Extreme Ground effects on the coil?

[nick_f]

Hi All,

I built my Hammerhead project and now I am at a stage where I do adjustments of the pots on board. The board seems to work ok, although the voltage in TP9 seems to vary a bit for no known reasons and it causes variations in the audio tone. I will check more later.
Now I just noticed something which is of concern for me: the width of the pulse in TP4 greatly varies with the ground proximity. I attach in here two screenshots, one with the coil suspended in the air, and the other one with the coil placed on the floor. I live in an unit and the floor is not actually the real ground, but I think this doesn't matter.
My question is what happens when I go outside if the width of pulse at TP4 varies so greatly with the distance from soil? Does that means the adjustment of the delay at TP6 is useless?

Regards,
Nicolae

[nick_f]

What a sucker I was! The drastic change in the width of the TP4 signal must be created by the reinforced steel bars in the floor! To confirm this, I brought a pot with soil next to the coil and obviously, there isn't even the slightest change in the width of TP4. It wasn't the ground effect, it was the steel effect after all

[Carl-NC]

I was going to suggest your floor must have some metal in it, but it would have to have A LOT of steel to cause that much shift! Apparently it does.

- Carl

[nick_f]

Hi Carl,

It's funny because I have very little experience with any metal detectors and I already got trapped in some of the well known things. For example, I had another unused coil under the table and a few on the table where I was testing the board and there was a lot of ringing at TP4. I decreased the dumping resistor a lot, and the ringing wasn't disappearing! Then I came across by accident to a post where somebody was mentioning this effect and I was able to get rid of ringing very quickly I also had a problem with the 7660, which was only working without the external sync. As soon as I was connecting the sync resistor, the output was dying. I noticed by accident that when I was turning off the power supply, at some stage, 7660 started working. I found out that it was working only for input voltages of max 4.5V or so. I replaced the 7660, solved the problem. I put an IC socket and tested again the first 7660, this one also works fine now even at 12V! Have no idea what was wrong, probably I didn't solder it right. Meantime I decreased the resistor R37 to 1k, it works well (I will try again with the original 10k).

Regards,
Nicolae