Raspberry Pi On Its Way

I got email from Newark today saying that they had shipped my Raspberry Pi. I should get it this week.

They also sent an invoice, having charged my credit card. The item itself is $35.00, shipping is $5.00, and tax is $2.28, for a total of $42.28. Way back in early March, I called Newark to place my order since someone noted that the huge shipping charge they were quoting at the time could be avoided with a phone order rather than an online order. Hopefully they’ve fixed the online ordering since then.

Otherwise, I think that I have enough RasPi accessories to get started with when it arrives. I picked up a used Tektronix oscilloscope and an LCD monitor with DVI interface from Mario DeSario at Audio Workshop in Bradenton, Florida. I have a HDMI to DVI adapter and an HDMI cable. I have a 2.5 amp powered USB hub to supply the RasPi and allow me to plugin more accessories. I’ve got a USB GPS and a USB sound interface. I’m looking forward to diving in on this.

I also saw on the RasPi forums that by installing Mono, a RasPi can run executable files produced in managed .NET code from Visual Studio. That is a cool capability. I might try doing the monitoring system software in C# and .NET.

I still need to do some work on the hydrophone side of the system. I want to do a bit of home-brewing there. Years ago, I heard Cornell’s Chris Clark talk about fabricating in the field what he called a “schlong-o-phone”, which was basically a microphone capsule potted in epoxy in a convenient flexible cylindrical latex mold, otherwise known as a condom. Some of the more convenient materials for filling a mold don’t actually work well for salt-water and electronics. For example, most RTV (silicone) formulations are waterproof in themselves, but do not bond well to cable insulation. Additionally, the relatively high viscosity means that best results require applying a vacuum to de-gas an RTV mold, and that’s outside my current state of gear. I’m going to experiment with a piezo disk and Plasti-Dip. I’ve priced urethane for molds, and it is indeed expensive, but it is also low viscosity (no bubbles in the mold) and should bond nicely to cable insulation. If the Plasti-Dip results are bad, I’ll likely give the urethane a try.

Checking stuff out will require at least a couple of the home-brew hydrophones, our trusty frequency generator, and the oscilloscope. I have a variety of pre-amp and amplifier DC-powered circuits to pair up with the hydrophones.

Something I’ve wanted to try out for years now is doing a reciprocity calibration, but I’m not quite there yet, either. Reciprocity calibration uses three transducers (whose characteristics may all be unknown) and yields an absolute calibration of a receiving transducer. If a transducer can serve for both receive and projection on short time scales, a self-reciprocity calibration can be done. In either method, precision measurement of the current going to the projector in the system is necessary. That’s the part that is most challenging. There are plenty of other challenges, though. For hydrophones, the distance between all the transducers needs to be enough to be in the far field, where spreading loss is predictable. At least a meter to 1.5 meters is what would serve as a minimum. Then, one needs to be far enough away from either the water surface or any other obstacles so that the calibration signals aren’t bouncing off other things and giving reverberent noise that would compromise the received energy levels. 1.5 meters distance from other surfaces would give about a millisecond’s worth of unblemished sound. So we’re talking a quiet body of water at least 4.5 meters wide by at least 3 meters deep just to get a millisecond of calibration data at a time. I think I’ll have to defer home-brew reciprocity calibration for a later date.

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Wesley R. Elsberry

Falconer. Interdisciplinary researcher: biology and computer science. Photographer. Husband. Christian. Activist.