SNIFFER XR-71

[Geo]

Geo,


bbsailor

Hi bbsailor.
Last 2 months i am very busy so i have not modify the XR71. I tried it with diferent coils and i show that the detector works better with an inductance of about 500uH and dc resistance 2.5..4 Ohm. With a 10in coil it detects a soda can at 80 cm on air (very good). At 100hz and full power the mosfet is litle hot (I can put my finger on it for long time) but i dont know what will hapen when i will increase the frequency at 400..500Hz. General is a very good detector for large objects especially with large coils.
When i will modify it i will send the results.
Thanks for your reply
Regards

[bbsailor]

More XR-71 Results

Geo,

My XR71 is very good for experimenting with different coils. One thing that you must do is put a heat sink on the coil driver MOSFET. With low resistance coils, below 1 or 2 ohms, the MOSFET can get very warm.

The initial design of the XR71 was for finding ferrous objects. Eric Foster designed a similar circuit for a company called Woodtec. The lumber industry uses Pulse Induction detectors to locate metal in logs before cutting it and ruining very expensive saw blades when they hit the metal.

The overall design of the XR-71 was to find large deep objects. The low frequency, long TX pulse width and long delay indicate this. For those who want to try to raise the frequency by reducing the value of R29 (frequecy limit resistor) you must also reduce the power limit resistor also called the TX pulse width by lowering the value of R42 or the resistor in series with the power control. The main thing to do is to keep the MOSFET operating no hotter than warm. If your cannot keep your finger on it, it is too hot. Another thing you can try to reduce the heat of the MOSFET when operating at higher frequencies is to use a coil with thinner wire or add a series resistor.

For making a coil to find smaller objects, try using AWG 30 which has .1 ohm per foot. Try making the coil 25 turns of AWG 30 at 11 inches diameter for a coil of 7.2 ohms for a frequency above 500 PPS. Also, AWG 28 or 26 will also work but it will have a lower resistance. Try to keep the resistance at about 3.5 ohms to keep the peak coil current at a few amps for frequencies at about 300 to 400 pps. Also, adding a small series resistor (2 to 4 ohms) has the same effect as using a smaller wire diamerter.

Because the pulse delay is so long in this machine, a shield is not necessary. This XR71 also has an adjustable secondary delay set to sample later to reduce noise by subtracting the secondary signal form the first signal which also cancels the noise as the target signal will only be in the first (or earlier) sample while the noise will be in both samples.

I did some experimenting to try to reduce the delay by clipping a variable 20K pot across R48 (27K). By lowering the pot value I as able to reduce the delay some. I suspect that C30 (1n5) could also be changed to a lower value also to reduce the delay.

If you have any other ideas to modify or adjust the delay please let us know?

I did a lot of experimenting and the circuit is still working. I thought that I would have blown something out by now. The key is to watch the current in your power supply. If you are going over about 200 ma, stop! Check the MOSFET temperature with your finger.

Another point: This XR71 design is not optimized to find coins although it will detect them. If coins are want you seek, look at Eric Foster's Goldscan IV schematic to see how this XR71 circuit could be modified or adjusted to find coins using a coil in the 10 to 12 inch diameter range. His coil is 435uH at 3.5 ohms with a 2.2 ohm series resistor and a MOSFET that has a .55 ohm on-resistance for a total of 6.25 ohms. If you get the delay down below about 25uS then a coil shield becomes more important, especially on wet sand. Also, as delays are reduced, the damping resistor value to achieve critical or slightly over-damping becomes more important so as to sample at a flat RX signal location.

Summary. If you MOSFET is getting too warm to touch at the TX frequency you choose, use thinner wire in your coil or add a series resistor of about 2 to 4 ohms and most importantly, add a heat sink!

bbsailor

[Geo]

Hi. Is there any news from Bbsailor about XR71?

[capinrita]

xr71 protel99se file

[QUOTE=fish1985]hi aban

I want to XR71 protel99se 's file.could you sent this file, please?

my email is [email protected]

Gold Fish 2

Guys, i found Gold Fish 2 on e-shop here:

http://mdetectors.com/product_info.php/cPath/26/products_id/37/language/en

Anyone has PCB or Schematic ?

[bbsailor]

Geo,


Now that I have my XR71 working, here are some XR71 and coil observations.
Anyone who has made modifications to this circuit, jump in and let us know what you did and the result. I'll share what comes to my mind with my XR71 operating for two days.

1. Almost any coil will work as this is a high power low frequency metal detector designed to seek out large objects with long time constants. Thicker wire in the range of AWG 22 to 24 will probably work better than wire in the AWG 28 to 30 range because at low frequency operation you want a higher peak pulse current.

