IGSL

[Davor]

Both BC 327 and 547 are fine for your purpose. Good Tx setup is achieved when oscillator is working in slight saturation. So you increase amplitude until it hits the maximum, and turn a potentiometer just a little bit more.

[liekwang]

?I could make more in the distance but also it could be in action against the wave of a hand if I play disc 1 and 2 can be low in depth and not in action against a wave of the hand and the bricks but poor depth

[liekwang]

?sensitive to the flick of the hand above the coil and bricks but the depth of 30 cm can not work as GB if the threshold I rotate to the right depth is decreased considerably and is not sensitive to the bricks

[Davor]

So you learned one of the functions.
What I suspect is that your GB is not adjusted properly, and that your coil shielding is not connected right. Please explain how you set your shield, and what wires you connected where in your Rx.

Basic adjustments:

Tx is supposed to operate in slight saturation. It is achieved when you find a point where oscillations start reducing, so you turn a bit back into saturation. The best operation point can be checked with an oscilloscope, but unlike the common wisdom, the best point to observe is the very trimmer. As an oscillator goes into saturation, the current through this trimmer inflects (I'm not sure if this is the right term because inflection in math is something completely different), and it can be observed with a scope. The key is to avoid negative currents, e.g. the inflection must not go beyond the positive rail.

To adjust GB initially, you need a piece of ferrite, any size, but preferably not a ring. In case you are using the original setup, you have 2 trimmer potentiometers to adjust so that both channels are silent when waving a ferrite in front of your coil. So you turn it one side, and you get a ferrous tone. Turn it slowly to the other side and ferrous tone is vanished - so your ferrous channel GB is set. repeat for the other channel.

To adjust discrimination potentiometers you need a hank of foil. Wave it in front of your coil and turn a discrimination potentiometer of one channel. Observe how a foil is detected at one potentiometer side, and not the other. Now turn a potentiometer from non-detection to a position where foil is starting to appear as a tone. Do the same with the other channel. Now your detector is set to discriminate coloured metals with one tone, ferrous with the other, and foil, small gold and poor conductors with a mixed tone.

[sini]

Hi Davor,

I would like to react to your post #1555. You wrote , that you made center tapped coil. I made center tapped Rx coil too, but for Tesoro eldorado. I made center tapped coil by winding of two same paralel wires. Is it correct ? what is advantage of center taped coil ? Can be omitted Rx coil shielding ? what about wet grass ? My Eldorado did not work well. With Eldorado I had problem with discrimination and air detection distance for one euro was only 20 cm.I left this project, so I focused on IGSL ( my air detection distance is same - 20 cm, but discrimination works fine )

[Davor]

Centre tapped coil works in differential mode and should be connected to a differential mode preamp. The centre tap goes to signal ground. Eldorado has a preamp similar to IGSL, so you just have to take care about the signal ground. It is indicated as connected to the inverting branch of a preamp in Eldorado, so just take care it is not connected to the coil end, but the coil tap.
It works fine for me, but I'm not entirely happy with the preamp configuration. I'll try some other resistor values, and if I'm happy with that it will be a final solution for my IGSL. I have to move on.

[6666]

Davor is this what you did to the Eldorado ?

[Davor]

Exactly. Only it was IGSL, but essentially it is the same thing. I'm about to try a combination of 1k5 and 100k in feedback. It is going to increase Rx/Tx angle, but in case of IGSL it seem as a proper course of action, as it will increase Fe discrimination range, and it will improve noise for a few dB. If it works fine I'll introduce it among my other mods.

A differential mode coil requires a differential mode input, and a single opamp configured in differential setup has a downside of loading a coil unevenly. Apparently it is not too much of a problem, but a better solution would be a real differential amplifier, or an instrumentation amplifier. The former can be realised with ordinary transistors that exhibit low Rbb and have a very low noise as a result. This solution is cheap in comparison with a real instrumentation amplifier, but also better for our purposes as very low noise instrumentation amps are not too easy to come by.

Anyway, If I'm happy with reduced resistors - I'll be pragmatic about it and won't tinker with it any more.

For a good example of a differential amplifier in front end see SMW. Sadly one of the inputs is directly connected to the signal ground there, but that would be a way to do it.

[sini]

Hi Davor,

I am sorry for my stupid questions:
Can be omitted Rx coil shielding in the center tapped coil ? What about wet grass ?
Another question: Does aluminium kitchen shield foil decrease sensitivity of Rx coil in comparism with Rx coil without shielding ?
Another question: In this forum I can read some recomendation for thickness of wire of coils - for example IDX Rx coil 0,15mm. Can I use wire 0,28 mm? Does it matter ?Does it change performance of the coils ( does less resistivity - higher Q change performance - especialy detection depth )?
Another question: Is graphite shielding better than aluminium kitchen foil shielding (detection depth )?
Thanks a lot for answering

[Davor]

Can be omitted Rx coil shielding in the center tapped coil ? What about wet grass ?
Yes. Either you shield both Rx and Tx, and take care that they are not forming any kind of short circuit, or you don't use any shield at all but use centre tapped Rx instead. In that case your front end must be configured as a differential amp.

Another question: Does aluminium kitchen shield foil decrease sensitivity of Rx coil in comparism with Rx coil without shielding ?
Yes, but so little that you won't be able to notice any difference. Loss is in order of ~5%

Another question: In this forum I can read some recomendation for thickness of wire of coils - for example IDX Rx coil 0,15mm. Can I use wire 0,28 mm? Does it matter ?Does it change performance of the coils ( does less resistivity - higher Q change performance - especialy detection depth )?
Wire thickness is more a problem for Tx if you use too thin wire, as Tx oscillators tend ti be in problems in case of too low Q. In Rx you'll hardly notice any difference, as Rx front ends are usually much noisier than the equivalent coil resistance. Going with a thicker wire will improve Q a little, but your coil will be much heavier. A good practice would be making both coils ~same weight. So go with square root of the ratio of number of turns to establish ratio of wire thickness.

Another question: Is graphite shielding better than aluminium kitchen foil shielding (detection depth )?
Yes, but it is lots messier and often makes more problems than it solves. It is a must if you are after small gold on difficult terrains and using a PI machine.

[liekwang]

davor actually do I have to wear paraday to a coil and what are the advantages of his musketeer if wear does not wear the same paraday paraday..??

[Davor]

Having a shield enables you to use your coil in single ended configuration while the E-field phenomena, such as wet grass, are reduced to acceptable levels. Single ended operation provides potentially 3 dB less noise than differential. IMHO if you are not certain what to do next - you better put a shield on.

[liekwang]

?If I wear a perisay al foil TX and RX can reduce sensintip and depth of sound becomes unclear just tik tik if I don't wear a perisay sound very clear and can increase the depth of up to 2 cm how precise his resistance to RX and TX coils for my current diameter 27 cm 46omh and RX. TX 2.7 omh ™

[Davor]

just tik tik must be a result of short-circuiting the foil shield

Make sure you understand the procedure of placing, AND INSULATING foil

[6666]

Davor
have you seen the Centre tapped coil differential front end of this design ?