Position measurement.

Yep. Years ago I designed some motion detectors with 40KHz transducers... slow as crap.

...Jim Thompson

To achieve the required accuracy, phase measurement, continuous wave and much higher frequency, that was typically used by Polaroid is necessary. In our vibration measurement we have achieved about =.1 micrometer amplitude resolution and have used 200kHz transducers. In 5cm distance even 1MHz could be used and the diplacement measurement accuracy would be significantly higher.

W. Bicz

Acoustical interferometry is working in real time, exactly as optical, but can be significantly cheaper and easier to use.

W. Bicz

Some of the piezo transducers are really horrible, but it depends on how they are made inside (the really crummy ones could be used in CW for doppler motion detection systems). But even the more appropriate ones ring for some time after you hit them, even if you damp them electrically (so any signal much shorter than a wavelength is going to get lost in the noise).

Polaroid originally used an electrostatic transducer (driven with a transformer) for their SX-70 camera so they could use a single transducer for send and receive. The minimum focus distance was pretty short, maybe 16cm/6" or something like that. These ones have a characteristic gold-plated polyester membrane visible through the front mesh.

I've used both types- one type with an analog PID controller for controlling the 'bubble' of hot plastic in blown-film extruders and one type for a small microcontroller-based proximity detector. I just bought on a whim an end-of-line parking device (IIRC similar to the one you found a year or two ago). It uses two 40kHz piezo transducers and an 8051 variant to measure distance and control three big red/green/yellow LEDs. EEPROM holds the customer preset distance, I think. Couldn't resist for $20.

Where did you get it?

I tossed the Genie closers, what crap, and my new openers don't have a convenient place to mount a magnetic switch.

I park my truck by eye, just watch for the rake handle to sway ;-)

But "N" regularly overshoots the walkway, making it a squeeze for me to get through the door into the house.

...Jim Thompson

Home Depot. Logix brand. I guess they didn't sell very well. I don't know how well it works, I've only taken it apart so far ;-) , but it looks pretty good inside design-wise, with typical low-end Chinese assembly.

Found a similar product called Park Zone at

...Jim Thompson

I don't shop Home Depot. They conned me ONCE.

...Jim Thompson

Single transducer system rather than dual, other than that, it's pretty similar, although my one didn't come with an AC adapter.

What's the story on that? I find them okay, though their handling of returns could be much better (faster, mostly). They, and their ilk, have driven most of the small competent hardware stores out of business.

One of the reviews on the system you mention says that, contrary to claims, the instructions say 18" is the minimum working distance, and in fact it needs that much room. It also needs space on all sides or it picks up false echos, and it doesn't work too well with curved bumpers, according to them.

Best regards, Spehro Pefhany

There weren't any real hardware stores around when the built near me. Sure they could order things if you could wait, or you didn't mind paying $12 for a $3 part. Real hardware stores started disappearing in the '70s, long before HD opened their first store. less people were doing their own repairs, and big contractors had salespeople calling on them and delivering things right to the jobsite for them. I saw several close in the '70s that had opened in the late 1800s, and had been family owned businesses, till there was no one left who wanted to run the store. Lose that connection to the store's past and most people went elsewhere.

Huh?

Cheers! Rich