A
Anonymous
Guest
Could anyone comment on the positive charactoristics of VCO. I hear its use is primarily for Gold hunting. However someone recently told me it can help in IDing an object? How? Any thoughts?
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A
Anonymous
Guest
Hi John,
Are you refering to the type of audio circuit? Some are variable frequency, where the pitch of the note changes as a target is approached. This is done by a VCO circuit. The other type is where the pitch stays constant and the loudness, or amplitude, changes. Some detectors use a combination of both. Aside from personal preference, I don't know if either method is better for IDing.
Eric.
Are you refering to the type of audio circuit? Some are variable frequency, where the pitch of the note changes as a target is approached. This is done by a VCO circuit. The other type is where the pitch stays constant and the loudness, or amplitude, changes. Some detectors use a combination of both. Aside from personal preference, I don't know if either method is better for IDing.
Eric.
A
Anonymous
Guest
Hi John,
I am somewhat familiar with the VCO concept on VLF's. In essence, as a target is approached, the audio level and the frequency of the tone will increase.
The amount of change in either is a function of the size and type of object approached. Those who have become very proficient on their detectors having this feature can distinguish one metal from another in many cases.
Obviously, this technique isn't foolproof, but it does have its merits. Generally, an iron object such as a washer or other small piece of iron will cause a much higher tone or frequency change than a similar sized non-ferrous object like a gold nugget or a gold ring, even though the volume change caused by the two objects may sound similar.
The reason is on a VLF, an iron object simply creates a stronger signal which thus causes a wider variation in pitch.
I have been experimenting with the concept with my PI. In my case, I have elected to basically use the level change out of my last opamp stage to change the audio tone generator.
The technique I used is crude to say the least since it presently causes the audio to go down, rather than the traditional up in frequency as a target is approached. However, my technique was quick and easy way to test the idea.
This rough form of experimenting did give me a better idea of what to expect and how effective it might be.
So far, it is difficult for me to tell differences in metals, but I haven't really done that much experimenting along those lines either. This type of experimenting will come later.
Instead, I was more concerned if the change in pitch would make it easier to determine a very weak target signal from any background noise.
I have determined that, at least for me, a change in the pitch, or frequency as it is commonly called, does make it much easier to distinguish a very weak signal. It also better defines a target, or in other words, makes it easier to hear or determine a buried object from other signals.
It is my guess that I will notice a difference in pitch change on similar sized objects when comparing an iron object to a non-ferrous one. However, this will be dependent upon the overall gain of the system and other factors which could easily mask the positive effects.
Reg
I am somewhat familiar with the VCO concept on VLF's. In essence, as a target is approached, the audio level and the frequency of the tone will increase.
The amount of change in either is a function of the size and type of object approached. Those who have become very proficient on their detectors having this feature can distinguish one metal from another in many cases.
Obviously, this technique isn't foolproof, but it does have its merits. Generally, an iron object such as a washer or other small piece of iron will cause a much higher tone or frequency change than a similar sized non-ferrous object like a gold nugget or a gold ring, even though the volume change caused by the two objects may sound similar.
The reason is on a VLF, an iron object simply creates a stronger signal which thus causes a wider variation in pitch.
I have been experimenting with the concept with my PI. In my case, I have elected to basically use the level change out of my last opamp stage to change the audio tone generator.
The technique I used is crude to say the least since it presently causes the audio to go down, rather than the traditional up in frequency as a target is approached. However, my technique was quick and easy way to test the idea.
This rough form of experimenting did give me a better idea of what to expect and how effective it might be.
So far, it is difficult for me to tell differences in metals, but I haven't really done that much experimenting along those lines either. This type of experimenting will come later.
Instead, I was more concerned if the change in pitch would make it easier to determine a very weak target signal from any background noise.
I have determined that, at least for me, a change in the pitch, or frequency as it is commonly called, does make it much easier to distinguish a very weak signal. It also better defines a target, or in other words, makes it easier to hear or determine a buried object from other signals.
It is my guess that I will notice a difference in pitch change on similar sized objects when comparing an iron object to a non-ferrous one. However, this will be dependent upon the overall gain of the system and other factors which could easily mask the positive effects.
Reg
A
Anonymous
Guest
Hi Reg,
What would be interesting is to change the loudness with the early sample (just as the Beachscan does), but change the pitch on a late sample. This would require another parallel channel in the receiver, going to the control voltage pin on the TLC555 audio generator.
