Osciloskope

[dbanner]

I dont think it gives accurate voltage using this scope, only displays waveform. Soundcard is AC coupled. Look at pic in post#26, this shows the freq, measured by software. Click on freq, tab on software to display spectrum analyzer window where freq. of wave is displayed. There are three jacks that are associated with soundcard, I used the microphone(pink), not green(headphone out) or blue(line in). I built a buffer circuit between soundcard and circuit under test. The titles all say Tx.

[dbanner]

Also i forgot to mention that buffer circuit has attenuation resistor, thats why it looks small(amplitude)

[BKY1337]

Ok, i can not get such a nice wave like you. I have played with the settings.

But its ok, thanks for your help. I will use the multimeter for the new coil and hope for 35 cm +.

[dbanner]

Here is better waveform of my TX, the timebase is strange, but its a nice sine wave as you can see, and it measures around 14.5khz. Maybe someone can help me understand why the timebase is off, yet the same software shows freq. of signal at 14.5khz

[dbanner]

Here is actual TX with correct timebase. Now it makes sense. There is something else oscillating at around 24hertz on this line? That smooth sine wave was too good to be true.

[green]

Here is better waveform of my TX, the timebase is strange, but its a nice sine wave as you can see, and it measures around 14.5khz. Maybe someone can help me understand why the timebase is off, yet the same software shows freq. of signal at 14.5khz

Looks like aliasing. 14418/1000 samples/sec would give a 418Hz signal. Sample rate needs to be greater than 2 times measured frequency. 10 or 20 times is a lot better

[dbanner]

I see, well this illustrates the need to get a real oscilloscope, if only a cheap 20mhz analogue. Ebay, here I come.

[BKY1337]

I checked my coils right now and i got 6,06 mH for RX and 6,50 mH for TX.

This means my coils are swapped. I used this coil in the past year. Detection depht was ok but it was chattering.

Thats the reason for my 13,7 kHz.

I need to calculate the frequency because maybe its my color pigment in the epoxy too.

[dbanner]

I'm surprised it even worked.

[BKY1337]

about 27 cm 1 Euro. :-D

[BKY1337]

When the RX is connected to TX it should be 13,95 khz with exactly 6,5 mH, but it is 13,6-13,7khz.
Do you know why? Is this because my caps are only 5% ?

[ripsdevala]

5% or 10% tolerance is only indicate the true value of the caps, bellow or up to 5% from the printed value... I think. The most important is the stability of the caps. Your problem at 27cm max for coins is same with me, i don't know how the other member can 30 to 40cm, maybe bad/fake chips

[BKY1337]

I guess you didnt saw that i swapped my coils and had 27cm :-D.

I could reach over 30cm at the tests (unswapped), but i accidently glued them wrong together.

I mean my frequenzy and the frequency depends on the capacitor value.

I want the 13,95 because my inductance is correct.

I can build the new coils with 13,95 (measured wit dmm) or 6,5 mH (measured with inductance meter) but not both.

Depht is not so important, quietness is more important and proper discrimination.

[dbanner]

Foil sheilding is not too good at all, graphite sheilding makes huge difference in detection range, as you can see from this very good video https://www.youtube.com/watch?v=XnrStRklcUQ. Look towards end of video where he demonstrates two differently sheilded coils. Also, he puts in a calibration screw into coil, so after setting with epoxy, fine adjustments can be made using scope for best nulling. Also quality of op-amps can make some difference.

[dbanner]

When the RX is connected to TX it should be 13,95 khz with exactly 6,5 mH, but it is 13,6-13,7khz.
Do you know why? Is this because my caps are only 5% ?

Yes, capacitor makes big difference to resonance, 6.5mH with 20nF gives 13.95khz. But with 22nF, gives13.3khz, so precision caps are better.