2. The more turns on the coil will make a stronger magnetic field but will also make a higher peak pulse voltage. Right now the XR71 has a battery current draw of 70ma to 100ma based on the pulse width (power) setting. Any changes to increase the frequency will have the following effects (1) increase the total battery current draw and (2) increase the peak voltage (3) need the pulse width range reduced to keep the battery power draw within limits and MOSFET heat down. If the MOSFET is getting warm that means that you have exceeded the voltage rating of that particular MOSFET. If it is hot, stop and fix it immediately! I installed a MOSFET socket by using a modified 8-pin DIP socket cut with a fine saw to use only three pins. The pin spacing will line up. Use the sockets that have flat pins. The round pin sockets will not fit the MOSFET leads. This allows you to quickly change the MOSFETS to see which one works best with the peak voltage being generated by what ever coil you choose to use. As the main frequency is made to operate in a higher range (400 to 500 hz), you can use coils with thinner wire as you will be wanting to limit the peak coil current to about 2 Amps (12 volts divided by 6 ohms (coil resistance, MOSFET-on resistance, coax cable rsistance)). Use a power supply and watch the current meter rise as the pulse width is increased or the frequency is increases. I would suggest adding a heat sink to the MOSFET. Here is another modification tip. A 3-pin header socket (.1 inch spacing) will fit the MOSFET pin spacing and will allow you to mount the MOSFET off the circuit board. These header sockets can usually be obtained in pin/socket sets with leads attached. Just solder the leads into where the MOSFET goes and plug in the MOSFET. If the MOSFET is warm, that is OK but if it is too hot to touch, something needs to be changed!

3. The damping resistor needs to match the coil better than the 2.2K resistor that is in the circuit now. This is not so much for obtaining a faster response but to keep the peak voltage within the limits of the MOSFET. Try using a 1 watt 1K metal oxide resistor or just put another 2.2K in parallel with the coil at the coil connector which produces the same result without needing to remove anything from the circuit board. Remember, coils operating at these low frequencies do not need a shield and will have the capacitance of the MOSFET dominate the total capacitance in the TX circuit. In any event, a scope is needed to observe the waveform at the output of the LF357 (or first amplifier stage).

4. If modifying the circuit to operate at a higher frequency, you also need to reduce the main delay to make the XR71 more sensitive to smaller targets with shorter time constants. Once you get under about 30 uS delay, shielding the coil becomes important. Adjusting the damping resistor value become more critical also. At 100Hz with long time constants an unshielded coil will work and not be affected by ground capacitance as the ground response has decayed by the time the sample is taken.

All of these things interact, so changing coils, MOSFETS, main TX frequency range, pulse width range, and delay will need to be observed and balanced.

This is my first low frequency machine, so I have many things to learn.

bbsailor

[Geo]

Hi Ivconic. I believe that a 60cm x 60cm square coil is very sensitive, easy to use and very easy to construct it. My xr71 works at 100Hz (no 400hz) so I need a coil bigger than 300uH (for my opinion). Sorry but I have not a simple one sided pcb. Bbsailor wrote me that he took the same pcb with me and now he try it. I think that he has a collection of coils so we waiting for his results

regards

[ivconic]

Hi Geo


Here is a little something with delay.
I read your report.Well, your XR71 is still good.Your report tell's me that it can
be a very usefull device. You have to find only a proper coil. If u have a simple
one sided pcb for it why do not post here.
Here is an idea for coil: diameter 45cm, 1.9ohm, form 19-35 windings, dependable on wire gauge!
regards!

[Geo]

Hi Aban,

i know PulseStar II and Lorenz 3 & 5. What is Gold Fish 2 ? Is it PI detector? Please send us some links and pictures of GoldFish 2 PCB. Have you some info about this device - PCB or schematics ...

Best Regards: FLEX

I think that Gold Fish 2 is a Pulse Star II clone.

The same for Bandito Impulse that is a Delta Pulse clone.

[aban]

GOLDFISH2

Yes,it is pi disc detector and it can detect at 5.5 meters in the ground.

GOLD FISH 2

Hi Aban,

i know PulseStar II and Lorenz 3 & 5. What is Gold Fish 2 ? Is it PI detector? Please send us some links and pictures of GoldFish 2 PCB. Have you some info about this device - PCB or schematics ...

Best Regards: FLEX

[aban]

DISCRIMINATE IN PI DETECTORS

PI systems can discriminate the metals ,i tested some of them:PSll and LOrenz and Goldfish2 pi detectors.The main role in this filed is the time of going to zero the voltage of the pulses and we must convert this time and period to voltage or frequency.This is kinds of parameter for ferous and noneferouse metal object in the time of refelected pulses and out put of Op-amp for example this time for gold X microsecond and for iron Y microsecond.

help

Any body can send me a both sides(but separate) of pcb for XR71?
I would like to try to make it.
Or any other good PI project.Even is better if it is one sided pcb.
my mail is:
[email protected]
Thanks in advantage.I want to exchange schematic....

[ismail surek]

hi aban

hi aban
thanks for pcb
can you send me pulse star 1 and 2 s shematic or add here
we didn't understand to IC name

please send me
[email protected]

[Geo]

OK,I would glad to upload information about these detectors in the froum.I said that the psl and ll have versions with kinds of action to detect the objects.I have some versions of them and i`ll load some of it in the froum.Version 3 to up can discriminates ferous and noneferous carefully like LORENZ deepmax with 700-1300 pulse per second.We can design the samplying and op-amp section to discriminate the F and NoneF metal object.

Hi Aban.
How the PI can discriminate? If we play with sample then we increase the ability to reject some small metals. This is no discrimination. Can you tell us how PSII disc the metals? Who are the parameters that the new digital PI detectors uses to discriminate the metals? Do you have any block diagram of a PI disc section?