TR locator / 2 box

[kece hon]

Hi everyone
a little help please
i followed the instruction to build TR locator / 2 box
i used this latest know working schematics
instead of transistors TIP31C TIP32C i used TIP41C TIP42C which are a bit robust

i used 16 mhz quarts which gives me 3.3 khz, i did try 32mhz the obtained coil frequency was about 5 khz= 4891 KHZ
last quartz i put gave me the exact frequency as mentioned in the project = 2048hz
both my coils TX and RX when measured using LRC meter gives 7.00 mh
TX= wire diameter is thicker 0.8 mm and coil resistance is 13 ohm
RX= wire diameter is 0.6 mm and coil resistance is 5 ohm


when calibrating ; i put the coils facing each other , beyond 30 cm distance there is no recepetion at all on the RX coil
the obtained results at 30cm on the scope are shown in the picture

Yellow is TX
Blue is reception RX


**problems**
i need more distance than 30 cm ( the advertised distance for two box is greater than 2 meters for big objects)
there is no significant audio change on the headset whenever i put a big object between the two coils or whenever moving them inward or outward

--Question--
Can you help me what is it that iam doing wrong ?? ****Thanks in advance****

[dbanner]

The circuit calls for 32kHz crystal

A T/R Two-Box locator operates on the principle of Induction Balance (IB).

Severe Resonance Mismatch:

Currently, your TX coil and capacitor are far out of resonance, which drastically reduces the strength of the transmitted magnetic field:

Your Setup: You have a 7.00 mH coil connected to a 0.2 µF capacitor (C12). This results in a resonant frequency of about 4.25 kHz.

Because your circuit is driving the coil at 2.048 kHz but the tank is tuned to 4.25 kHz, very little current flows through the TX coil, resulting in a incredibly weak magnetic field.

To make a 7.00 mH coil resonate at 2.048 kHz, you need a capacitance of approximately 0.86 µF.

Incorrect Coil Orientation:

You mentioned that you "put the coils facing each other."

The TX and RX coils must be mounted perpendicular (orthogonal) to each other (typically one flat/horizontal and one upright/vertical) at opposite ends of a rigid frame (usually 1.5 to 2 meters apart).

You must physically adjust the angle of one coil slightly until you find the "null point"—the exact position where the RX coil receives zero (or minimum) direct signal from the TX coil. Only when the system is perfectly nulled will an external metal object disturb the field and trigger a loud audio change.



The oscilloscope screenshot confirms exactly why your detector is not performing as expected.

Looking closely at the on-screen data, the yellow waveform (TX) is being driven heavily into standard square-wave/switching distortions rather than a clean, high-amplitude sine wave, and the blue waveform (RX) reveals that your receiver is heavily saturated.

• CH1 (TX) shows a frequency of 2.048 kHz. This is your digital micro/quartz clock driving the transistors.
• CH2 (RX) shows a frequency of 4.307 kHz.

This heavily confirms the mathematical resonance mismatch. Because your 7.00 mH coil and 0.2 µF capacitor inherently want to vibrate at~4.25 kHz - 4.3 kHz, the RX circuit is actually picking up the sharp, higher-frequency harmonic edge spikes of your square wave rather than your true 2.048 kHz base signal. When a tank circuit is forced to run so far outside its resonant frequency, power transfer drops to near zero.

Re-tune the TX and RX Tanks:

You must force the hardware tank frequency down to match your digital driver. Add capacitance in parallel to your existing C12 and C3 capacitors until you reach roughly 0.86 µF. Alternatively, you can temporarily test a standard 1 µF film capacitor on both tanks; this will bring your resonant frequency closer to ~1.9 kHz, which is much closer to your 2.048 kHz driver than your current setup.

Change Coil Orientation to "Null" the Signal:

Right now, your blue RX channel is reading 168 mV at 30 cm away while facing directly. When you mount these coils onto a two-box chassis frame (spaced roughly 1.5 to 2 meters apart), orient them orthogonally (one flat, one vertical). Watch Channel 2 on your scope and mechanically tilt one coil until that 168 mV reading drops as close to 0 mV (the null point) as possible.

A T/R Two-Box locator operates on the principle of Induction Balance (IB).

