Threshold Pot doesn't work

[h9361]

Minelab's DVT uses short high-voltage pulses combined with long low-voltage pulses to implement an improved ground balance. You can increase sensitivity by just increasing the TX voltage but power (heat) will also increase. You can also increase sensitivity by increasing either the TX pulse width or the TX pulse frequency, but again power (heat) will also increase. If you just want to find small targets (small gold nuggets) then you can decrease the pulse width and increase the frequency to improve sensitivity. I would try this before increasing the TX voltage. If you want excellent sensitivity to both small and large targets then you probably need a multi-pulse design.

Thank you for your guiding.
I already tested both high frequency (5 KHz) and high pulse width, my results were not very impressive.
Now, i want to test the increasing TX voltage. Is it possible to tell me how i can do it in my HHD?
Thank you.

[h9361]

Minelab's DVT uses short high-voltage pulses combined with long low-voltage pulses to implement an improved ground balance. You can increase sensitivity by just increasing the TX voltage but power (heat) will also increase. You can also increase sensitivity by increasing either the TX pulse width or the TX pulse frequency, but again power (heat) will also increase. If you just want to find small targets (small gold nuggets) then you can decrease the pulse width and increase the frequency to improve sensitivity. I would try this before increasing the TX voltage. If you want excellent sensitivity to both small and large targets then you probably need a multi-pulse design.

What is interesting is that the mosfet of GPX has not any heat sink!

[Carl-NC]

Now, i want to test the increasing TX voltage. Is it possible to tell me how i can do it in my HHD?

Just increase the voltage. You need to look at the voltage ratings of all the components connected to -VB such as caps and the inverter, make sure they are rated above the new battery voltage. Then watch your heat dissipation.

What is interesting is that the mosfet of GPX has not any heat sink!

Yes, if you don't avalanche the MOSFET it won't get hot. That's true of any PI design.

[h9361]

Just increase the voltage. You need to look at the voltage ratings of all the components connected to -VB such as caps and the inverter, make sure they are rated above the new battery voltage. Then watch your heat dissipation.



Yes, if you don't avalanche the MOSFET it won't get hot. That's true of any PI design.

Thank you dear Carl.
Please forgive me for my questions.
So, do you think i only need to increase voltage my battery?

I checked HHD (attached), only speaker and IRF740 parts are connected to -vb.
I am going to use 15-16V for battery, have you any suggestion for change value of the part (connected to -vb) for this voltage?

[Carl-NC]

Check the voltage ratings of C1, IC1, and C10.

[h9361]

Check the voltage ratings of C1, IC1, and C10.

Dear Carl
I tested and my result was very good.
I have a question about some PI machines.
As you know, some PI uses a special push button as Retune/Reset. Off course, some PI calls it as re-balancing.
Is it possible to tell me what the push button does ( from technically)?
Thank you.

[Carl-NC]

In the HH design C19 and R34 form a "SAT" circuit which AC-couples the signals and removes DC offsets. This requires constant coil motion to detect. If you short C19 then the signal becomes DC-coupled and you then have non-motion operation. However, DC offsets and drift can be a problem so you then add a retune circuit, which is a sample-hold feedback that can be reset as needed. It's been discussed in other threads but I don't know which ones offhand.

[h9361]

In the HH design C19 and R34 form a "SAT" circuit which AC-couples the signals and removes DC offsets. This requires constant coil motion to detect. If you short C19 then the signal becomes DC-coupled and you then have non-motion operation. However, DC offsets and drift can be a problem so you then add a retune circuit, which is a sample-hold feedback that can be reset as needed. It's been discussed in other threads but I don't know which ones offhand.

Thank you for your explanation.

[h9361]

In the HH design C19 and R34 form a "SAT" circuit which AC-couples the signals and removes DC offsets. This requires constant coil motion to detect. If you short C19 then the signal becomes DC-coupled and you then have non-motion operation. However, DC offsets and drift can be a problem so you then add a retune circuit, which is a sample-hold feedback that can be reset as needed. It's been discussed in other threads but I don't know which ones offhand.

Hello
Dear Carl
I hope you are passing good days.
Please forgive me for my questions.
I have a question about Mosfet of PI machines.
As you know, HHD is using IRF740 (with breakdown 400V) because we have a flyback voltage near it if we set TX pulse width near 100us.
As you know, the higher the flyback voltage, the greater our depth!
Now there is a main question about GPX5000. I checked GPX mosfet and its model is IRF9630!
The breakdown voltage of the mosfet is 200V (6.5A)! So, i conclude that the flyback voltage of GPX is under 200V, but how can it detect target from higher depth with max 200V flyback?!

[Carl-NC]

Sorry, I don't know.

[h9361]

Sorry, I don't know.

I checked gpx on scope in normal mode.
Three short pulses 35us and a long pulse 120us.
I remember that you measured flyback voltage with 50us tx pulse width, it iwas 350v. But gpx uses 120us pulse width, so i think its flyback voltage is more than 350v.

[h9361]

Sorry, I don't know.

Dear Carl
I have a question about Gain of threshold part (HHD).
As you know, Gain of IC8b is 100. Do you think i can increase it for more sensitivity in audio?

[Carl-NC]

You can try. Any of the gain stages can be increased, up to the point something bad happens. You'll have to determine that for yourself.

[h9361]

You can try. Any of the gain stages can be increased, up to the point something bad happens. You'll have to determine that for yourself.

Hello
Dear Carl
I hope you are in a good health.
I need to your help, i hope you accept to guide me, thank you.
Actually, i am working on a project (testing) and need to connect my HHD output to a 12DC/220AC Relay so that the relay output (220) is disconnected if my HHD has output.
I attached a relay schematic but i do not know how to connect it to the my HHD. Is it possible to guide me?
Thank you.

[Carl-NC]

If "output" means the audio output, then HH normally has an output all the time because of the threshold. Probably the best way to do it is to use a comparator with an adjustable threshold, like this:


The comparator can be an LM393. R1 & C1 are a low-pass filter to convert the speaker pulses to a reasonable DC level. You will have to figure those out but I would start with R1=100k and C1=1uF. The pot can be, say, 100k. R2 is an offset resistor that can start out at 0 and you can figure out what to use depending on how much range the pot needs. Simply adjust the pot so that the relay energizes at a certain output level.