Build a seven-key spoon piano

A grounded metal spoon will act as a movable switch. Touching a foil key connects its input to ground, and the buzzer plays that key’s note. Lift the spoon to stop.

You will build seven separated keys, use internal pull-up resistors to define untouched inputs, and turn the dial to bend pitch. Gather cardboard, seven foil or copper-tape strips, tape, a metal spoon and eight insulated clip leads.

Before you begin

Build and test one grounded contact before seven. INPUT_PULLUP means untouched HIGH, contact with ground LOW. Array index -1 is a sentinel, not a valid key.

Your goal: Scan inputs and resolve competing events. 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? Build a pedestrian crossing.

Use A0, A1, A2, A3, D2, D4 and D5, in that order. Analog-labelled pins A0–A3 also work as digital inputs on the Uno.

This version deliberately uses seven keys. D6 is omitted because this shield also connects it to a button circuit with a pull-down; it is not a free input for the pull-up arrangement used here. A4 remains the onboard light sensor and A5 is the dial.

Disconnect USB and remove the traffic-light module or any external LED wiring before building, because D4 and D5 are needed as inputs now.

Cut seven foil strips about 2 by 5 centimetres. Tape them in a row on cardboard with clear gaps, so neither the foil nor clip jaws can bridge neighbouring keys. Label the positions C, D, E, F, G, A and B.

These are low-voltage switch contacts. Keep them away from the shield’s VCC pads; they will be connected only to the listed input pads and the grounded spoon.

Use one signal-coloured clip lead to connect the C foil key to the shield’s A0 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use one signal-coloured clip lead to connect the D foil key to the shield’s A1 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use one signal-coloured clip lead to connect the E foil key to the shield’s A2 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use one signal-coloured clip lead to connect the F foil key to the shield’s A3 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use one signal-coloured clip lead to connect the G foil key to the shield’s D2 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use one signal-coloured clip lead to connect the A foil key to the shield’s D4 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use one signal-coloured clip lead to connect the B foil key to the shield’s D5 edge pad. Keep this clip clear of adjacent foil, clips and pads.

Use the black lead to connect the metal spoon to a shield ground pad. This gives every key a return path when the spoon touches it. No wire goes to VCC.

Check all seven input connections, confirm the foil strips do not touch, and refit the buzzer jumper if necessary. Reconnect USB.

INPUT_PULLUP gives each untouched key a weak internal connection to the high level. The grounded spoon completes a stronger path to ground, so a touched key reads LOW. This is intentionally the opposite of the onboard D7 button’s active-high circuit.

The key and note arrays are paired. The program scans them in order and plays the first touched key, because this buzzer can play one tone at a time. Bridging two keys therefore does not create a chord.

Read this part of the actual starter

void loop() {
  int selected = -1;
  for (byte i = 0; i < KEY_COUNT; ++i) {
    if (digitalRead(KEY_PINS[i]) == LOW) {
      selected = i;
      break;
    }
  }
  const int bend = map(analogRead(POT_PIN), 0, 1023, -80, 80);
  const int frequency = selected < 0 ? 0 : static_cast<int>(NOTES[selected]) + bend;
  if (frequency != playingHz) {
    if (frequency == 0) noTone(BUZZER_PIN);
    else tone(BUZZER_PIN, frequency);
    playingHz = frequency;

These external keys use INPUT_PULLUP, so contact with ground reads LOW. The scan stores the first pressed index then breaks. -1 means none. The frequency expression checks selected before indexing NOTES; this avoids reading NOTES[-1].

Work through one case

If keys 1 and 3 are pressed together, index 0 wins because it is found first. With no key selected frequency is zero and noTone stops sound.

Your turn: complete the trace

With no key selected, selected=-1 and frequency=___. With keys at indices 2 and 5 both LOW, selected=___.

If your answer differs

Trace the condition before evaluating the selected branch.

Set the pitch-bend limits to 0 and 0 to keep standard notes regardless of dial position. Next restore the bend range and replace one entry in NOTES with 523 for a higher C.

Explain the complete switch path from input, through foil and spoon, to ground.

• A note plays untouched: look for bridged foil or a clip touching ground; check that none of the keys uses D6.
• One key is silent: inspect its clip contact with both foil and the correct pad.
• All keys are silent: check the spoon’s ground lead and the buzzer jumper.
• The highest-priority key wins when two touch: expected behaviour of the first-match scan.

Run a controlled experiment

Predict two simultaneous keys, release both, then change the bend range. Explain how this external-key polarity differs from shield D7.

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.

Independent check — try before revealing

Why must selected<0 be checked before reading NOTES[selected]? How does this differ from shield D7?

Touch the grounded probe to a spoon contact; do not attach anything to a person or mains. This is a contact keyboard, not a body-resistance sensor.

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.