Once the system is perfectly resonant at 2.048 kHz and mechanically nulled out, passing a metal object between or underneath the coils will instantly disrupt the balance, causing the RX voltage to jump up on your scope and trigger the audio circuit.

[ivconic]

Hi,
Your main issue is tuning.
The crystal doesn't set the frequency; the coil and capacitor do.
If TX and RX aren't on the same frequency, signal dies fast. That's your 30 cm problem.
Also, 7 mH coils are too small.
One working build used 18 mH (TX) and 22 mH (RX) with 0.27 uF and 0.33 uF caps. Try bigger coils.
Last thing: balance RX to a null (almost zero signal) before testing metal.
Cheers!

[ivconic]

Also, one more important thing: this is a 2box design, so the mechanical balance between the TX and RX boxes is critical.
In a 2box, the TX and RX are basically one big air-coupled coil system. The signal you get when there's no metal is called "air coupling."
You must mechanically adjust the distance or angle between the two boxes until that air signal drops to a minimum (null).
If the boxes are misaligned or too close, the RX picks up strong direct signal from TX, and then a metal object won't make any noticeable difference; exactly what you're seeing.
Try moving the boxes closer/further apart, or tilting one slightly, until the RX signal is as low as possible before testing with metal.
Some 2box units also have a small trimmer or adjustment screw for this purpose.
By the way, I've seen some DIY builds where guys made really simple mechanical setups for fine coil adjustment; like a threaded rod with a nut, a small hinge with a screw, or even a wooden lever with a bolt.
Nothing fancy, just something that lets you move one coil in tiny steps and lock it in place.
That kind of crude but precise adjustment makes a huge difference on a 2box, because even a millimeter off the null and you lose sensitivity.

[kece hon]

The circuit calls for 32kHz crystal

A T/R Two-Box locator operates on the principle of Induction Balance (IB).

Severe Resonance Mismatch:

Currently, your TX coil and capacitor are far out of resonance, which drastically reduces the strength of the transmitted magnetic field:

Your Setup: You have a 7.00 mH coil connected to a 0.2 µF capacitor (C12). This results in a resonant frequency of about 4.25 kHz.

Because your circuit is driving the coil at 2.048 kHz but the tank is tuned to 4.25 kHz, very little current flows through the TX coil, resulting in a incredibly weak magnetic field.

To make a 7.00 mH coil resonate at 2.048 kHz, you need a capacitance of approximately 0.86 µF.

Incorrect Coil Orientation:

You mentioned that you "put the coils facing each other."

The TX and RX coils must be mounted perpendicular (orthogonal) to each other (typically one flat/horizontal and one upright/vertical) at opposite ends of a rigid frame (usually 1.5 to 2 meters apart).

You must physically adjust the angle of one coil slightly until you find the "null point"—the exact position where the RX coil receives zero (or minimum) direct signal from the TX coil. Only when the system is perfectly nulled will an external metal object disturb the field and trigger a loud audio change.



The oscilloscope screenshot confirms exactly why your detector is not performing as expected.

Looking closely at the on-screen data, the yellow waveform (TX) is being driven heavily into standard square-wave/switching distortions rather than a clean, high-amplitude sine wave, and the blue waveform (RX) reveals that your receiver is heavily saturated.

• CH1 (TX) shows a frequency of 2.048 kHz. This is your digital micro/quartz clock driving the transistors.
• CH2 (RX) shows a frequency of 4.307 kHz.

This heavily confirms the mathematical resonance mismatch. Because your 7.00 mH coil and 0.2 µF capacitor inherently want to vibrate at~4.25 kHz - 4.3 kHz, the RX circuit is actually picking up the sharp, higher-frequency harmonic edge spikes of your square wave rather than your true 2.048 kHz base signal. When a tank circuit is forced to run so far outside its resonant frequency, power transfer drops to near zero.

Re-tune the TX and RX Tanks:

You must force the hardware tank frequency down to match your digital driver. Add capacitance in parallel to your existing C12 and C3 capacitors until you reach roughly 0.86 µF. Alternatively, you can temporarily test a standard 1 µF film capacitor on both tanks; this will bring your resonant frequency closer to ~1.9 kHz, which is much closer to your 2.048 kHz driver than your current setup.

