Read the knob

Read an analogue input and relate a changing number to the knob’s centre contact.

Predict: What number would you expect near halfway between the knob’s stops?

Before you begin

Know the knob is A5; distinguish print from println.

Your goal: Turn a voltage into data. Show this with a prediction, a tested change and an explanation using the program’s names.

Retrieve one idea: what input, state or output did you change in the previous project?

Need a reminder? Two lights, one sequence.

Use an Arduino Uno with the Crack the Code shield. The shield already connects its LEDs, knob, light sensor and buttons; this activity needs no jumper wires or breadboard.

If the shield is not fitted, disconnect USB power, align every shield pin with the Uno sockets, and press evenly without bending the pins. Reconnect the Uno with a USB data cable. Leave the traffic-light module disconnected for this activity.

Open the example below. Read the English plan, then choose Arduino C++ from the language selector and Arduino Uno from the board selector. Select Verify to compile. Select Pair board…, choose the Uno’s serial port, then select Upload. Use Chrome or Edge on a desktop computer with this HTTPS page for browser upload. If browser upload is unavailable, copy the complete C++ sketch into Arduino IDE, choose Arduino Uno and its port, and upload there.

Each upload replaces the previous program. Edit the supplied C++ when trying the challenges, then verify and upload again.

After upload, select Open Serial Monitor and set 9600 baud. Close other serial programs first so only one application owns the port. The Plot tab can display the labelled numeric readings.

Inside the potentiometer, a moving contact touches a resistive track between the supply and ground. The shield connects that centre contact to A5. The Uno converts its voltage into one of 1024 possible readings, numbered 0–1023.

The voltage calculation assumes a 5.0-volt reference. It is an estimate from the raw reading, not a calibrated voltmeter: the actual USB supply can differ. The knob changes the input voltage; the code reports it.

Read this part of the actual starter

void loop() {
  int reading = analogRead(potPin);
  float volts = reading * 5.0 / 1023.0;
  Serial.print("knob:");
  Serial.print(reading);
  Serial.print(",volts:");
  Serial.println(volts, 2);
  delay(50);
}

analogRead(A5) samples the knob voltage as an integer from 0 to 1023 on this Uno. Local reading is replaced each loop. float volts = reading * 5.0 / 1023.0 estimates voltage using a nominal 5 V reference. print continues a line; println ends it.

Work through one case

For reading 512, the estimate is 512 × 5 / 1023 ≈ 2.50 V. The labelled serial line is knob:512,volts:2.50. It is not a separate voltmeter measurement.

Your turn: complete the trace

For readings 0, 512, 1023, estimated volts are 0, ___, ___ using the supplied 5 V reference.

If your answer differs

Check whether the intermediate calculation can keep a fractional value.

Hold three positions and compare your readings with another group. Change the 50-millisecond wait to 250 and explain why the graph updates less often without changing the knob’s range.

Run a controlled experiment

Collect five knob positions. Predict the midpoint and compare the live serial plot against your table. Explain why repeated readings can differ by a small amount.

1. Save a copy of the working starter. Reset the board so stored state begins from the declared values.
2. Write the expected result before editing. Change only the named factor; keep wiring and other settings fixed.
3. Edit the C++ in the editor, Verify, then Upload to the connected Uno. The starter simulation does not execute your edited C++.
4. Repeat the same input sequence. Record input, expected output, observed output and an explanation. Use labelled serial values where the sketch provides them.
5. If the result differs, inspect the relevant condition and pin before changing another factor. Restore and upload the saved starter to recover.

Core task: explain one changed case. Optional extension: choose a boundary or timing case and justify the extra test. Use a paper trace or annotated screenshot when physical manipulation is inaccessible; distinguish predictions from measurements.

If the graph reacts to shadows rather than turning, check that the code reads A5, not A4. If the console is unreadable, match its speed to 9600 baud. Why are there 1024 values but a largest value of 1023?

If nothing changes: check the power light, successful upload and the selected Uno port. Disconnect power before reseating a shield. Read the first compiler error before changing several lines at once.

Independent check — try before revealing

Why would replacing 5.0 / 1023.0 with a separately calculated integer ratio 5 / 1023 break the conversion?

The number is an ADC reading, not an angle or voltage until a conversion is applied. The physical reference voltage can differ from exactly 5 V.

If your explanation missed a condition or stored value, add that column to your trace and try a new input. A working upload alone does not answer this check.