Buttons & Potentiometer

Momentary Push Buttons

A push button is a momentary SPST switch — conducts only while held, returns to open on release.

Pinout of a standard 4-pin tactile button:

The four pins are arranged in two pairs. Each pair is internally connected. The button bridges the two pairs when pressed.

1 ── 2
|    |   ← button
3 ── 4

Pins 1&2 are always connected. Pins 3&4 are always connected. Pressing the button connects 1-2 to 3-4. This often confuses people since it looks like four independent pins.

Wiring with pull-up:

Vcc ─── 10kΩ ─── Signal pin (reads HIGH when open)
                    │
              [BUTTON]
                    │
                   GND

Press button → pin connects to GND → reads LOW. The resistor prevents a short between Vcc and GND.

Most microcontrollers have internal pull-ups, so the external 10kΩ becomes optional.

Debouncing

Mechanical contacts bounce on make and break — the circuit rapidly opens and closes a few dozen times in under 20ms.

For a human reading a display: invisible. For a microcontroller at MHz: reads as many presses.

Software debounce (state machine approach):

bool lastState = HIGH;
bool stableState = HIGH;
unsigned long lastChange = 0;
const int DEBOUNCE_MS = 20;

bool readButton() {
  bool current = digitalRead(BUTTON_PIN);
  if (current != lastState) {
    lastChange = millis();
    lastState = current;
  }
  if (millis() - lastChange >= DEBOUNCE_MS) {
    stableState = lastState;
  }
  return stableState;
}

Hardware debounce: an RC filter (100Ω + 100nF) smooths the bounce before it reaches the pin. Simpler but wastes components.

Potentiometer

A potentiometer (“pot”) is a resistor with three terminals: two ends and a wiper that slides along the resistive element.

End A ───[resistive track]─── End B
                  │
               Wiper (W)

As a variable resistor (rheostat): use one end + the wiper. Resistance varies from 0 to max.

As a voltage divider: connect Vcc to one end, GND to the other, and read the wiper. Wiper output scales from 0V to Vcc as you turn the knob.

Vwiper = Vcc × (position / max_position)

Turn it fully: 0V. Halfway: Vcc/2. Full: Vcc.

Common values

10kΩ is the most common value for general use. High enough not to waste significant current; low enough to drive typical ADC inputs.

Tapers

• Linear taper (B): resistance changes uniformly with rotation. Good for precise control, position sensing.
• Audio taper / log taper (A): resistance changes logarithmically. Matches how human hearing perceives volume — we hear in decades, not linear increments.

That’s why “audio pot” is a thing. A linear pot on a volume knob feels like it jumps from quiet to loud in the first 10% of rotation.

Uses

• Volume controls
• Brightness adjustment
• Calibration trim
• Joystick position sensing (two pots, one per axis)
• User-set thresholds for comparator circuits