Change Coil Orientation to "Null" the Signal:

Right now, your blue RX channel is reading 168 mV at 30 cm away while facing directly. When you mount these coils onto a two-box chassis frame (spaced roughly 1.5 to 2 meters apart), orient them orthogonally (one flat, one vertical). Watch Channel 2 on your scope and mechanically tilt one coil until that 168 mV reading drops as close to 0 mV (the null point) as possible.

A T/R Two-Box locator operates on the principle of Induction Balance (IB).

Once the system is perfectly resonant at 2.048 kHz and mechanically nulled out, passing a metal object between or underneath the coils will instantly disrupt the balance, causing the RX voltage to jump up on your scope and trigger the audio circuit.

Thank you for your very informative feedback
although i did try several values of capacity as you mentioned , i hardly got a small improvement
iam aware of the mechanical balance afterwards , in the original post it says that optimal distance must be found first
thats why i haven't made it up yet to the mechanical part
i will try again and get back to you , sir , Thanks

[kece hon]

Also, one more important thing: this is a 2box design, so the mechanical balance between the TX and RX boxes is critical.
In a 2box, the TX and RX are basically one big air-coupled coil system. The signal you get when there's no metal is called "air coupling."
You must mechanically adjust the distance or angle between the two boxes until that air signal drops to a minimum (null).
If the boxes are misaligned or too close, the RX picks up strong direct signal from TX, and then a metal object won't make any noticeable difference; exactly what you're seeing.
Try moving the boxes closer/further apart, or tilting one slightly, until the RX signal is as low as possible before testing with metal.
Some 2box units also have a small trimmer or adjustment screw for this purpose.
By the way, I've seen some DIY builds where guys made really simple mechanical setups for fine coil adjustment; like a threaded rod with a nut, a small hinge with a screw, or even a wooden lever with a bolt.
Nothing fancy, just something that lets you move one coil in tiny steps and lock it in place.
That kind of crude but precise adjustment makes a huge difference on a 2box, because even a millimeter off the null and you lose sensitivity.

Thank you , i already made the hinge (screws ..) to adjust the distance for balacing
i will try several values of capacity and get back to you sir
it took more that 150 turn to achieve 7 mh
i doubt i can get to 18 Mh , do have any suggestion ?
Thank you

[ivconic]

Thinner wire might do as well, right?

[pechkata]

[kece hon]

Thinner wire might do as well, right?

Thanks , ill try
does it make a difference if coil windings are not tidy and organized? will it affect in resulting inductance?

[kt315]

,,,

[kece hon]

,,,

**YR/R Locator — Device / Transmitter**

A compact metal detector designed to locate medium-sized and large objects on the bottom, including in the coastal zone. Small and minor metal fragments are detected with reduced sensitivity.

**Detection range:**
- Roofing-iron sheet, 60 × 60 cm — 1.5 m
- Roofing-iron sheet, 150 × 150 cm — more than 2.0 m

**Detection area:** at least 1 m².

The detector operates in static mode.

**Power supply:**
- 9 V
- Two “Krona” batteries

**Current consumption:** 25–35 mA

**Controls:**
1. Range
2. Sensitivity
3. Balance

Battery-discharge indication is provided.

**Indication:**
- Audible
- Meter/needle display

**Preparation for operation:**

1. Set the **“Sensitivity”** control to maximum.
2. Set the **“Coarse adjustment”** control to minimum.
3. By moving the transmitter/receiver handle, adjust the unit until the transmitter and receiver signals are aligned, observing the correct polarity.

All adjustments should be made in the absence of metal objects within a radius of at least 2 m of the metal detector.

It should also be taken into account that the mineral composition of the soil affects the detector’s sensitivity. Mineralized soil and certain types of sand can affect detection performance.

[kece hon]

,,,

Thank you, its almost the same as the remarks from the comments inthe forum during construction
I couldn't get it to resonate although i put several values of capacity
i only got the resception to about 1khz - 8khz its oscilating very much whatever capacity value i put
i did follow one recommendation to put 0.82uf on both sides to get near resonance
but i dont know the exact equation of calculus to obtain the capa value
the coils are 40cm by 30cm
Tx= 2048 Khz coil ind= 7mh needed capacity =?

is the above calculus right ??
i need all the help i can get Thanks again sir

[kece hon]

This is the scope readings with cap 0.8uf on TX and none on Rx