~90 min. Build a voltage divider that drops 5V to 3.3V using two resistors. Measure the actual output; compare to predicted.
Goal: build and verify a voltage divider; understand the math; build intuition for why this circuit is so common
Estimated time: 90 minutes
Prerequisites: lab 1.1 complete; familiar with Ohm's law
Steps
Step 1: Pick resistor values (15 min)
You want Vout = 5 × (R2 / (R1 + R2)) = 3.3 V. So R2 / (R1 + R2) = 0.66. One clean choice: R1 = 1 kΩ, R2 = 2 kΩ (= 3.3 / 5 × (1 + 2)). Verify: 2/(1+2) = 0.667; × 5 V = 3.33 V
If your kit doesn't have a 2 kΩ, use 2.2 kΩ. Output is 5 × 2.2/(1+2.2) = 3.44 V; close enough for the lab
Step 2: Build on the breadboard (20 min)
Wire +5 V supply (USB or batteries; ~5 V) to R1; R1 to a tie-strip (this is Vout); the same tie-strip through R2 to ground. Use color discipline (red for +; black for ground; yellow for Vout)
Step 3: Measure Vout (10 min)
Set the multimeter to DC voltage. Probe Vout to ground. Note the reading. Compare to your predicted 3.33 V. The difference is mostly resistor tolerance (±5% typical)
Step 4: Measure current through the divider (15 min)
Break the circuit between R1 and R2. Insert the meter in series. Current should be ~1.67 mA (5 V / 3 kΩ). Record. Higher-current dividers waste more power; lower-current dividers are more sensitive to loading
Step 5: Add a "load" and see Vout change (20 min)
Connect a 10 kΩ resistor (or your kit's LED with a current-limit resistor) from Vout to ground. This is a "load" on the divider's output. Measure Vout again. Notice it has dropped. Why: the load is now in parallel with R2, lowering the effective R2 and dropping the output
This is the "stiffness" concept: a divider built with smaller resistors is "stiffer" (less affected by loads); a divider built with larger resistors is more efficient (lower current) but less stiff
Step 6: Document (10 min)
In your lab notebook: record the resistor values you used; the predicted Vout; the measured Vout (no load and with load); the current through the divider; one sentence on what "stiffness" means in this context
Expected output
- Working voltage divider on the breadboard
- Measured vs predicted Vout (within 5%)
- Observation of the loading effect
Common pitfalls
- Mixing up R1 and R2 in the formula: R2 is the bottom resistor (between Vout and ground). Vout = Vin × R2 / (R1 + R2). Getting this backward gives you Vin × R1 / (R1+R2), which is also a valid divider but with the opposite ratio
- Power supply that is not actually 5 V: a USB cable from a laptop might supply 4.8-5.2 V depending on the laptop's regulation. Measure your actual supply first; then predict Vout from the actual Vin
Stretch (optional)
- Build a three-resistor divider with two tap points (e.g., 5 V → 3.3 V → 1.65 V). Verify both Vouts
- Try a divider where R1 is a fixed resistor and R2 is a potentiometer. Turning the pot varies Vout from 0 to (almost) Vin. This is how a volume knob works
Lab 2.1 v0.1. The voltage divider returns in every analog circuit you read for the rest of your career.