TM 808 White's

[pito]

Post #219
He doesn't have harmonic, he has interferences from 50Hz power lines.
This is sine with harmonics, second and third

[pito]

Post #218 if it's just an Rx coil without the amp the results aren't perfect but ok now you can work with those coils and you should get good results. A can of Coke, at least 50 cm.
When checking the distance (6 m or 8 m), keep your small oscilloscope away from the Rx coil.

Post #214, how far away do you get reactions to a can of Coca Cola? You have 66 cm, keep it, you don't need 80 cm, increasing the distance it is easier for balancing, 10 mV instead of 40mV = not much benefits.
Now you can add amplifier U5A.

Your Rx coil is in the air, so there is no metal close to it, metals close to Tx do not much interfere with nuling.

[Skippy]

I have no experimental experience of two-box machines, but from what I know about conventional DD & concentric coils, this is worth knowing:
The 'null' output signal is not going to be exactly in or out of phase with the TX signal. It can be resolved ( example with synchronous detectors ) into X and R signals, one of which is in/out of phase with TX, one which is 90 or 270 degrees away. When you null the two-box coil ( by adjusting the angle of one coil relative to the other ) you will find you are almost exclusively adjusting the R. So even when you have the 'null' at minimum, you still have some residual X.
The way to remove that is more complex. One solution can be using a real 'target' , fixed in place in 'just the correct location'. The target can be a piece of metal, ideally with target-frequency ( corner frequency ) much higher than the detector, or it could be a 'model' target, made from a loop of wire, or a loop of wire with a resistor in series with it, to change the L/R and hence target frequency.
This is all more complex on a two-box, because of all the existing metal junk that is already there : cables, PCB full of parts, battery, shafts etc etc.
So nulling the '808 properly is a tough job.

Another problem with viewing the RX signal on a 'scope screen: It is not just a nice sine wave, there will be some 'other stuff', such as 3rd and 5th harmonics, high-frequency ringing. Though the appear on the scope, they are usually not important, as they are not at the fundamental ( approx 6650Hz ) freq. Trying to 'null' them is a pointless effort.

[berreizeta]

Up to 6 meters there is a good signal and frequency, at 8 meters the signal is 10mV

https://youtu.be/LtcAQp7NTmI

[berreizeta]

Thanks Skippy, that's what I was thinking, with all the metal around me it must be difficult to adjust.

I'm going to move everything to a place with less "iron"..

For the tests I am taking into account that the centers of the coils are 80cm from each other, which I think is the distance between centers of the original,
regardless of what the coils are: round, square, etc.

[Skippy]

A little bit of advice, guys:
This TM808 device is a Metal Detector. If you have ANY metal near it , it will be detected .... there will be a signal coming from the search coil. If you want any of your tests and experiments to have any meaning, you need the search coil to be A LONG WAY AWAY from any metal objects.
If you get an experimental 'null' from your coil, remember it is a null WITH all the metal junk INCLUDED. If that junk moves, for example you pick up your pliers and put them down elsewhere, or you move your oscilloscope 2cm to the left, your coil signal WILL change.
All this is annoying enough with conventional 25cm search coils. I am sure a two-box will respond to many more objects.

[berreizeta]

Freq.: 6.533Hz

TX Coil : 759uH, 782nF

RX Coil: 29mH, 20n47

[berreizeta]

On the collector of Q1 I have 22.5V

At 6 meters one coil in front of the other I get 10mV

This is the minimum signal at 66cm perpendicular ~40mV

[pito]

[QUOTE=berreizeta;n414771]I'm going to make a new RX toroid, the one I use has too much resistance 7.8ohm
and what is the inductance of it ?

[pito]

For comparison Rx antennas: use your yellow loop add capacitor connected in parallel that will be not perfect but good Rx template, then compare Rx with toroid with this one.

[pito]

Without losing the frequency: 30mV,

if I lower the signal further the frequency goes away...

frequency losing = synchronisation losing = ok = good balancing, then you connect amplifier U5A

[berreizeta]

I'm going to make a new RX toroid, the one I use has too much resistance 7.8ohm

I will try to get 2.1ohm and 17.5mH for RX

[berreizeta]

Without losing the frequency: 30mV,

if I lower the signal further the frequency goes away...

[pito]

Adjust the coils to the minimum signal, without losing the frequency :

https://youtu.be/E_VlChTRuSI

Sensitivity is very low, that can be to low transmitter power, or Rx not in rezonanse. Put Rx and Tx 10m or 5m apart, adjust for max voltage on RX , at 10m you need about 10 mV.

Maybe there is someone who have access to TM808 an can measure emitted power of Tx for comparison ?

[pito]

Adjust the coils to the minimum signal, without losing the frequency :

https://youtu.be/E_VlChTRuSI

and what is your minimum in volts?