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?
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:
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.
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!
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 :)
ANSWER BASED ON YOUR TITLE:
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
1.Model:LFS-DC04 2.Frequency:2.7GHz 3.Size:40*22mm 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
ANSWER BASED ON YOUR TEXT:
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!