Eric:
Back in the mid '80's I build one like that, except that it had synchronous demodulators like a VLF. Called it the "Codfisher", and therein lies a tale.
Back in the bad old days, we were striving for a fully static discriminator. (You might justifiably ask "why"?, but that's another story.) Well, anyway, I figured out that VLF didn't have sufficient stability or ground rejection to do that, so I began developing PI. Whenever I mentioned PI to him, Jim Lewellen (president of Fisher) said, "PI is no good". That's all he knew about PI, was that it was "no good". So I had to develop it without official sanction. Fortunately, Jim Lewellen had the wisdom and self-discipline to pretend he didn't know about unapproved projects.
Well, when I finally had PI to the point where there was something worthwhile to at least illustrate its potential, I knew I had a sales job ahead of me.
I sloppy-wound a coil on a cheap reed basket, fastened it to a rather crooked small tree limb, and for an electronics housing used a small wooden box of the kind they sell dried salted codfish in. For a threshold knob, glued a Pepsi screw cap to the pot shaft. Loop cable was old lamp cord. Our graphic artist, who was not above plotting fun, put a label on it dubbing it the "CodFisher".
It ran on a 9 volt transistor battery, would discriminate out aluminum poptabs, and in the all metals mode would find coins out to about 8 inches. By the standards of those days it wasn't too shabby a performer.
I took it to the local fish store and there, the owner Fatima and I agreed to put a price tag on it of $10, and put it in an out of the way place where customers would not see it. She didn't know Jim, so our agreement was that if someone walked into the store asking to see a metal detector, offer it to him.
The next day I told Jim that they had an interesting looking metal detector for sale at the fish store, maybe he ought to go take a look at it. Well, a week later, it was still there, so I asked him if he'd gone and checked it out.
He said, "Darn it, Dave, I know you're up to something here, just tell me what it is." I told him, "Yep, I'm up to something, but in order to find out what it is, you're just gonna have to go to the fish store and find out."
A couple days later his curiosity finally got the best of him. He went, he saw, it conquered.
The only way to get him to take PI seriously was to make him pay ten bucks out of his own pocket for it.
It was one of his prized possessions-- kept it on the top of the bookshelf in his office. I wonder if he took it with him when he retired.
--Dave J.
