I have been thinking about and casually researching for this project a few months now. But today I started fiddling with some hardware, so it seemed a good point to start a blog on it.
I’m currently in St.Arnaud near Melbourne, a long way from my base in Italy. A couple of months ago I ordered a bunch of electronic components to get started playing with the analog parts of the system, starting with an ELF/VLF receiver. Of those the only thing that arrived before I set off over here was a Bitscope gadget to allow a computer to be used as an oscilloscope and/or data capture. Coincidentally that shipped from Australia, arrived in good time, unlike the stuff from Farnell in Europe.
I didn’t bring any other kit with me, but my darling Raven took me to a component shop the other day (Jaycar) where I picked up some bits & pieces.
I had planned to try some of the VLF receiver circuits in Radio Nature, which seems like the bible on this stuff. The ones I was looking at have a 2N3819 FET at the front end (the circuits can be found around vlf.it, the site of the book’s author). Unfortunately Jaycar didn’t have these, but gave me what they said was an equivalent. It wasn’t – just a regular bipolar. But I’d also bought a few TL084 quad jfet op-amps, so I should be able to get something together that’s functionally equivalent, if perhaps a bit noisier.
Here’s my prototyping setup:
Bottom right is a bit of scrap wood with hot-glued to it: 2xPP3 batteries; a speaker; breadboard; Bitscope.
The Bitscope’s hooked up to an old laptop of Raven, running Ubuntu. My first try with this, doesn’t seem like the signal’s getting through and their site just happens to be down today. Grr!
Still got things to play with though. I want to get the sound of VLF coming out of the speaker, and I want it to be portable to get away from power lines at first. So I got an LM386 250mW amp chip, as used in some Radio Nature circuits. Haven’t played with one of these before – I’d have remembered, it’s atrociously unstable. The basic (inverting) configuration from the data sheet oscillates happily. But I was able to get something more stable by tweaking the non-inverting config (in data sheet as “AM Radio Amplifier”).
Hopping ahead a little on the breadboard I’ve also got a white noise generator. I’m going to make a 50 Hz active (bootstrapped) notch filter to rid the signal of the worst of mains hum, hopefully allowing the receiver to be used in (or near) the house. I should be able to tweak the notch when the Bitscope’s working, with noise as input.
PS. Not long after posting this I remembered archive.org and found the BitScope troubleshooting tips. My problem was just permissions on /dev/tt1USB (?). So I had a crack at the notch filter. No joy with that yet, but my problem could well be inexperience with the ‘scope, there are a lot of controls.