Probe PCB from Silverdog

[Qiaozhi]

Silverdog sent me a blank PCB for the probe circuit from "Inside the METAL DETECTOR" as he was having problems getting it to oscillate. Consequently I thought I would write some brief notes to guide anyone else attempting to build this design, and to include some useful updates to the circuit.

The prototype circuit featured in the book was built on stripboard, and used components that happened to be available in my parts box at that time. Although the component values do match those in the prototype, it would be advisable to lower the value of R1, as it is currently at the high end of the tolerance range. For anyone wanting to experiment with the design, you can try replacing R1 with a 5k multi-turn preset to allow testing with various oscillator amplitudes.

Finer adjustment of the threshold level can also be achieved by lowering RV1 to 1K.

I also took the oppotunity to add a piezo buzzer (+5V type from Murata) across D4/R5 to provide a quite loud audio response when a metal target is detected.

Silverdog has experimented with the coils wound on a ferrite rod with good results, and you may find it interesting to try some larger coils as well. There is no need to place both coils inside the search head. L1 could be a choke placed inside the enclosure, and L2 can be a mono loop in the search head. However, if both coils are placed close together (as in the original prototype) then the coils need to be in anti-phase, otherwise the oscillator will fail to start.

One final modification I made, in this second unit, was to replace the original DPDT switch with an SPST type. In this case the -ve supply from the battery is switched, and the center pin is connected to 0V. This then allows C4 to be discharged via R6 when the unit is turned off, thus resetting the threshold. Please see schematic below.

Here are few photos of the completed unit.

[dfbowers]

[I have not played with your probe circuit yet (but did buy your book), and I noticed some similarities with the old White's T/R, probably "off resonance" as well?
I have build several of these and noticed that it's very pickey about the tuner resistor. Once you figure it out by trial and error, the circuit is fun to play with.
Also, the tuner resistors seem to be highly dependant on the transistor gain, so unless you get a similar transistor every time you are back to trial and error.

One question I have to anyone else that is familiar with the attached circuit: How does the AGC feature of this detector work? Apparently it does work well as it is not affected by the ground.

Don

[Qiaozhi]

[I have not played with your probe circuit yet (but did buy your book), and I noticed some similarities with the old White's T/R, probably "off resonance" as well?

The TX circuit certainly has a similar topology, which is probably due to them both being Hartley oscillators that require two coils (or a center-tapped coil).

I'm not sure why the design you posted is labelled as a "T/R", since it clearly does not have a receive coil, but relies on an amplitude change in the TX oscillator when a metal target is present. The CD4024 is being used as a frequency divider, so that the 12kHz frequency is divided down to 375Hz to produce an audio tone. This tone then varies depending on whether a metal target is present or not. I'm not sure how the AGC part of this circuit works, but it appears to be accomplished via the 100k resistor between Q5 and the pulse input.

The ITMD probe circuit is different, as it compares the output of a fast peak detector with a similar slow peak detector, thus providing a switch-on-and go probe with no external adjustments required. The output (in the book) is an LED, but a piezo buzzer can be added to give an audio indication of a target.

Whites produced another design where the TX oscillator (Hartley configuration) was similar to the probe and to the CM-1000. This was called the AF101, and cost £65 (including VAT) in 1993.

[dfbowers]

I bought a White's TR Coinmaster Series 2 off of ebay, almost in mint condition but it didn't work. Someone sent be the schematic to help me troubleshoot. Turned out to have a bad CD4024. Not sure why White's labeled it "TR".

[Dixix]

I also bought the very interesting book. (My compliments to the authors!). And I built the pinprobe (p. 33) Unfortunately, the small device does not work. Too bad. I suspect that the coils do not work. How can I check that? (I'm electronics novice). Thanks for a tip!
regards
Dirk

[Qiaozhi]

I also bought the very interesting book. (My compliments to the authors!). And I built the pinprobe (p. 33) Unfortunately, the small device does not work. Too bad. I suspect that the coils do not work. How can I check that? (I'm electronics novice). Thanks for a tip!
regards
Dirk

The probe circuit does actually work, but I would advise changing R1 to a 5k multi-turn preset (as mentioned above).
Are you using the same inductors specified in the book, or have you wound your own?

[Dixix]

Hi Qiaozhi
Thank you for the quick reply! Yes, I'll replace R1. I have 2x RF Choke 1mH used as described in the book. I have all a question: Is it allowed a picture of my PCB to show here? I have developed an own PCB according to the plan on page 38th With the "Sprint Layout". Thus, the PCP is smaller. And fits better in an ABS case. I can send you a picture of the layout of the plan (p.3?
Thank you for your patience.
regards
Dirk

[WM6]

Is it allowed a picture of my PCB to show here?

Dirk

This is free forum (yet) Dirk, so it desirable and welcome to show different approach and solutions. More of those - better.

[Qiaozhi]

Hi Qiaozhi
Thank you for the quick reply! Yes, I'll replace R1. I have 2x RF Choke 1mH used as described in the book. I have all a question: Is it allowed a picture of my PCB to show here? I have developed an own PCB according to the plan on page 38th With the "Sprint Layout". Thus, the PCP is smaller. And fits better in an ABS case. I can send you a picture of the layout of the plan (p.3?
Thank you for your patience.
regards
Dirk

Yes, no problem. Please post the PCB layout.

By the way, if you place both coils close together then make sure they're in anti-phase, otherwise the oscillator will fail to start.

[WM6]

I also took the oppotunity to add a piezo buzzer (+5V type from Murata) across D4/R5 to provide a quite loud audio response when a metal target is detected.

You mean this (?):



Can you suggest simple way of how to connect piezo transducers instead of buzzer?

[Qiaozhi]

You mean this (?):

Yes, that's correct. It gives a good loud audio signal.

It must be the type that beeps when you connect it to a battery, and not the piezo speaker type that justs
clicks.

[WM6]

It must be the type that beeps when you connect it to a battery, and not the piezo speaker type that justs
clicks.

Thanks.
In fact I am asking you for simple sulution that can be replacement for self-beeping buzzer and allow us to use beside piezo speaker or piezo transducer, crystal earpice too.
Something like this for dinamic speaker:

[Qiaozhi]

Thanks.
In fact I am asking you for simple sulution that can be replacement for self-beeping buzzer and allow us to use beside piezo speaker or piezo transducer, crystal earpice too.
Something like this for dinamic speaker:

Obviously you will need some form of audio oscillator, such as a 555, and then gate the timer output from the comparator.
Have you tried the solution in your circuit diagram?

[WM6]

I am trying to go as simple as possible, preferably one transistor solution, but using piezo transducer or crystal earpiece, cause of low consumption. 555 is one solution, bot not the most elegant.

I didn't check circuit posted above it is as example of simplicity. This is what I checked as working, but 2 TR and dynamic speaker:

[Qiaozhi]

Hi Qiaozhi
Thank you for the quick reply! Yes, I'll replace R1. I have 2x RF Choke 1mH used as described in the book. I have all a question: Is it allowed a picture of my PCB to show here? I have developed an own PCB according to the plan on page 38th With the "Sprint Layout". Thus, the PCP is smaller. And fits better in an ABS case. I can send you a picture of the layout of the plan (p.3?
Thank you for your patience.
regards
Dirk

Something just occurred to me. As you're using your own PCB, I assume you've seen the errata list?
There is a small error in the schematic for the probe, as follows:
Page 38 – Fig. 4-5 – Off-resonance probe has incorrect pin numbers on IC3. These should be VDD=8, VEE=4, INV=3, NINV=2 and OUT=7. The PCB layout is correct.