2 Minute Ultrasonic Alarm

About: I am robotics engineer and a teacher whose passion in life is breaking down the maths and academic jargon of programming and robotics into something that everyone can understand, primary school kids include...

The ultrasonic sensor works by sending out sound waves and counting how long it takes for them to come back ( having been reflected from the nearest object.

In this circuit the ultrasonic sensor HC-SR04 plugs straight in to the Arduin, with pins 7 and 4 being used and GND and Vcc. The distance is calculated and every time the distance is less than a given threshold an alarm is raised, thus turning on an LED and a buzzer


•Arduino Uno

•5mm LED

•Piezo Buzzer

•Ultrasonic Sensor HC-SR04



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    4 Discussions


    4 years ago on Introduction

    Hi Robotix_au, I like ur instuctable very much. I programmed exactly d way u showed but my LED remains on after few seconds and serial monitor shows 0 cm continuously. Can u help me plz?

    2 replies

    Reply 4 years ago on Introduction

    Unfortunately the HC-SR04 has horrible quality control. I throw away fully a quarter of the ones I buy. This is why the code has a safety feature of connecting GND first then VCC.

    To test your sensor, I suggest you copy the code I have in the video below at around 6 minutes:

    If it still shows 0, your sensor is busted :(