I built the metal detector with the intention of using it to find a surveyor's pin in the bush near my parents cabin, so did we find the pin?

Nope.

found some (iron rich?) rocks that set off the metal detector though.

We were unable to find the surveyor's pin for a couple reasons:

1. It seems like there may not actually be a metal surveyor's pin on the property (at least not on the corner we were looking at)
2. it was a lot more difficult to use the metal detector in the bush than I had expected
3. the metal detector doesn't work that well

Was There Actually a Surveyor's Pin?

We know for sure that there is a metal stake driven into the ground on one corner of the property, supposedly put there by a surveyor, since we've found that stake previously. As a result we assumed that there would be a surveyor's pin on all of the corners of the property. However, after an unsuccessful search for it we did a bit more research, and it seems like pins are only placed if the property owners request a survey for and no pins were placed when the government initially surveyed the land as we had assumed. Therefore, it seems like there is a pin in the opposite corner of the property since the far neighbour must have done a survey at some point and driven a stake then. Since the property line that's under dispute is between us and the near neighbour it seems like there's probably never been an official survey, and there is likely no pin to look for.

Additionally, the ground near were the corner is supposed to be according to the government maps of the area is pretty rocky, and as a result its probably not possible drive a steel peg into the ground there anyway. In talking to some other people afterwards I've also heard that surveyors can sometimes use naturally occurring landmarks (such as large identifiable rocks) instead of a pin, so its possible that even if a survey had been carried out there wouldn't be a pin left.

So while my failure to find a metal object that doesn't exists isn't entirely my fault, I think that there is still a lesson to be learned here which is: make sure the solution you've come up with is actually a solution to the problem at hand...

Metal Detecting in a Forest is Harder Than I Thought

Taking electronics outside is always way more of a challenge than I think its going to be, batteries run out faster than I think they will, things that felt light inside start to feel pretty heavy after 15 minutes of walking around, displays are hard to read in sunlight, wires get knocked loose, and on and on and on...

Metal detecting is apparently no exception. First of all,  hiking with the detector turned out to be a lot more work than I thought it was going to be. I figured that since it was lighter than my backpacking backpack it should be easy to hike with. What I didn't realize was how often I'd use my hands for balance on uneven ground, and how much carrying something that's delicate and awkwardly shaped effects that. As a result just hiking with the metal detector was a more challenging than I'd anticipated.

The other big problem is that it turns out to be sort of difficult to swing something around in a forest because you keep hitting trees (revolutionary I know). I think if I cared less about just whacking the detector coil into things it wouldn't be so bad, but I spent a lot of time being careful not to smack things since I was worried that I'd break some part of the metal detector. I think this problem could probably be mitigated by a more robust mechanical design, but hey, I used what I had on hand.

Its Hard to Find Metal When Your Metal Detector Doesn't Work That Well

For being built over a couple of evenings out of random junk, with basically no plan in mind, and no prior experience building metal detectors I think that the metal detector work surprisingly well. Unfortunately, 'working surprisingly well for something made from literal garbage' is not the same as working well...

The biggest issue with the metal detector is that it's just not that sensitive, so you need a really big piece of metal to get an easily identifiable signal. Something like a large nail sitting on the surface just isn't enough to get a really clear signal, instead you need something like a large bolt, or piece of angle iron or something. This also means that the detector doesn't have great penetration depth into the ground either, since the signal that you hear is really dependent on how close the object is in addition to how big it is. Between these two effects it turns out that most of the metal things that the detector picks up on are pretty easy to see on their own. (it does do a good job of picking up rebar in concrete though, which is neat)

I have some brief Ideas about how to improve the detector's sensitivity, but I think that ought to be its own post.