Hand-wave lamp dimmer with an HC-SR04 and PWM on Arduino: design worksheet and code

A ₹605 lamp you dim with a hand movement. An ultrasonic sensor faces up, the sketch maps the distance from 5 cm to 30 cm onto a PWM level, and a low-voltage LED lamp follows your hand. A short smoothing filter keeps the light steady, and a hold gesture locks the level.

Scope: beginner. This is a design worksheet with illustrative costs. It has not been bench-tested by this site. The mapping table was computed by script. This page drives a small LED, or a low-voltage LED strip through a logic-level MOSFET, and nothing on mains. Ultrasonic sensors have a blind zone close to the face and are less reliable on soft or angled surfaces, so hands held flat work best.

How it works

The sketch maps distance d to brightness with b = (d − 5) / 25 × 255, clamped to the range 0 to 255. At 5 cm the lamp is off, at 17.5 cm it is half bright and at 30 cm it is full. Beyond 35 cm the sketch treats the reading as no hand and keeps the last level.

Raw ultrasonic readings jump by a centimetre or two. An exponential filter, smoothed = 0.8 × smoothed + 0.2 × new, removes most flicker with a small lag. Holding the hand still within 1 cm for two seconds locks the level until the hand leaves, so you can step away.

Prices are illustrative figures for Indian sellers in November 2026, before shipping, and have not been checked against current listings.

The numbers

The eye sees brightness roughly on a curve rather than a straight line, so a straight PWM ramp looks too bright too early. Squaring the 0 to 1 level before scaling, level² × 255, gives a gentler rise at the dim end; try both and keep the one that looks right to you.

Power and wiring

The sketch

// Hand-wave dimmer: Nano + HC-SR04 + LED on a PWM pin
// Design worksheet: tune the near and far distances for your lamp.

const int TRIG = 7, ECHO = 8, LED = 9;
const float NEAR_CM = 5.0, FAR_CM = 30.0, GONE_CM = 35.0;
const float ALPHA = 0.2;                    // smoothing: higher reacts faster
const bool GAMMA = true;                    // gentler dim end

float smooth = FAR_CM;
float held = 0;
unsigned long stillSince = 0;
bool locked = false;
int level = 0;

float ping() {
  digitalWrite(TRIG, LOW);  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  unsigned long t = pulseIn(ECHO, HIGH, 25000UL);
  return t == 0 ? 999.0 : t / 58.3;
}

void setup() {
  pinMode(TRIG, OUTPUT); pinMode(ECHO, INPUT); pinMode(LED, OUTPUT);
}

void loop() {
  float d = ping();
  if (d < GONE_CM) {
    smooth = (1 - ALPHA) * smooth + ALPHA * d;
    if (fabs(smooth - held) > 1.0) { held = smooth; stillSince = millis(); locked = false; }
    else if (!locked && millis() - stillSince > 2000) locked = true;
    if (!locked) {
      float x = constrain((smooth - NEAR_CM) / (FAR_CM - NEAR_CM), 0.0, 1.0);
      if (GAMMA) x = x * x;
      level = (int)(x * 255);
    }
  } else {
    locked = false;                          // hand gone: keep the last level
  }
  analogWrite(LED, level);
  delay(40);
}

When the hand leaves, the lamp keeps its last level, so you can walk away. To switch the lamp off, bring the hand down to the near mark. The 25 ms timeout on pulseIn limits the sensor to about 4 m, and a missed echo returns 999 so it is treated as no hand. The sketch has been proofread but not compiled on this site.

Test plan

1. Distance check: hold a flat card at 10, 20 and 30 cm and compare readings with a ruler.
2. Smoothing: hold a hand still and see that the light does not flicker.
3. Lock: hold still for two seconds and check the level holds.
4. Gamma: compare GAMMA on and off in a dark room.
5. Surfaces: try a flat hand, a fist and a sleeve.
6. Stray objects: place a shelf nearby and check it is not detected.

What usually goes wrong

Upgrades

• Colour: use an RGB strip and map height to hue.
• Gestures: detect a quick wave to toggle the light.
• Two sensors: one for brightness, one for warmth.
• Wall switch: mount in a box beside the lamp.

Cost and parts notes

The ₹605 total is an illustration and does not include shipping. Use the cost calculator to compare sellers.

Common questions

Last reviewed: November 2026. This is a design worksheet with illustrative costs. It has not been bench-tested by this site; library names, pins and limits must be checked against your board and the current library versions, and measurements from your build are welcome.