How to complete my surf pi detector?

[skylly]

I have build a surf pi, but there is some problem in it.
when I tune the offset trimmer to make the pin 6 of ne5534 to 0 Voltage, whatever how I tune the delay trimmer， the detection is very low , about 1 cm.
If I tune the offset trimmer to make the pin 6 of ne5534 to about 2 Voltage, there is a ringing sound from the loudspeaker when i don't place metal near coil, Nonetheless it can detect a metal at about 10 cm.
the coil is about 220uH, the dumping resistor is 360R.





I have record the ringing sound like this, when there is no metal approach to coil。
0008.zip

[WM6]

Where did you buy this version of Surf PI PCB?

[Tepco]

I have build a surf pi, but there is some problem in it.
when I tune the offset trimmer to make the pin 6 of ne5534 to 0 Voltage, whatever how I tune the delay trimmer， the detection is very low , about 1 cm.
If I tune the offset trimmer to make the pin 6 of ne5534 to about 2 Voltage, there is a ringing sound from the loudspeaker when i don't place metal near coil, Nonetheless it can detect a metal at about 10 cm.

Extend coil connection to detector using some suitable cable and move coil away from rest of the circuit (and any metal nearby). With coil too close detector will respond to it's own operation causing oscillations at a rate of detector response speed.

[skylly]

Where did you buy this version of Surf PI PCB?

I use this .lay file (I forget downloaded from which website) to the local circuit board mill to provide some sample PCB.
surfmaster.zip

btw: The file I uploaded is slightly modified: bc556=>2n3906, bc337=>mpsa13。

[skylly]

Extend coil connection to detector using some suitable cable and move coil away from rest of the circuit (and any metal nearby). With coil too close detector will respond to it's own operation causing oscillations at a rate of detector response speed.

Thanks for your reply.
OK, I will find some Coaxial Cable and try again.

[sinclairuser]

also the 1000uf cap near the fet needs to be installed, nice board by the way.