Threshold Pot doesn't work

[h9361]

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.

Thank you dear Carl for your information.
Is C1 electrolyte type?
About LM393, Pin 4 is GND and Pin 8 is VCC. In HHD, +VB is GND. Do you think i connect -VB to pin4 and +VB to pin8?

[Carl-NC]

I would use ceramic for C1. R2 can start at 0. Pin4 = -VB, pin8 = ground.

[h9361]

I would use ceramic for C1. R2 can start at 0. Pin4 = -VB, pin8 = ground.

Thank you for your kindness.

[h9361]

I would use ceramic for C1. R2 can start at 0. Pin4 = -VB, pin8 = ground.

Hello
Dear Carl
I hope you are in good health. Please forgive me for my questions, i again need your help.
Please see the attached, it is a part of a DISC and PI machine (it is belongs to Mr. dfbowers). The red box is output of DISC and the Blue box is a tone frequency divider.
Before, i tested the DISC (schematic) on my HHD, my result was good. Now, i want to connect it to my HHd so that my tone is changed as soon as the DISC has an output, like the attached schematic (divider tone frequency).
I also added audio part of HHD at right side (in attached). As you know, My HHd is using 1K for tone. I prefer to 500Hz (divider) if DISC output is generated.
Do you have any suggestion for this ( generating low tone if DISC generates output)?

[h9361]

I would use ceramic for C1. R2 can start at 0. Pin4 = -VB, pin8 = ground.

I hope you see my post
and forgive me for my request.
thank you

[h9361]

I would use ceramic for C1. R2 can start at 0. Pin4 = -VB, pin8 = ground.

Dear Carl
I think i should adjust my HHD Audio as VCO ( like attached), then i use a frequency divider for my DISC output.
Since your HHD VCO also used a frequency divider, i don't know how to add an extra divider to it so that i have a lower tone for Iron detection.
I hope you can suggest me a solution.
Thanks.

[Carl-NC]

You can either run two 555 timers to generate both tones, or run a single 555 timer and use a divider chip (such as a 74393) create the low iron tone. If you want to have a VCO response for normal targets then use two 555 timers: one as a VCO and the other as a fixed iron tone. Use a comparator on the DISC channel to select the tone.

[h9361]

You can either run two 555 timers to generate both tones, or run a single 555 timer and use a divider chip (such as a 74393) create the low iron tone. If you want to have a VCO response for normal targets then use two 555 timers: one as a VCO and the other as a fixed iron tone. Use a comparator on the DISC channel to select the tone.

Thank you for your valuable guidance.
I tried to make a schematic with two simple 555 timers ( one Iron and one non-ferrous ).
But i do not know how to switch between output of the two timers using my DISC output.
The only method that reach to my mind was to use 4066, but i still don't know how to use it.
I put the schematic in attached, i hope you can guide me how to switch between the timers output.
Thank you.

[Carl-NC]

Offhand, I don't know. In HHD you have U8 directly driving a VCO which then directly drives the speaker. Most likely you will need to completely redesign the audio back-end to add the iron tone. Since the HHD audio was designed to support either the VCO audio or a fixed tone audio it may be possible to alter the circuit so that the DISC signal switches between them. For example, apply the DISC signal to IC7c-pin6 and when DISC goes HIGH it will disable the VCO drive. But then you also need to change the VCO into a fixed-frequency iron tone generator, maybe use IC7d for that. This will take some work to figure out and I don't have the time to do that, sorry.

[h9361]

Offhand, I don't know. In HHD you have U8 directly driving a VCO which then directly drives the speaker. Most likely you will need to completely redesign the audio back-end to add the iron tone. Since the HHD audio was designed to support either the VCO audio or a fixed tone audio it may be possible to alter the circuit so that the DISC signal switches between them. For example, apply the DISC signal to IC7c-pin6 and when DISC goes HIGH it will disable the VCO drive. But then you also need to change the VCO into a fixed-frequency iron tone generator, maybe use IC7d for that. This will take some work to figure out and I don't have the time to do that, sorry.

Thank you dear carl for your tips.
I have a question about 2F coil (attached). As you know, it has two serial mono coil with different inductance and damping values. My question is about calculation of the values.
For better understanding, HHD uses 680R + 500uH inductance. Do you think we should divide the values in two parts? for exp, first coil (300uH+180R) and second coil ( 200uH+510R), sum of them will be 500uH+680R?

[Carl-NC]

The coils couple with each other so if you make one coil 300uH and the other 200uH you will end up with more than 500uH. I would suggest 100-150uH for each coil individually. The damping resistors are effectively in parallel so they will need to be higher than 680Ω, probably in the range of 1k-2k. Maybe someone has posted the L & R values used in the White's DF coil, I don't remember them. Otherwise you will need to build it & figure it out.

[h9361]

The coils couple with each other so if you make one coil 300uH and the other 200uH you will end up with more than 500uH. I would suggest 100-150uH for each coil individually. The damping resistors are effectively in parallel so they will need to be higher than 680Ω, probably in the range of 1k-2k. Maybe someone has posted the L & R values used in the White's DF coil, I don't remember them. Otherwise you will need to build it & figure it out.

Thank you for your information, i will test it.
Generally, is it possible to tell me your opinion about advantages and disadvantages of this coil rather than simple mono coil?

Dear Carl, i have a request about one simple idea. As you know, my TP4 voltage will increased if we detect a metal target.
But i want to add a simple led (with a simple threshold or gain pot for controlling sensitivity ) to my HHD so that the led is turned on if TP4 voltage is decreased ( reverse than detecting a metal target).
In fact, I want to notice whenever the voltage of my coil drops, do you think it is possible?

[Carl-NC]

The DF coil is not as good on large deep targets as a large coil, and not as good on small shallow targets as a small coil. But it is more consistent across all targets.

I'm sure it's possible to add an LED to TP4, but under what conditions do you think the voltage will go negative?

[h9361]

The DF coil is not as good on large deep targets as a large coil, and not as good on small shallow targets as a small coil. But it is more consistent across all targets.

I'm sure it's possible to add an LED to TP4, but under what conditions do you think the voltage will go negative?

According to my real test (i saw more and more), some vacuum void strongly decrease the voltage so that your threshold sound (pit... pit....) will be completely silent.
As you know, we have to adjust threshold as silent for solving mannerization. So, we can not detect such void.

[Carl-NC]

OK, but then if you will get a negative response if:

1. you slightly lift the coil off the ground
2. you sweep the coil over uneven ground
3. you sweep over a negative hot rock
4. you sweep over a target (the "bounce-back" response)

I think the LED will be constantly flashing. I thought you added ground balance to the HHD, that should have solved this problem.