Eric.
What would be interesting is to change the loudness with the early sample (just as the Beachscan does), but change the pitch on a late sample. This would require another parallel channel in the receiver, going to the control voltage pin on the TLC555 audio generator.
Eric.
A
Anonymous
Guest
Just out of interest Eric has anyone ever tried creating an audio waveform based on the sampled voltages ? If your detector is doing 2000 pulses per second and you feed the sampled voltage straight into the audio amplifier starting with say a 10 usec delay, then take the sample on the next pulse slightly later at 15 usec and so on until after 10 pulses you start again at 10 usec then you would create a 200 hz audio signal shaped like the flyback signal - a target would make the signal louder and possibly different targets would have a different sound due to the different harmonics.
Nick
Nick
A
Anonymous
Guest
I 've recently built my first PI detector (mark Stuart just because I was able to modify the PIC source if necessary).
Apart of the many problems I faced and still not solved and the poor sensitivity, I was wondering if it could be possible have both amplitude and frequency changes with a simple modification.
I find annoying listen those click all the time even with a slow frequency, so I'd like to have some advise on how use the target signal to modify VCO and audio level. I've tried something but I didn't get to any result till now
Thank you
Stefano
Apart of the many problems I faced and still not solved and the poor sensitivity, I was wondering if it could be possible have both amplitude and frequency changes with a simple modification.
I find annoying listen those click all the time even with a slow frequency, so I'd like to have some advise on how use the target signal to modify VCO and audio level. I've tried something but I didn't get to any result till now
Thank you
Stefano
A
Anonymous
Guest
I think poor sensitivity is going to be hard to improve with that design as it is basically the same as the goldpic which I bought and built. They use a sample and hold instead of the differential integrator which isn't going to give the performance since the S&H won't cancel noise the way an integrator does. The Goldpic was fun to play with but isn't much on performance. Eric bought one, maybe he has some suggestions.
Good luck,
Charles
Good luck,
Charles
A
Anonymous
Guest
VCO seems to be something people love or hate. Personally, I like it. The human ear is actually more sensitive to small variations in frequency than it is to small variations in loudness, so VCO audio should give a slight edge with weak signals.
- Carl
- Carl
A
Anonymous
Guest
Hi Charles,
I built both the Gold Pic and the Mark Stuart PI. I decided I didn't want to try to reengineer the Gol Pic, but did modify the Stuart PI.
Actually, when I got done, it was really quite sensitive. Of course it took reprogramming a pic, adding another amplifier stage which also provided an autotune and a few mods to the power supplies to get there.
However, like you said about the diff integrator, the single sample did suffer from the earth field effects.
I think if a person were to use a figure 8 coil, much of that would be eliminated.
Unfortunately, then a person would have to modify the power supplies even more than I did to have a really stable detector.
I do have to say, it was a very good learning experience though.
Reg
I built both the Gold Pic and the Mark Stuart PI. I decided I didn't want to try to reengineer the Gol Pic, but did modify the Stuart PI.
Actually, when I got done, it was really quite sensitive. Of course it took reprogramming a pic, adding another amplifier stage which also provided an autotune and a few mods to the power supplies to get there.
However, like you said about the diff integrator, the single sample did suffer from the earth field effects.
I think if a person were to use a figure 8 coil, much of that would be eliminated.
Unfortunately, then a person would have to modify the power supplies even more than I did to have a really stable detector.
I do have to say, it was a very good learning experience though.
Reg
A
Anonymous
Guest
Hi Charles,
I am still trying to improving the circuit but unfortunately with no good result.
I've decoupled operationals and 555 supplies with a 100 ohm and 10uF capacitor but the audio tone has still frequency shift when set to a higher frequency than the suggested 1 Hz, and I'm even speaking of indoor (in shack) air-test so not earth field effects.I re-wrote the source and now I can change any time with 5usec step. I found several suitable sets of data, but almost any of them allows to detect a 2 cent of euro coin (19 mm copper coin) at 12-14 cm at the moment.
I would really like to know something more about the modifications you made expecially that of the amplifier and auto-tune stage.
Thank you in advance for any further help.
Stefano
I am still trying to improving the circuit but unfortunately with no good result.
