Position Sensor: Rotary Encoder

Working Principle

An incremental rotary encoder generates two square-wave signals (Channel A and Channel B) that are 90° out of phase — called quadrature encoding. By detecting which channel leads, the MCU determines rotation direction. Counting pulses gives angular displacement.

Analogy: Imagine two turnstiles side by side, offset so one always clicks a half-step before the other. Which one clicks first tells you which direction someone is walking.

Electrical Characteristics

Parameter	Value (KY-040 module)
Supply Voltage	3.3 V – 5 V
Pulses per Revolution	20 (detents)
Output	2 digital channels (A, B) + push button
Bounce Time	~5 ms (requires debouncing)

Code Example

/*
 * Rotary Encoder with Interrupt — Arduino
 * Wiring: CLK → Pin 2 (INT0), DT → Pin 3, SW → Pin 4
 */
#define CLK_PIN 2   /* Must be interrupt-capable pin */
#define DT_PIN  3
#define SW_PIN  4

volatile int position = 0;

void encoderISR() {
    /* Read DT at the moment CLK fires */
    if (digitalRead(DT_PIN) == HIGH) {
        position++;   /* Clockwise */
    } else {
        position--;   /* Counter-clockwise */
    }
}

void setup() {
    Serial.begin(9600);
    pinMode(CLK_PIN, INPUT_PULLUP);
    pinMode(DT_PIN, INPUT_PULLUP);
    pinMode(SW_PIN, INPUT_PULLUP);

    attachInterrupt(digitalPinToInterrupt(CLK_PIN), encoderISR, FALLING);
    Serial.println("Rotate encoder. Press button to reset.");
}

void loop() {
    static int lastPos = 0;

    if (position != lastPos) {
        Serial.print("Position: ");
        Serial.println(position);
        lastPos = position;
    }

    if (digitalRead(SW_PIN) == LOW) {
        position = 0;
        Serial.println("Reset to 0");
        delay(300);  /* Simple debounce */
    }
}

Real-World Applications

Absolute Encoders: No Homing Required

Absolute encoders retain their position even after power loss — no homing sequence needed at startup. The AS5600 is a 12-bit (4096 positions/revolution) magnetic absolute encoder that reads a diametrically magnetised magnet via a Hall-effect sensor array. It outputs position over I²C, analog voltage, or PWM. Other options include Gray-code optical absolute encoders and SSI (Synchronous Serial Interface) industrial types.

Optical Encoders: High Resolution

Optical encoders use a slotted disc and photointerrupter pairs to achieve resolutions of 100 to 10,000+ PPR — orders of magnitude higher than the KY-040’s 20 PPR. They have no contact bounce (non-mechanical) and extremely long lifespans. Industrial optical encoders (CUI AMT103, Omron E6B2) are standard in CNC machines, robotics, and servo motor feedback loops.

Encoder Technology Comparison

Feature	KY-040 (Mechanical)	Optical Incremental	AS5600 (Magnetic Absolute)
Resolution	20 PPR	100–10,000 PPR	4096 positions (12-bit)
Position Type	Incremental (relative)	Incremental (relative)	Absolute (survives power-off)
Output	Quadrature A/B + switch	Quadrature A/B + Z index	I²C, Analog, PWM
Contact Bounce	Yes (needs debounce)	No (optical, non-contact)	No (magnetic, non-contact)
Max RPM	~120	6,000+	~4,500
Cost	~$1	$10–50	~$3–5
Best For	UI knobs, menus	CNC, servo, robotics	Gimbal, robot joint, valve position

Limitations

• No absolute position (incremental): The KY-040 and optical incremental encoders reset to 0 at power-up; use an AS5600 or other absolute encoder if power-off position retention is needed.
• Contact bounce (mechanical): KY-040 needs hardware (RC filter) or software debouncing; optical and magnetic types are bounce-free.
• Low resolution (KY-040): 20 PPR is fine for UI controls but insufficient for precision positioning; choose optical or magnetic for >100 PPR.
• AS5600 magnetic interference: External magnets or ferrous materials near the encoder chip can corrupt readings.