Polish Pi

[Qiaozhi]

Wonder if it an underdamped wave form. Scope looks like it's on 50us/div(maybe not?). Wonder if maybe a target signal?

Look at the 560R image. The waveform would settle in about 25us if it was critically damped.

[green]

Look at the 560R image. The waveform would settle in about 25us if it was critically damped.

You might be right. I'm not used to seeing coil decay taking over 50us. Would be interested in see a scope picture of coil volts not amplifier out with 560R and 1000R damping.

[kt315]

The 560R image shows an underdamped waveform. The 1k resistor image shows even more underdamping. I would guess that the correct damping resistor value is 470R.
You can see in the 1k image that the preamp saturates to the negative rail during flyback, and then saturates to the positive rail before decaying to 0V. The correct value should make the preamp output saturate to the negative rail, followed by a smooth decay to 0V. The bottom line is that 1k is too high, and 560R may cause some instability.

i think his coil is not 350-380uH but less. with 560 Ohm he has to get near-to-good picture.

[surfdetector]

Look at the 560R image. The waveform would settle in about 25us if it was critically damped.

This is a prime example of why it is easier to just use a damping jig to identify the correct ohm value.

[Qiaozhi]

This is a prime example of why it is easier to just use a damping jig to identify the correct ohm value.

... and also a prime example of not understanding what you're looking at.
The signal must decay smoothly to 0V from the negative rail for this circuit, not the positive rail. A misunderstanding can lead someone to think that the damping resistor value of 1k is the correct one.

[green]

Tried a spice simulation to try and understand why decay took so long. Some thoughts, maybe not correct. Maybe someone could correct where I'm wrong.

Coil decay lot faster than amplifier out. Amplifier BW/gain, 50MHz/3700=13.5kHz controls decay rate not coil decay.
Rd can be adjusted so decay starts near zero(very fine adjustment). Probably doesn't improve detection over starting decay above zero reply #176. Starting above zero might be better, takes a bigger target signal to over range.

LTC6244 specs about twice as fast as the 357 used in the detector.

[Carlos]

Us? la lM318P
Carlos

[Bayu]

Green Pin 10 4011
Yellow pin 5

[surfdetector]

[ATTACH]54603[/ATTACH]
[ATTACH]54603[/ATTACH] Green [ATTACH]54603[/ATTACH] Pin 10 4011
Yellow pin 5

@Bayu, what did you use as your schema for the VCO? Please show us.

[Mr.Jaick]

my experience on two sample delay resistors:
i tried 20k for both (27uS sample delay) with a 50cm spiral flat coil about 350uH
detects gold very good but as soon as you try to go over some clean ground it does not work, too sensitive to capacitive effect of ground
have to shield it
but increase the resistors up to 27k both, it works like a champ
40uS delay if i remember correctly and very little ground effect
still catches gold
12gram bracelet 18k : 80cm
2gram ring 14k(not sure) : 17cm

[Qiaozhi]

12gram bracelet 18k : 80cm
2gram ring 14k(not sure) : 17cm

That's a huge bracelet and ring!
I assume you actually meant 80mm and 17mm?

[dbanner]

i think his coil is not 350-380uH but less. with 560 Ohm he has to get near-to-good picture.

I concur. The coil itself is under the so called " inhabitable zone".

You may want to wind a new coil, and don't be surprised to see different results.

[Mr.Jaick]

That's a huge bracelet and ring!
I assume you actually meant 80mm and 17mm?

those are the detection depths
we're not giants

[Qiaozhi]

those are the detection depths
we're not giants

That's funny.

When you say "detects gold", most people immediately assume you mean gold nuggets. Rings (whether made of gold or not) enhance the flow the eddy currents, and are the easiest targets to detect.

[Mr.Jaick]

That's funny.

When you say "detects gold", most people immediately assume you mean gold nuggets. Rings (whether made of gold or not) enhance the flow the eddy currents, and are the easiest targets to detect.

That is indeed
"nuggets" oh boy...
detecting rings with this detector is a big achievement on it's own