Whites Surf PI schematic

[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 ?????

[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.

[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]

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.

[maxm3s]

q1 -4,37 v

My transistor q1 is giving -4.37 is this correct? any sound in speaker

[kt315]

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...

Hi All,
Below is a repost of an article I did a while back on my Tech forum.
"There are many factors that influence the design of a PI coil. However, let's decide on two starting parameters i.e. 100uS pulse width and 2kHz pulse rate. One very important factor is what the earliest sampling time is required to be, as this does have a major effect on the maximum useable coil inductance. The greater the inductance and the higher the current, the longer it takes to reduce the field to zero and the later the sample delay has to be. If you are designing a detector for beach and shallow water hunting a good minimum sample delay to aim for is 15uS. With the right components in the transmitter circuit you can then use a coil inductance of 300uH. I have found it best to wind the coil with 10/0.1 PVC insulated stranded wire (10 strand, 0.1mm strand diameter) as this minimises eddy currents generated in the wire cross section. For a 10in coil the winding resistance is about 4 ohms and with an additional 4.7 ohm 4W resistor in series (in the drive circuit) gives a coil time constant (L/R) of about 30uS. For a 100uS TX pulse width (3.3 times the coil TC) the current will have reached over 95% of its maximum value at switch off, which is a satisfactory situation.
Another important factor to decide at the outset is the type of battery and the required battery life. At 2kHz the current consumption with the above values will be about 250mA at 12V which really requires the use of 10 C cells as a minimum. You can of course design for less current and smaller batteries by using a shorter TX pulse, a higher inductance coil with higher series resistance but there will be some performance trade off. Some PI detectors, particularly earlier designs, do not use a series resistor in the coil circuit to limit the current. Is this case the pulse current is often limited by the coil inductance i.e. the current is switched off well before it has reached its final value which would be determined by the coil resistance. Alternatively, the pulse repetition rate can be lowered until the current is of an acceptable value. There are disadvantages to both of these techniques:- Inductance limiting results in the magnetic field still changing rapidly in a positive direction at the point of switch off. This can cause some signal loss due to switch on eddy currents cancelling those generated at switch off. In pulse frequency limiting, the response time and noise averaging of the detector may not be as good.
The shunt, or damping, resistor, will be determined by the resonant frequency of the coil plus stray and cable capacitance. Not being able to display formulae yet (hope to soon as scanner arrives on Monday) you could look up in a radio or electronic text book the formula for a critical damping resistor for an LC with a given resonant frequency. Or you could just try it experimentally. Start with 1k ohms and look at the receiver output on a scope. The receiver (again it depends what front end I.C. you use) should recover cleanly with no ringing before the point of sampling. Add more resistors in parallel until this is achieved. You should end up with maybe 500 ohms across the coil as described above. Use a good quality 0.5W metal film as the peak emf at switch off can reach several hundred volts and I have found that smaller wattage carbon film resistors can break down and go noisy. I have tried a cermet preset in series with a fixed resistor to give an easily variable adjustment but again the high voltage can cause burning at the wiper contact and eventual failure.
Eric.

[daverave]

3000Hz is uncomfort freq for human sensing... 800hz is too... if you do not believe me you will see... better freq in 200-300Hz range... in Surf you can not tune TX generator on 3000Hz and hear 200Hz... you hear _the TX generator_sound_directly. try to either add a VCO circuit... this part you may plagiarize from Hammerhead schematic... or you may add a part with CD4024 divider, in tesoro's schematics manner, and to divide the 3000Hz to 3000:16=187...

i didnt realize that the search frequency and audio frequency are the same....now it makes sense to me....maybe i just leave it as it was designed and experiment with hammerhead which ive already completed but not set up as yet...i really want to learn about detectors as i find them so interesting compared to my old profession of tv/video repair which i hardly do now.

[Watanabe]

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.

Hello Tepco .
What is the performance of your " Surfenstain " ?
How far to detect a currency ( 1 euro or equivalent ) .
Have some video of the operation ?


Thank you.

[Tepco]

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.

[kt315]

3000Hz is uncomfort freq for human sensing... 800hz is too... if you do not believe me you will see... better freq in 200-300Hz range... in Surf you can not tune TX generator on 3000Hz and hear 200Hz... you hear _the TX generator_sound_directly. try to either add a VCO circuit... this part you may plagiarize from Hammerhead schematic... or you may add a part with CD4024 divider, in tesoro's schematics manner, and to divide the 3000Hz to 3000:16=187...

[daverave]

maybe im a dummy but im only learning to make detectors...i have a lot to learn and it takes time....i just thought higher frequency better for small gold items and better pinpointing and search speed....im learning each day from you guys and by reading....my back ground is tv and video engineer and not metal detector electronics...so im having to adapt myself to this.

[kt315]

if i wanted an adjustable frequency say up to 3000hz and adjustable tx drive what components do i need to add to the 555 circuit ????

Dave, you are a dummy... sorry but it is true. you COULD build Hammerhead of Carl Moreland, but you want only pull in general stream you see in forum. HH _has_ VCO, Surf is _not. but yes, Surf is simplest in realization. if you will re-tune Surf freq say up to 3000Hz you will hear mosquito sound. so leave out your 'super duper surf mods nobody still do not know them but I am forward in this f-ing sh-t hi tech electronics' I ask you now... you can increase PULSE WIDTH to 100us, may be 200-300. enough. if you will use nice modern japan accu batts...

[daverave]

if i wanted an adjustable frequency say up to 3000hz and adjustable tx drive what components do i need to add to the 555 circuit ????

[starmoml]

help me

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

[daverave]

thanks Joop...greetings from UK