I'm new to all this. I bought an Arduino Uno recently and an Ultrasonic distance sensor. My mom's cats keep walking down into the basement and doing their... business. Anyhow, I want to create a radar system which detects movement past this door. I'd like to have an alert sent to my computer or Android. Is this within a basic Arduino level? Other than the Arduino what else might I need?

  • The widely available ultrasonic modules are more suited to detecting object presence (ie distance) than specifically motion, but they are at least cheap and easy to experiment with. You could also consider an IR beam-break for presence. In terms of an alert, would a local audio transducer be workable? If not you can look at bluetooth, or wifi, perhaps even running your project on an Arduino-ified ESP8266 instead of an actual Arduino in the latter case. May 28, 2015 at 18:22
  • That would be a sonar, not a radar. May 28, 2015 at 20:08
  • Of course, you could do what I did when my kids were babies ... put a gate across the stairs.
    – SDsolar
    May 17, 2017 at 9:20

6 Answers 6


I'm not convinced an ultrasound detector will detect a cat. They're fluffy and (usually) soft, so the ultrasound might not bounce off.

Instead I suggest you just try a gadget that annoys cats and keeps them out of the area. It's an ultrasound emitter, and the noise is claimed to be objectionable to cats. I've seen them sold in gardening stores, for keeping cats away from the bird table:

Opinions on cat scarers

  • Ultrasound should detect a cat - the question is more, if it will detect a cat amid background clutter at similar ranges. Generally for the cheap pulse-in/pulse-out sensors, you have to arrange the situation so that the first substantial reflection is the object of interest. Jun 29, 2015 at 0:56
  • If the other side of the stair provides a reliable reflection, then the reading would change with any obstacle in the way.
    – SDsolar
    Aug 24, 2017 at 23:41

Ultrasonic sensors can only measure the distance to the nearest object in their cone. Are you sure you can detect a "small" thing like a cat?

I think the only feasable way is to use optical recognition (motion sensor with a camera - even a webcam - and a board like a raspberry pi).


I think that's quite possible. As mentioned, not technically radar. I don't know if cats could hear it, but then if it scares them off, job done, right?

I don't know if sending the alert to a PC would help or not - you might just consider an audible alarm using a cheap 8-ohm speaker. Otherwise, USB, bluetooth, wifi, or SMS for notification. A PC or phone (on all the time) to receive the data. Some thought about what you will do with it.

As a deterrent, a water pistol is ideal, you'd need a motor - solenoid? - to fire the water pistol. Harmless, but very distasteful to cats.


Update: Inspired by @SDSolar, I checked it out - yes, cats can hear that frequency.

Cats have excellent hearing and can detect an extremely broad range of frequencies. They can hear higher-pitched sounds than humans or most dogs, detecting frequencies from 55 Hz up to 79 kHz.

(from https://en.wikipedia.org/wiki/Hearing_range#Cats )

This means they will probably learn to associate the sound with the water pistol (yes, cats CAN be trained, if they are motivated). This means if you remove your device, they will probably come back, but if you leave the sensor (active), but remove the water, then it will continue to work!

  • Cats can't hear 40 KHz.
    – SDsolar
    May 17, 2017 at 10:12
  • According to wiki (as per my update), they can. Citation? May 17, 2017 at 21:16
  • 1
    I stand corrected. I am very surprised. I didn't know any animal could hear that except maybe bats. But I found this reference that confirms what Wikipedia says, sort of. This one says up to 59 KHz: pets.thenest.com/frequencies-cats-hear-8238.html TNX for the info. Learn something new every day. So perhaps the mere presence of the sensor would be a deterrent? Probably not for cats, unless the water pistol or a loud noise is triggered when they get in the critial range.
    – SDsolar
    May 18, 2017 at 8:15

As a project building a cat detector sounds interesting, but as a cat owner I can promise you that if you actually go through with it the cats will see it, play with it and trash it within 5 minutes. If it is successful and you do manage to scare them out of the basement they are just going to find somewhere else to 'do their business' and that could be in your bed.

However, your sensor will need to scan a large field of potentially rapidly moving objects. A beam wouldn't be suitable because they could step over or under it. I think PIR would be best for this task, but PIR sensors might not be sensitive enough to detect the little darlings.

Another idea maybe a gas sensor based project to provide an audible warning when a human enters the area that there are 'landmines' in the proximity and extreme caution needs to be used, if you know what I mean :)



Here is a microwave RADAR motion-sensing module that you might find useful. You could mount it right by the stairs.


LFS-DC04 2.7GHz Microwave Radar Module DC 5V 360 Degree High Level Signal New

enter image description here

Product Introduction:

4.Input Voltage:DC 5V
5.Output Type:level signal output 
6.Connection Method:VCC,GND connect with positive negative input, OUT,GND signal connect with output 
7.Installation Height:1-3.5m
8.Sensing Ditance:5-8m
9.Delay Time:30s(quick testing mode is 4s)
10.Sensing Angle:360 degree
11.$1.59 price via the slow boat


Ultrasonic can work - you will want to position it so your software can tell whether there is a cat/no cat, like pointing it across a step. With no cat it would give a reliable distance reading from something on the other side of the step. With the cat in the way, that reading would change.

Yes, this is in the realm of Arduino, for sure.

Here is a Google search to get you started:


To tell the difference between a cat an a human you would want a second sensor, above the cat's height, also.

Or you could use a DS3231 RTC module to decide when it is night-time so you wouldn't need to discriminate between cats and people at all.


Or there's an off-the-shelf solution: Innotek ssscat is a bottle of compressed air and a motion detector. The spray is harmless but it sounds like a hiss, which is Cat for GTFOuttaHere!

  • That is cool. A bit pricey but I would love to see one in action. If they were a quarter of the price I would use them as perimeter guards around my solar arrays to keep the critters away, including the family dog. Upvote.
    – SDsolar
    May 18, 2017 at 8:11

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