I've decoupled operationals and 555 supplies with a 100 ohm and 10uF capacitor but the audio tone has still frequency shift when set to a higher frequency than the suggested 1 Hz, and I'm even speaking of indoor (in shack) air-test so not earth field effects.I re-wrote the source and now I can change any time with 5usec step. I found several suitable sets of data, but almost any of them allows to detect a 2 cent of euro coin (19 mm copper coin) at 12-14 cm at the moment.
I would really like to know something more about the modifications you made expecially that of the amplifier and auto-tune stage.
Thank you in advance for any further help.
Stefano
A
Anonymous
Guest
I must take issue with you on one point. When it is done right a sample and hold will beat the spots off an integrator. Getting rid of the noise is a seperate issue. There is no question that the detector becomes MUCH more sensitive if you use a very fast sample gate time. I use a 1uS sample.
The fast sample provides excellent signal seperation. Unlike an integrator, a sample and hold has a very high speed response to a target. This is critical if you want to experiment with discrimination.
The fast sample provides excellent signal seperation. Unlike an integrator, a sample and hold has a very high speed response to a target. This is critical if you want to experiment with discrimination.
A
Anonymous
Guest
Hi Dave,
I am inclined to agree with you that a sample hold is better. It just takes a few more parts to accomplish the same thing as a differential integrator. Your design seems like a great way to go.
The Stuart PI does use a sample hold, but doesn't take a second sample, so I started to build one, but I used a perfboard. Somehow I managed to get something wrong and blew a PIC. I should have built a pc board but I HATE MAKING PC BOARDS. Because the power supplies also needed a lot of work also, I gave up at the time.
At the time I was experimenting with the Stuart PI, I did take the sample time down but not to 1 usec. I could only go down so far due to the speed of the pic. I needed to go to a faster PIC to get down any shorter.
As I went down in sample time, I didn't notice any noise increase, but did notice a sensitivity increase. However, my observations were less than scientific, so there may have been a slight increase in noise that I didn't obviously hear in the headphones.
Stefano if you read this, I just turned the sample on, then the next instruction, I turned the sample off.
One other thing I liked about the PIC was I did do a little experimenting with time between pulses and found I could reduce this time quite a bit with no obvious loss of sensitivity, nor did it seem to affect the speed at which I could sweep the coil.
Reg
I am inclined to agree with you that a sample hold is better. It just takes a few more parts to accomplish the same thing as a differential integrator. Your design seems like a great way to go.
The Stuart PI does use a sample hold, but doesn't take a second sample, so I started to build one, but I used a perfboard. Somehow I managed to get something wrong and blew a PIC. I should have built a pc board but I HATE MAKING PC BOARDS. Because the power supplies also needed a lot of work also, I gave up at the time.
At the time I was experimenting with the Stuart PI, I did take the sample time down but not to 1 usec. I could only go down so far due to the speed of the pic. I needed to go to a faster PIC to get down any shorter.
As I went down in sample time, I didn't notice any noise increase, but did notice a sensitivity increase. However, my observations were less than scientific, so there may have been a slight increase in noise that I didn't obviously hear in the headphones.
Stefano if you read this, I just turned the sample on, then the next instruction, I turned the sample off.
One other thing I liked about the PIC was I did do a little experimenting with time between pulses and found I could reduce this time quite a bit with no obvious loss of sensitivity, nor did it seem to affect the speed at which I could sweep the coil.
Reg
A
Anonymous
Guest
try turning the connections to the coil the other way round
sound like you have it wrong.... check decades of all resisotrs you used
give the code i reworked a try
i also added a sheet of hardware mods to inplace
if you need to cut the pop from the speaker from the vco this is easy to do ... by changing
c5 c7 r11 to get a different freq
the voltage swing you need to preseve as this is encoded with the single from the present find...
so is realy must be the source of the poor sensativity
remember to use a 22cm coil with 17T of .5mm wire this is the best one {wound left to right in circles clockwise....
sound like you have it wrong.... check decades of all resisotrs you used
give the code i reworked a try
i also added a sheet of hardware mods to inplace
if you need to cut the pop from the speaker from the vco this is easy to do ... by changing
c5 c7 r11 to get a different freq
the voltage swing you need to preseve as this is encoded with the single from the present find...
so is realy must be the source of the poor sensativity
remember to use a 22cm coil with 17T of .5mm wire this is the best one {wound left to right in circles clockwise....