Digital BFO-like (but not really) - discussion on feasibility

[Teleno]

Teleno: Have you studied the ETI561 project?

http://www.geotech1.com/cgi-bin/page...i561/index.dat

That seems to be one of the most advanced BFO variants I've seen published. In addition to being technically interesting, it may be a good reference point for establishing what a BFO can achieve, when comparing it to your designs.

Yes I did, but thanks anyway for the pointers.

[Carl-NC]

Speaking of "Fun with BFOs," I was looking through some old notes of mine, back to the late 90's when I first started playing with metal detector circuits. In 1999 I drew out an idea for a multi-frequency VLF BFO. Last week, 18 years later, I finally built the thing. It works, at least on the bench. If I get a strong itch, I'll stick it in a box and take it outside. It's strictly a novelty circuit, lets me brag that I have multi-frequency VLF BFO. Maybe the only one in the world.

[homefire]

Is it multi channel too Carl ? Trade Secret?

[Dave J.]

Back in the Pleistocene era, Gardiner (no relation to our John Gardiner) patented and manufactured multifrequency eddy current loss detectors. Circuit-wise they have a lot in common with a multifrequency BFO.

[Joe(TX)]

Speaking of "Fun with BFOs," I was looking through some old notes of mine, back to the late 90's when I first started playing with metal detector circuits. In 1999 I drew out an idea for a multi-frequency VLF BFO. Last week, 18 years later, I finally built the thing. It works, at least on the bench. If I get a strong itch, I'll stick it in a box and take it outside. It's strictly a novelty circuit, lets me brag that I have multi-frequency VLF BFO. Maybe the only one in the world.

I really like the BFO's..I know that the circuit is obsolete but that pleasing sound is hard to beat. Once you get your circuit in a box and take it for a test drive I would like to see some pics.

[Carl-NC]

Is it multi channel too Carl ? Trade Secret?

Yes, 2 PLL channels that are BFO-mixed.

Back in the Pleistocene era, Gardiner (no relation to our John Gardiner) patented and manufactured multifrequency eddy current loss detectors. Circuit-wise they have a lot in common with a multifrequency BFO.

I had forgotten about that, thanks for the reminder.

[homefire]

You must be using Anti Phase 180 Apposed coming out of Each PLL ?

[Tec]

BFO using Arduino

Here is one I made on the weekend: "The simplest Arduino BFO"

The hardware is simple, but the code is smart (attached)


The coil is a standard coil that I would use for a PI detector (30 turns, 19cm) (Haven't tried other coils)
The frequency is 16kHz (Feel free to try others)

The Tank oscillator is actually driven by the arduino: Every cycle an interrupt is generated, and the oscillator is topped up.

The frequency is measured at 16MHz resolution, a few times a second, so it measures changes of tiny fractions of a Hz. 1 part in a billion or so.

Performance:

It is sensitive for a BFO.
But like every BFO I have made, it is also sensitive to the ground and surroundings.
Nice clear difference between Iron (decrease frequency) and non-Iron (increase frequency)

Other notes:

Sensing the amplitude is probably possible, every cycle:
ie. with some additional circuitry, could control the amplitude of the sine wave exactly, by controlling how much the tank circuit is topped up.
If an eddy current steals some power, we will see a drop in amplitude and increase in the amount required to top up.

For example.

Arduinos are awesome.

Tec

[Teleno]

Here is one I made on the weekend: "The simplest Arduino BFO"

That's the spirit, well done!

You may increase the precision of the freq. measurement if you trigger a timer1 capture in sync with the analog comparator. This is done by setting the ACIC bit in the ACSR register. Then at each interrupt you read the frozen count from the ICR1 register, which is no longer affected by interrupt latency.

[kosacid]

@Tec i was working on the same but i was using a pickup coil and phasing it at 180 the best i got was iron non iron using the arduino nano

[6666]

Good work Tec.

[homefire]

Hope you don't mind. Converted .ino file to Txt so we can see how you are doing this.