Ultrasonic Range Finder keeps sending -1 code

I’m working with my students to have them write code in RobotC to have their robots move around the room like a Roomba, i.e. when they get close to a wall or object, they stop and change direction. We are using the Ultrasonic Range Finder for this, and for some of the students it just jumps to -1 and doesn’t budge, which, does present problems when writing code to have it change when, say <5 cm… I have looked online for reasons why the URF might show a -1, but couldn’t seem to find any answers.
Thanks in advance!

-1 is the value the ultrasonic returns when there is nothing within range. It isn’t detecting any distances.

According to the VEX website, the ultrasonic range finder has a usable range of 3.0 cm - 3.0 m (1.5 in - 115 in).

If there is nothing within that range distance from the sensor, then you have your answer. You may need to program your Roomba so that it drives blindly until something is within the acceptable range.

If there IS something within that range distance from the sensor, and it is still registering -1, you probably have an issue with your setup, or possibly a faulty sensor.


this was a very common issue when i was in IQ. the sensor is really sensitive, so avoid touching the net on the front because if the metal pieces inside misalign, it doesn’t work

Thanks! That’s actually been very helpful and we’ve had to make adjustments to our code (more conditionals) but we’re on track now!

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