Whites Surf PI schematic

[daverave]

Dave, if you will notice something from Eric Foster or anybody on hight impedance coil in your english web-www-zine share this info in forum.
those detectors with the coil are working in 3-10kHz range like you so are looking for now... I taken this his post from PI TECHNOLOGY forum... it is in english and this is VERY clearly for you maybe... not for me... as your english is perfect... my gut says me... it is what you want to learn deeply...

the eric foster text that you sent is very interesting...im i right in believing that when the coil switches off then an opposite magnetic field is produced that reduces any field within the target item so loosing some depth on that item ???? also what do you think about using a series resistor with the coil ???? i can see now about consideration with tx on time and battery consumption...maybe its more important to get the circuit noise down and external interference than to increase tx power.

[Qiaozhi]

the eric foster text that you sent is very interesting...im i right in believing that when the coil switches off then an opposite magnetic field is produced that reduces any field within the target item so loosing some depth on that item ???? also what do you think about using a series resistor with the coil ???? i can see now about consideration with tx on time and battery consumption...maybe its more important to get the circuit noise down and external interference than to increase tx power.

When the coil current switches off, the magnetic field collapses. It is this changing magnetic field that generates eddy currents in any nearby metal targets. What Eric is saying is that a [smaller] changing magnetic field (of opposite polarity) also occurs during the coil ON period. By adding a suitable series resistor to the coil, and/or with a sufficiently long switch-on time, the changing current during the ON period can be made to essentially flatten out. This allows some time for the On period eddy currents to diminish in amplitude before the coil current is switched off. If you switch the coil current off while the ON period current is still rapidly increasing, then the subsequent eddy currents generated in any nearby metal targets will be partially canceled, since they are of opposite polarity.

[daverave]

so am i right in thinking that we are only interested in the currents produced when coil switches off ???? i would imagine also that we can sample earlier if a series coil resistor is used reducing the back emf to maybe 200v instead of 400v as in many designs.

[daverave]

Here is the one “Surfenstain” I built many years ago, with independent audio frequency adjustment and stronger audio output. I lost, or even never made schematic, manually routed, but board is 100% error free, just assemble as is. Tested fine at 10kHz, you can modify everything independently, this is original timing, just change components.

what was the proformance like using high frequency...were there any problems in searching with this detector ?????

[Qiaozhi]

so am i right in thinking that we are only interested in the currents produced when coil switches off ????

This is correct for a conventional PI detector, such as the Surf-PI. Some experimentation has been done to try and make use of eddy currents generated during the On period, although I'm not aware of any commercial detectors that incorporate this feature.

i would imagine also that we can sample earlier if a series coil resistor is used reducing the back emf to maybe 200v instead of 400v as in many designs.

Reducing the coil current will allow earlier sampling, as mentioned by Eric, as will reducing the coil inductance. Basically, the quicker you can get rid of the flyback pulse, the earlier you can sample.
Of course, you might think that fitting a low value damping resistor would be the solution (i.e. overdamping) but this only diverts energy into heating the resistor, instead of stimulating the target.

[6666]

Sean_Goddard made an interesting comment, by adding his comment to Erics information in post #794 makes good reading

If you put an oscilloscope ACROSS the coil, you will actually be measuring the "equivalent" current through it (Ohms law?) So set the pulse widtch until the signal JUST starts to clip, at that point you know you have maximum current flow. Any more and you are just wasting battery power, any less and you are not developing the maximum field for your coil.

Optimising the PW in this way will give you best depth and best battery life (barring external factors not mentioned).

[daverave]

Sean_Goddard made an interesting comment, by adding his comment to Erics information in post #794 makes good reading

thats a very interesting comment..that is worth experimenting with and see the clip point on the scope

[starmoml]

help

My circuit does not work. PLEASE guide me
voltage basis of IC circuit.؟؟؟

how much voltage and Fercance TP ???????

[Mohamed Ghitas]

hello everyone
in my circuit when i plug power it made discontinuous beeb
and the Q6 gets hot
here is the circuit



Note:
i replaced Q6:BC337 with KSP13 ---> MPSA13
and replaced Q1:327 with 2N3906 and i inverse its polarity according to datasheet

any idea what could be wrong ?
iam using 10 ohm speaker

Thanks in advance

[chemelec]

10 Ohm Speaker.

Better Yet, Use a 32 Ohm or a Much Higher Ohm Speaker.
This is one reason Why I use the TL026 Transformer.

[Mohamed Ghitas]

Better Yet, Use a 32 Ohm or a Much Higher Ohm Speaker.
This is one reason Why I use the TL026 Transformer.

maybe this is the cause of Q6 hot ??

[Fabian]

This PCB has an error, Q6 is in wrong position. Must be reversed.

[Mohamed Ghitas]

This PCB has an error, Q6 is in wrong position. Must be reversed.[ATTACH]26625[/ATTACH]

i replaced Q6:BC337 with KSP13 ---> MPSA13 so its placed correctly

[Fabian]

Sorry. You're right. I have not read the previous post altogether. Just saw the picture and remembered that mistake.

[Mohamed Ghitas]

ok now the Q6 is not heating iam supplying my circuit with computer power supply
the discontinuous beeb stops when i touch the power supply metal body and the circuit works
but with very low sensitivity so is there ghost inside my power supply :P

i think there is some problems with ground connections

any advice ??