Circuit board rendered by OSH Park.
I’ve learned a bunch about making circuit boards at work recently, so I decided to re-do the ol’ strobe guitar tuner project from 3 or 4 years ago. The original still works… sort of. I have to power it from a 3x AA battery holder that’s clipped to the power terminals with alligator clips. It doesn’t tune the low E string properly for some reason. The input’s inflexible, and it’s hard to reprogram.
The hardware has been updated in the following ways:
If all goes extraordinarily well, the AUV will have the following modules sitting in the hull:
Rather than wiring everything together, I plan to give each module its own power supply and use an optical communication protocol to connect the modules to the master controller. The inside of the hull will be cleaner and more solid than if everything was wired together, and waterproofing will be easier. Here’s how it works:
The old guitar tuner I made works fine, but I’m thinking of some improvements:
I considered replacing the microcontroller with a 555 timer, but I don’t think the 555 timer will generate sufficiently precise frequencies because of the tolerances in the resistors and capacitors.
A ±10 V power supply
I’ve covered how not to power an acoustic modem, and how I’d like to power an acoustic modem, now it’s time to tell how I’m doing it right now.
To restate and clarify the problem: I want to use an acoustic transducer to transmit and receive data via pressure waves. The transducer I have transmits a 40 kHz carrier wave, and can be driven by up to 20 V. That means that when transmitting, the circuit must alternate a 20 V signal back and forth between the transducer’s two input pins, once every 25 microseconds. Additionally when receiving, the transducer’s output signal needs to be amplified into a usable signal. Continue reading
I mentioned before that I’m basing my acoustic transducer off the Devantech SRF04 ultrasonic ranger. I’m removing the piezoelectric transducers from a dead SRF04 unit and using them for my modem. The transducers, which are the part that generate and receive the acoustic pressure wave that carries data, have a range of about 6 metres or so (one way), are driven with up to 20 V, and resonate at around 40 kHz.
The puzzle I’m working on right now is how best to actuate the transducer to generate a 40 kHz pulse. The SRF04 does it using a chip that’s intended to convert 5 V logic signals into the ±12 V signals used for the RS-232 serial protocol. Unfortunately most RS-232 converter chips aren’t made to power an acoustic transducer, and they aren’t able to provide enough current to generate a strong signal.
How not to generate a ±10 V power supply
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