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Voltage Divider Lab

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About This MicroSim

A voltage divider is two resistors in series. The output is taken from the point between them, and it is always a fraction of the input:

\[ V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2} \]

Here \(R_1\) connects to the input voltage and \(R_2\) connects to ground. Clock builders use dividers to shrink a 5 V signal down to something a Pico pin can read, because the Pico's GPIO pins are not 5 V tolerant.

The bar-graph voltmeter on the right has three zones:

  • Green (up to 3.3 V): safe for a Pico pin.
  • Amber (3.3 to 3.6 V): a little above the Pico's 3.3 V logic level. The pin will survive, but there is no safety margin, so pick a slightly larger \(R_1\).
  • Red (above 3.6 V): too high. This can damage the pin.

The chapter's worked example (5 V, 1 kΩ, 2 kΩ) gives 3.33 V, which lands just inside the amber band. Try nudging \(R_1\) up one step to see how easy it is to add margin. The resistor sliders step through the standard E24 values from 100 Ω to 10 kΩ, so every setting is a resistor you could actually buy.

How to Use

  • Move Input voltage Vin to set the voltage at the top of the divider (0 to 5 V).
  • Move R1 (top) and R2 (bottom) to choose standard resistor values from 100 Ω to 10 kΩ.
  • Read Vout on the bar graph and the verdict under it: Safe for Pico, Just over 3.3 V: no margin, or Too high!
  • The formula line along the bottom shows the calculation with your numbers, and the Current readout shows how much current flows through the two resistors.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

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<iframe src="https://dmccreary.github.io/clocks-and-watches/sims/voltage-divider-lab/main.html"
        height="497px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objectives

By the end of this activity, students will be able to:

  • Apply the voltage divider formula to calculate an output voltage.
  • Evaluate whether a divider output is safe for a 3.3 V Pico input pin.
  • Choose resistor values that bring a 5 V signal into the safe range with some margin.

Grade Level

9-12 (Clocks and Watches with AI)

Duration

10-15 minutes

Prerequisites

  • Ohm's Law (V = I × R).
  • Knowing that the Pico uses 3.3 V logic and its pins are not 5 V tolerant.

Activities

  1. Check the worked example (3 min): Set Vin = 5.0 V, R1 = 1.0 kΩ, R2 = 2.0 kΩ. Students calculate Vout by hand first, then compare with the MicroSim. Discuss why 3.33 V lands in the amber band.
  2. Find the ratio (4 min): Students find three different R1 and R2 pairs that give between 3.0 and 3.3 V from 5 V. What do the pairs have in common? (The ratio R2 / (R1 + R2) is about 0.6 to 0.66.)
  3. Evaluate designs (4 min): Give students two designs that both give 3.2 V, one with 100 Ω resistors and one with 10 kΩ resistors. Using the Current readout, which wastes less power? Which would you choose for a battery clock?
  4. Swap the resistors (2 min): What happens if R1 and R2 are wired the wrong way round? Students predict, then swap the slider values to check.

Assessment

  • Students correctly calculate Vout for any Vin, R1, and R2 shown in the MicroSim.
  • Students design a divider that turns 5 V into 3.0 to 3.3 V and justify their resistor choice.
  • Students explain why larger resistor values waste less current but give the same output ratio.

References

  1. Voltage divider (Wikipedia) - Derivation of the divider formula with examples.
  2. E series of preferred numbers (Wikipedia) - Why resistors come in standard values such as 1.0k, 2.0k, and 2.2k.
  3. RP2040 datasheet - The electrical limits for the Pico's GPIO pins.
  4. Logic level (Wikipedia) - What 3.3 V and 5 V logic levels mean.