step by step building

[waltr]

Possibly those coils will still work.

Use the method in the TGSL 101.pdf document that uses the TX oscillator to check the coils.
The TX coil should oscillate at about 14.5kHz and the RX coil connected to the same oscillator should run at about 13.9kHz.

The ABSOLUTE Frequency is NOT as important as the difference.

The Finial TEST is when you have both RX & RX coils connected to the TGSL circuit and with your O'scope compare the TX signal with the RX Pre-amp signal and NULL the coils (as shown in the TGSL 101 doc). What is required is the correct Phase shift of about 20 degrees.
If you get this then the TGSL will work.

[windaube]

The Finial TEST is when you have both RX & RX coils connected to the TGSL circuit and with your O'scope compare the TX signal with the RX Pre-amp signal and NULL the coils (as shown in the TGSL 101 doc). What is required is the correct Phase shift of about 20 degrees.
If you get this then the TGSL will work.

my osciloscope has only one channel

I noticed that the coil was on a metal table and the measurement was wrong....
with the coilq on the floor I read :

RX : 6.9mh
TX : 5.2mh

I am thinking of removing one or two turns of the rx coil to have 6.5mh and remake the tx coil with 3 or 4 more turns
this solution is correct ?

for the tgsl it still does work, I am waiting for the pcb, normally I will receive them next week

[waltr]

Ahh... yes, any metal near the coil will change it's inductance.
Even the floor may have nails, screws, wiring in or under it.
I use a wood stool, with no metal it it, or cardboard boxes to ensure coils and away from any metal, by at least a meter.

Before removing winds from the coils wait for the TGSL and build ONLY the TX oscillator and test what frequency the coils run at.
Or do like I did and built the TGSL oscillator circuit on some perf board for coil testing.

Are the coil round or 'D' shaped? This effects the inductance.

From the picture you posted here:
https://www.geotech1.com/forums/atta...3&d=1593974609

the coils are 'D' shaped. In that picture the 'straight' sides of the coils bend outward too much.
Get then straighter and re-measure the inductance.
These will need to have less area 'inside' the over lap to obtain correct null.

[windaube]

Or do like I did and built the TGSL oscillator circuit on some perf board for coil testing.

Are the coil round or 'D' shaped? This effects the inductance.

you are a genius !!

I made the TX assembly and now it is very very easy to adjust the shape to obtain the desired frequency

my measuring device is really bad, I think the inductance reading was wrong

[6666]

Whats the device you are using to show the frequency ?

[windaube]

Whats the device you are using to show the frequency ?

this is "DSO138", low cost oscilloscope ( 10 - 15 $)
https://fr.aliexpress.com/item/4001117005820.html

[Qiaozhi]

this is "DSO138", low cost oscilloscope ( 10 - 15 $)
https://fr.aliexpress.com/item/4001117005820.html

You can also buy the DSO138 on ebay at ridiculously low prices.
I was almost tempted to get one ... just because they're so cheap.

[ivconic]

You can also buy the DSO138 on ebay at ridiculously low prices.
I was almost tempted to get one ... just because they're so cheap.

... also ridiculously useless instrument too. Had one. Sold it.
Actually it is splendid instrument for fresh beginners to learn bare basics.
Q. are you feeling like one?

[Qiaozhi]

... also ridiculously useless instrument too. Had one. Sold it.

That's why I decided against buying one. I knew it would be just a toy, and would soon end up in the odds & ends box.

[waltr]

you are a genius !!

I made the TX assembly and now it is very very easy to adjust the shape to obtain the desired frequency

my measuring device is really bad, I think the inductance reading was wrong

The frequencies look very good and I see you straightened the flat part of the 'D'.
Looks like your coils are good.

Guessing you did adjust the coil shape while looking at the oscillation frequency.

[windaube]

... also ridiculously useless instrument too

I knew it would be just a toy

yes yes it's really cheap, but ... I tested with a frequency generator and it read the frequency correctly
to make a coil I am satisfied

Guessing you did adjust the coil shape while looking at the oscillation frequency.

yes it is the best solution !
I did not understand before, this is the quick and efficient method ever

[6666]

yes yes it's really cheap, but ... I tested with a frequency generator and it read the frequency correctly
to make a coil I am satisfied


yes it is the best solution !
I did not understand before, this is the quick and efficient method ever

Thanks for the info, I have 5 devices that read the frequency of sine waves quite nice, but were they all seem to have a problem is with triggering on square waves and the mark to space ratio is large, for example when I was building the 555 oscillator in Eric's pin pointer he published, all the counters had a problem determining what part of the square wave to trigger on, so to get the correct frequency, I had to look at the wave form with my real CRO and measure the mark to space ratio and apply some math which I hate , hence my interest.

[windaube]

Hello

I finished the assembly but the sensitivity is bad, only 5cm for a big piece of metal

without the rx coil I am at 14.5 but when I connect the rx coil the frequency changes to 14.3
on pin 7 (u101) before nulling I have 14.3khz

tx coil only : 14.5khz
rx coil only connect to tx : 13.95khz

when everything is wired : 14.3khz in TX and 14.3khz in pin 7 u101

rail 8v : +8v
rail -5v : -6V

do you have an idea ?

[windaube]

I replaced the cable for a real usb2 cable, now I have 14.5khz on pin 7 u101

after the nulling coil I have 0.25V on the pin7 u101.
a coin of 1euro at 10cm does not change the wave, a 50cl aluminum bottle changes the wave at about 5 - 7 CM

I think the lack of sensitivity is caused by the coil

[waltr]

Sounds like the first cable has too much capacitance.
I use Belden-M (8723) cable but USB cable should be good.

You will not see much change on the RX signal at U101 pin 7 and need the target to be close or large There are two more Gain stages after the pre-amp and before the comparator.
Do you have the rest of the circuit built yet?