Mike, this is one of those cases where it would be helpful if you could tell us the application of the motion sensor you're looking for. For example, is this for intrusion detection? Or to annunciate when someone walks into your shop? Or detect a car that has come into your driveway? Or detect your cat approaching your aquarium? A squirrel approaching your bird feeder? Or what?
If you want to detect someone entering your shop while you're out of sight, then the cheap Chinese junk that KeithM linked might work for you (as long as you add the relay you'll need to switch power to a beeper). Don't be surprised if you get the occasional beep when someone
hasn't entered. In fact, don't be surprised if it happens often. The sensors might be good quality, but they aren't suitable to use in an intrusion alarm system.
If you're interested in an intrusion detection kind of application, or you want to minimize your false alerts, then you might want to use a low-end (inexpensive) PIR designed for intrusion detection. You can pick up one cheap off eBay. This Honeywell Aurora model PIR is a good example and they're reliable. There are more expensive models but this one will work for about (in my guesstimation) 95% of the applications.
http://www.ebay.com/sch/i.html?_fro...C0.A0.H1.TRS5&_nkw=Ademco+aurora+PIR&_sacat=0
My field of expertise is in the security alarm industry, primarily intrusion detection. I've worked extensively with all the technologies you mentioned, microwave (MW), ultrasonic (U/S) and passive infrared (PIR), as well as "electric eye" or Active Infrared, aka the IR beam that Bluejets mentioned.
All of the
Motion sensors are a lot more complex than most people realize. Making a "motion sensor" is simple.
What is considerably more difficult is making a sensor that detects _only_ something moving, and within the area you want to sense motion.
All the different technologies have their strengths and their weaknesses, and PIR sensors have pretty much dominated the motion sensing field for the past 30+ years, not because they sense motion better, but because they're better at _not_ tripping when there's no intruder moving in the coverage area. False alarms have always been the biggest technical problem in the alarm industry, and PIRs solved a huge percentage of that for area coverage.
I could probably write a 5-page essay on this, but I don't even know yet why you want the motion sensor, so I'll just point out a common factor you should be aware of.
Motion sensors don't have a well-defined range where they stop sensing something moving: The PIR I linked above, the Aurora, claims a coverage area of 35 feet out and 45 ft across: That's the range that it will
reliably detect a human big enough to be an intruder (IOW, not infants) moving across its field. But it doesn't ignore anything beyond that range--it will probably detect people moving as far out as 40 ft. and even farther if they're really big (e.g. 250 pounds), or hot from exercise (for a PIR), carrying sound-reflective material (for an U/S), or wearing MW-reflective material (like an tin-foil hat, for a MW), or if any of them are moving faster than a walking pace, which is the speed the sensors are generally calibrated for. If the intruder is a horse galloping across the patio, an Aurora PIR might catch it at 70 ft. or farther; an elephant beyond 100 ft.
...and a flying moth would be invisible at 25 ft, but might trip it at 6 inches, if it flies into its field of coverage.
Since I don't know what app you want the motion sensor for, I don't know if any of this is useful to you. Let us know the application, and we can narrow down our input to relevant information.
