Project 04 · Voltage divider

Ohm’s law as an instrument.

A known 1kΩ, an unknown resistor, and A0. Students stop treating resistance as a textbook number and start measuring it.

2 × 45 minBest physics crossoverGuard against divide-by-zero

Known and unknown resistors in a divider — R1 known between 5V and A0. R2 unknown between A0 and GND.

Vout = Vin × R2 / (R1 + R2), rearranged to R2 = R1 × (Vin / Vout − 1). Arduino reads Vout as a 0–1023 count.

ohmmeter.ino

const int analogPin = A0;
const float Vin = 5.0;
const float R1 = 1000.0; // known resistor, ohms

void setup() {
  Serial.begin(9600);
}

void loop() {
  int raw = analogRead(analogPin);
  float Vout = (raw * Vin) / 1023.0;
  if (Vout < 0.01) {
    Serial.println(0);
    delay(500);
    return;
  }
  float R2 = R1 * (Vin / Vout - 1.0);
  Serial.println(R2);
  delay(500);
}

ohmmeter.pde

import processing.serial.*;

Serial myPort;
float resistance = 0;

void setup() {
  size(420, 220);
  myPort = new Serial(this, Serial.list()[0], 9600);
}

void draw() {
  background(255);
  fill(20);
  textSize(20);
  text("Measured resistance", 40, 70);
  text(nf(resistance, 1, 1) + " ohm", 40, 110);
}

void serialEvent(Serial p) {
  String val = p.readStringUntil('\n');
  if (val != null) {
    resistance = float(trim(val));
  }
}

Basic Display ohms as text.

Intermediate Color by range (low / mid / high).

Advanced Analog gauge with arc().

Challenge Compare with color-code tolerance; log a batch.

Next: Irrigation