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Is it possible to write code in the sketch that checks the functioning of the ultrasonic sensor? Say if the sensor stopped working then the arduino will alert the user in the serial monitor.

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    it is possible to write code to do almost anything that is within the computing power of the Arduino. ..... interfacing with ultrasonic sensors is well within the capability of any Arduino. – jsotola Aug 25 '18 at 18:05
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    Maybe sensing the trigger with a second ultrasonic sensor, or simply monitoring the values for not expected ones. Is there a special reason, why you want to sense this? Some kind of fault tolerance? In this case the simpliest way may be to just use 2 sensors and compare the values. – chrisl Aug 25 '18 at 18:07
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    Step one is to define "not working". Step two is to work out how you can identify that condition, and what extra hardware you may need to achieve it. – Majenko Aug 25 '18 at 21:06
  • What is the active signal to trigger the ping ? HIGH or LOW? What is the active echo signal ? HIGH or LOW? What is the maximum distance you are detecting the object at ? What is the frequency, how often , of measurements? – Jan Hus Aug 26 '18 at 3:55
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First of all, this question is hard to answer because you don't provided a name of ultrasonic sensor that you are using. Also as @Majenko pointed out, the key here is to determine the state that you think sensor is not working correctly.

But if assume that you're using standard trig/echo ultrasonic sensor (HC-SR04 or similar) interface - the only possible solution here is to monitor the output(echo) signal of the sensor. Broken sensor will not respond on trigger pulses, so you'll not get the echo pulses. However, these sensors also do not send a signal if the object is out of range of the sensor.

Thus, it is possible to determine the breakdown by monitoring the output signal from the sensor only if you know that the object will always be in range. Otherwise - you can not determine for certain because of what you did not receive a signal, whether it is a sensor failure, or the object is too far from the sensor.

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Detecting OUTPUT trigger failure - grounded. Add INPUT pin connected via small signal diode ( 1N914 / 1n4148) to TRIGGER pin. Read its value every time you set the TRIGGER HIGH and it should read HIGH when working correctly and LOW when the device TRIGGER pin is faulty AKA grounded by the device.

Detecting INPUT - echo pin failure - grounded. Connect delay circuit (RC) between TRIGGER and ECHO pins. (I would put diode in into it too just to make sure you do not ground TRIGGER from faulty ECHO pin)

Calculate the RC delay to emulate value lower than your minimal expected distance. Every time you send "ping" you should get TWO echoes - one from the "delay circuit" and one for real. If you get NONE - your ECHO pin is faulty AKA grounded.

Perhaps adding current monitoring to device supply could also be used as failure indicator. The device current draw MAY be different when "pi9nging" or when idle / not operational.

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