Late 1960's BFO

[WM6]

Looks like it works to me.

Don't worry, it works.

You only need to know, that if working (measuring) frequency is 500kHz and you need coil for 6kHz MD, measurement accuracy will be worse than 1% (as in technical data).

[HECTOR]

Working frequency of your L meter is according technical data you posted about 500kHz (switching down to 500Hz at bigger L measurement).
It is OK for BFO measurement but less ok for VLF measurement if you try to follow someone coil data measured at 1kHz working frequency of most cheap L-meters.
But if you can control working frequency of your L/C meter over USB cable it can be ideal.

Not sure I follow what you're saying. It seems to me that this meter tunes itself to whatever inductor that it's connected to. If there's an internal clock frequency, I don't know where to access that measurement. With no inductor connected, it pulses hi-lo once per second. With an inductor connected, it assumes the resonant waveform of that inductor. Is this not typical?

Here are the readings for my 4 Tesoro coils. Couldn't find anything to compare them to. Spent over an hour looking for inductance data for Tesoro HOT coils. Only found uMAX #'s.
These will be posted in my 4 coil shootout thread.
10X12DD Pins 1,3 1.289mH Pins 2,4 22.13mH
9X8C 1.258mH 19.68mH
5.75"C 1.212mH 20.90mH
5.75"DD 1.142mH 19.37mH

So, if anyone else has any of these coils and an expensive L meter, let's compare notes to see how accurate this thing really is. Looks like it works to me.

[WM6]

Working frequency of your L meter is according technical data you posted about 500kHz (switching down to 500Hz at bigger L measurement).
It is OK for BFO measurement but less ok for VLF measurement if you try to follow someone coil data measured at 1kHz working frequency of most cheap L-meters.
But if you can control working frequency of your L/C meter over USB cable it can be ideal.

[HECTOR]

I think labelling coil in mH is ok (at least for classical BFO).

1Hz working frequency for L/C meter? Pretty unbelievable. Do you have technical data?

The test was done with no inductor connected. I thought that it was crazy low too.
Here are the specs provided on ebay. No paperwork came with the unit.
100% brand new and high quality
modle:LC100-A
Supply power: +5v ,mini USB interface
Measurement accuracy:1%
Capacitance measuring range: 0.01 pF - 10uF
The minimum resolution: 0.01 pF
Big Capacitance measuring range ：1uF-100mF/minimum resolution:0.01UF
Inductance measuring range: 0.001 uH - 100mH
Big inductance measuring range: 0.001 mH - 100H
The minimum resolution: 0.001uH
Test frequency range: L/C about 500KHZ/Big inductance 500HZ
Effective display digits: 4 digits
LCD display mode: 1602
Powered by Mini USB cable(included)

[WM6]

Thank you very much Sir!
The new meter appears to be operating at about 1Hz@1V (looks just like a 555 output on my scope)

Note:In the last schematic, the search coil is labeled mH and it should be uH.

I think labelling coil in mH is ok (at least for classical BFO).

1Hz working frequency for L/C meter? Pretty unbelievable. Do you have technical data?

[HECTOR]

Hi Hector,
very diligent work,
what is working frequency of your new L/C meter?

Thank you very much Sir!
The new meter appears to be operating at about 1Hz@1V (looks just like a 555 output on my scope)

Note:In the last schematic, the search coil is labeled mH and it should be uH.

[WM6]

Hi Hector,
very diligent work,
what is working frequency of your new L/C meter?

[HECTOR]

Hi Skippy
The #'s are in!
I squeezed the base into the case a little and it varied from 46-48uH

Updated schematic.
This should make a functional breadboard model

[HECTOR]

Hi Skippy
The #'s are in!
I squeezed the case a little and it varied from 46-48uH

[HECTOR]

I had a spare moment, so I made my own copy of Hectors original search-coil. Tech details are: 10 turns of 16/0.2mm PVC insulated copper wire, with nominal circumference of 38cm, nominal diameter of 122mm. Heavy braided tinned-copper screen stripped from screened 6-core power cable, 10mm gap.. The 10 strand bundle was 6mm diameter average, and with the braid included, total diameter was 7mm. Measured inductance was 25.8 microhenry, with or without the shield connected to one end of the coil. This is exactly as predicted, which is nice. Construction was a bit tricky, getting the last 2 turns in was a slow process.

Your timing (and build accuracy) is amazing!
Just got my new L/C meter from errr... OK China. Read some reports that it's accurate. Didn't want to give them any money but couldn't pass up $26 with free shipping.
I'll pull the PCB coil and test it very soon.
Wanted to test all 4 of my (Tesoro 4 pin) coils first, to include with the data in my 4 coil shootout.
Thanks Skippy!

[Skippy]

I had a spare moment, so I made my own copy of Hectors original search-coil. Tech details are: 10 turns of 16/0.2mm PVC insulated copper wire, with nominal circumference of 38cm, nominal diameter of 122mm. Heavy braided tinned-copper screen stripped from screened 6-core power cable, 10mm gap.. The 10 strand bundle was 6mm diameter average, and with the braid included, total diameter was 7mm. Measured inductance was 25.8 microhenry, with or without the shield connected to one end of the coil. This is exactly as predicted, which is nice. Construction was a bit tricky, getting the last 2 turns in was a slow process.

[HECTOR]

Cut the trace that connects the audio transformer to common ground and moved the speaker wire to the isolated lead. No noticeable changes.
Changed Q1 with a 2N2222 metal can type. No noticeable changes.
Wound another small 2cm coil and was able to get full speaker volume with this one. Going to do some more trial and error coil building and see if I can come up with very small a DD type that will tune properly. Any guidance for making a coil that will be most sensitive to small items would be appreciated.

Changed all transistors with 2N2222A with no problems.

[HECTOR]

Cut the trace that connects the audio transformer to common ground and moved the speaker wire to the isolated lead. No noticeable changes.
Changed Q1 with a 2N2222 metal can type. No noticeable changes.
Wound another small 2cm coil and was able to get full speaker volume with this one. Going to do some more trial and error coil building and see if I can come up with very small a DD type that will tune properly. Any guidance for making a coil that will be most sensitive to small items would be appreciated.

[HECTOR]

Hi Gwil
Good work

Yes, ditto.
Wouldn't have made the much more logical and easier to read transformation without Gwil.
My original rendition was a top view actual layout. That's the only way that I could come up with to trace the circuit and keep track of things while doing it in multiple stages.
Once again, thanks everybody who got involved in this project
Is it time to move on and create a far better pinpointer than anything else on the market? I'm convinced that most detectorists would prefer a more variable and smooth tone, rather than being tweeted at here and there. The ability to switch to an LED ladder would make it a leader in the field for those who like to be stealthy
I bet some of the brainiacs here could make a REALLY small example of this circuit that would fit on your index finger and be fully functional

[peterr88]

Hi Gwil
Good work