I mentioned the US40KT-01 transducer pair from Meas Spec before, but it’s an air sensor. One could cover it with a potting compound to waterproof it. My source suggested 3M Scotchcast 2130, which is intended for electrical insulation but apparently has some desirable acoustic properties (viz being close to the acoustic impedance of water). He cites Acoustic and dynamic mechanical properties of a polyurethane rubber [PDF] by Mott, Roland, and Corsaro as a good resource on this stuff, but I haven’t read it yet.
I also came across some really cheap waterprooftransducers that may be suitable. Unfortunately they both have high directivity, whereas I’d like at least an omnidirectional transmitter. Another possible issue is that they’re made for outdoor applications, not underwater applications. How waterproof is “waterproof”?
This guy has a super-cool post on driving the little waterproof transducers. Instead of using a step-up transformer, he uses an inductor and a MOSFET. The inductor gets charged from a 9 V battery through the MOSFET. When the MOSFET is opened the magnetic field collapses and it sends about 300 mA of current through a diode to power the transmitter. This is called the inductive flyback method. Awesome!
One final note: I found what look like the transducers that are on the ranger modules I’m using. Less than US$3 per pair.
I feel pretty dumb talking to nobody like this. My domain name doesn’t even work yet, but I guess logging is what engineers do. And I’m imaginative enough to see the utility of it: it’ll be nice down the line to have a log to review, writing stuff down helps flesh out ideas, and an open design process will make it a heck of a lot easier to produce open documentation. It will be tough to expose all my bad decisions and half-baked ignorance (and mixed metaphors), but I can suck it up.
My first project is to build an acoustic modem. This follows the principle of multiplying work: the modem doubles as my class project for ELEC 571: Underwater Acoustics. We’ve been using the Devantech SRF04 ultrasonic ranger in the mechatronics lab, and it strongly informs my design. The SRF04 actually uses an RS-232 chip to generate ±9 V levels, which actuate a piezoelectric transducer. I took a couple of the transducers from a broken SRF04 to use for my project.
I tried using an RS-232 chip that we had lying around in the lab, but those things have draconian current limits. There’s no way I can power a whole circuit off of one. I’m probably going to try it again soon though.
My solution: a DC/DC converter (AP34063N8L) to step up a 7.4 V lithium polymer battery up to 20 V. Taking half the output as the reference voltage will produce a ±10 V power supply. With that I can power pretty much anything I want. Continue reading →