Steps

Step 1: Look up forward voltages (10 min)

Typical forward voltages by color (read your kit's LED datasheet if you want precise values):

Red, orange, yellow: ~1.8-2.2 V
Green: ~2.0-3.0 V (depending on type; older green = 2 V, newer high-brightness = 3 V)
Blue, white: ~3.0-3.5 V

Note these in your notebook

Step 2: Calculate resistors for 5 V supply (15 min)

For each LED color: R = (Vsupply - Vforward) / I_target. Target current 15 mA (a comfortable brightness; below the typical 20 mA max)

Example (red LED, 5 V supply, 15 mA): R = (5 - 2) / 0.015 = 200 Ω. Use the nearest available value: 220 Ω

Step 3: Calculate resistors for 9 V supply (10 min)

Same calculations with 9 V. Notice the resistors are larger (because more voltage to drop)

Step 4: Build all six circuits (30 min)

You have three LEDs × two supplies = six circuits. Build each one on the breadboard, measure the actual current, record. Use the multimeter set to DC mA, inserted in series with each circuit

Tabulate: circuit | predicted current | measured current | observed brightness

Step 5: Verify Ohm's law in each circuit (15 min)

For each circuit, measure the voltage across the resistor and the voltage across the LED. Sum should equal Vsupply. Resistor voltage / R = current; compare to your direct current measurement

Step 6: Try too-small a resistor (briefly!) (10 min)

WITH SAFETY: pick the red LED with the 9 V supply. Try with a 100 Ω resistor (too small; predicted current would be 70 mA). The LED is brighter. Measure the current. Then immediately disconnect, before the LED overheats

Repeat your reading. The LED may have momentarily exceeded its rating; you should not have damaged it but you saw what could happen. Resistors and LEDs are forgiving over short durations; over long durations or with much-larger over-current, they fail

Expected output

Tabulated calculations for six LED/supply combinations
Measured currents matching predicted (within meter accuracy and resistor tolerance)
Brief over-current demonstration (with safety)

Common pitfalls

Forgetting to subtract Vforward: R = Vsupply / I is wrong; the LED drops 2 V (or whatever); only the remaining voltage is across the resistor
Using a too-small resistor by mistake: even if it doesn't burn out the LED, the LED runs hot and the lifetime drops dramatically
Different LED brightness with different colors: a red LED at 15 mA looks brighter than a blue LED at 15 mA, because eye response varies with color. Brightness is not just current

Stretch (optional)

Drive an LED through an even-larger resistor (say, 10 kΩ at 9 V). The current is ~0.7 mA. The LED is dim but still visible. Modern high-efficiency LEDs are usable at much lower currents than the data sheets suggest
Plot brightness (subjectively rated 1-10) vs current. The brightness response is roughly logarithmic; 30 mA is not twice as bright as 15 mA

Lab 5.1 v0.1. The LED-resistor pairing is the most common analog calculation in microcontroller projects.