Article
Touch-Free Smart Trash Can with Arduino
🗑️
A self-opening trash can lid is a small project with a big “wow factor” — you wave your hand near it, the lid pops open, and you never touch anything. It’s also a genuinely useful lesson in handling noisy distance readings and keeping a servo from jittering on flaky input — a pattern you’ll run into again in plenty of other robotics projects.
How it works
- The HC-SR04 sends out an ultrasonic pulse and times how long it takes the echo to bounce back, which gives a distance reading.
- If that distance drops below a threshold (say, 15cm), the Arduino treats it as “something’s near.”
- A four-state state machine (
IDLE -> OPENING -> OPEN -> CLOSING) drives the servo, so the lid doesn’t flap open and shut every time the sensor hiccups on a noisy reading. - Once open, the lid holds for a few seconds; if nothing is detected continuously during that window, it moves on to closing.
Parts list
- Arduino Uno or Nano
- HC-SR04 ultrasonic distance sensor
- SG90 servo (or a bigger servo if the lid is heavy, with an external supply)
- A trash can with a lid that can be lifted by a simple lever mechanism
- A separate 5V supply for the servo if you’re using a beefier one (a servo’s startup current draw can brown out and reset the Arduino if they share a USB power source)
Sample code
#include <Servo.h>
const int TRIG_PIN = 9;
const int ECHO_PIN = 10;
const int SERVO_PIN = 6;
const int OPEN_DISTANCE_CM = 15; // below this distance, treat something as "near"
const int SERVO_CLOSED_ANGLE = 0;
const int SERVO_OPEN_ANGLE = 90;
const unsigned long OPEN_HOLD_MS = 3000; // hold open for at least 3 seconds
const unsigned long DEBOUNCE_MS = 300; // must stay clear for 300ms before switching state
Servo lidServo;
enum State { IDLE, OPENING, OPEN, CLOSING };
State state = IDLE;
unsigned long stateEnteredAt = 0;
unsigned long lastDetectedAt = 0;
long readDistanceCm() {
digitalWrite(TRIG_PIN, LOW);
delayMicroseconds(2);
digitalWrite(TRIG_PIN, HIGH);
delayMicroseconds(10);
digitalWrite(TRIG_PIN, LOW);
long duration = pulseIn(ECHO_PIN, HIGH, 30000); // 30ms timeout ~ 5m range
if (duration == 0) return 999; // no echo, treat as "far away"
return duration * 0.034 / 2; // cm
}
void setState(State s) {
state = s;
stateEnteredAt = millis();
}
void setup() {
pinMode(TRIG_PIN, OUTPUT);
pinMode(ECHO_PIN, INPUT);
lidServo.attach(SERVO_PIN);
lidServo.write(SERVO_CLOSED_ANGLE);
Serial.begin(9600);
}
void loop() {
long distance = readDistanceCm();
bool objectNear = distance < OPEN_DISTANCE_CM;
if (objectNear) lastDetectedAt = millis();
switch (state) {
case IDLE:
if (objectNear) setState(OPENING);
break;
case OPENING:
lidServo.write(SERVO_OPEN_ANGLE);
setState(OPEN);
break;
case OPEN:
// only move to closing once the minimum hold time has passed
// AND nothing has been detected within the last DEBOUNCE_MS
if (millis() - stateEnteredAt > OPEN_HOLD_MS &&
millis() - lastDetectedAt > DEBOUNCE_MS) {
setState(CLOSING);
}
break;
case CLOSING:
lidServo.write(SERVO_CLOSED_ANGLE);
setState(IDLE);
break;
}
delay(50); // ~20 samples/sec, smooth enough without making the servo twitchy
}Common pitfalls
- Calling
pulseIn()without a timeout — if nothing is in range to reflect the pulse, the call can hang the whole sketch waiting indefinitely. Always pass the third timeout argument like the code above does. - No debounce for fast-moving objects — if you decide to close the lid based on a single reading, a hand or bag brushing past momentarily will make the lid flap open and shut. The state machine above only closes after
DEBOUNCE_MSof no detection. - Sharing one USB power source between the Arduino and a heavier-duty servo — the servo’s inrush current can brown out and reset the Arduino mid-motion. For anything bigger than an SG90, give the servo its own 5V supply and tie the grounds together.
- Mounting the sensor too low, at trash level instead of hand level — it’ll end up triggering on the garbage already inside the can. Aim the HC-SR04 outward, at roughly the height a hand would reach.
Where to go from here
Add a load cell under the base to warn when the can is nearly full, or wire in an ESP32 to send a Telegram notification when it’s time to take the trash out.
Tags:#arduino#sensors#automation
Comments