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### Topic: DIY ultrasonic sensor (Read 1 time)previous topic - next topic

#### fuh

##### May 07, 2011, 03:23 am
Any if you guys has built an ultrasonic sensor ?
I've just replied to a related topic and started wondering that this should be a good week-end project.

Searching here at the forum got this.
It says that the shortest measurable distance is ~30cm (1 foot), I would like to measure stuff up to 5cm (~2 inches) or even lower.. what modifies the "short-range" of such sensor ?

#### AWOL

#1
##### May 07, 2011, 10:51 am
It's a while since I did one from scratch, but here are some of the pitfalls:
1) Cheap transducers tend to ring after you drive them, so you give them, say 10 cycles of 40kHz, but they actually transmit more like 15 or 20 because of ringing. This may limit your measurement rate. I have never seen a smiple way to damp them.
2) wavelength of 40kHz in air is about 8mm. Without phase detection, it is tricky to get better resolution than this.

#### fuh

#2
##### May 07, 2011, 10:38 pm

2) wavelength of 40kHz in air is about 8mm. Without phase detection, it is tricky to get better resolution than this.

I was thinking about water level measurement suing a chep DIY sensor, the rate it's not that important.. but the "short" range is.
As I'm not a native speaker I assume resolution is not the same as range, so what the meaning of resolution onto this context ?

I'm afraid, Dave; My mind is going; I can feel it..
It just happen to love 2001. :-)

#### AWOL

#3
##### May 07, 2011, 10:51 pm
Quote
As I'm not a native speaker I assume resolution is not the same as range, so what the meaning of resolution onto this context

Resolution is the fineness of measurement.
If your timer can measure down to, say, 4 microseconds (in air) that means (in theory) that you can resolve just over 1mm in range. (Wavelength about 8mm at 40kHz, or 25 usec).
That's the theory, but how rapidly the receiver responds to the incoming pulse is a different matter.
You may find that you don't get significant signal until after ther first four or five received cycles.

Water is tricky, and I confess I haven't done any ultrasound in water.
However, the speed of sound in water is about 4.5 times that in air - nearly 1500m/s, so timing is even more critical.

Quote
I'm afraid, Dave; My mind is going; I can feel it..
It just happen to love 2001. :-)

Probably the first and last time I cried at the movies.

Congratulations! You're probably the first person to get the joke, and a non-English speaker at that!
I'm hugely impressed.
Some people even think my name is Dave.
As if!

#### fuh

#4
##### May 08, 2011, 12:24 am
-offtopic-
I love Kubrick films, one may not say to like movies if haven't seen a Kubrick's film.
HAL was one of my favorite characters, how can someone do not know it and the "Dave" speech ?
Did you know that HAL was rated 13th on the greatest villains ?
-offtopic-