Help with a RACHEL AND STEVE HAGEMAN, BFO

[6666]

This could be a bit easier to build no pesky varicaps

http://freecircuitdiagram.com/2008/11/24/simple-metal-detector-circuit-beat-frequency-oscillator-bfo/

[Qiaozhi]

Perhaps you can help me figure something out. I want to get this BFO working (since i have all the parts now...) but I want to make the search coil larger then 4 inches because my steps are longer then 4 inches. Around where I do most of my detecting, 6 inches into the dry sand is deep enough. Anything deeper then that is usually an old beer can.

BFOs are usually constructed with small coils because it increases the stability. Although I have seen one with a 32" coil!

So here is my question. Should wind a search coil to the demetions I feel I could use, then make an L1 coil that matches the uH of the search coil? Is this thinking correct?

That's probably the easiest method to use. You can use the Coil Calculator here ->

http://www.geotech1.com/forums/showthread.php?t=17909

to determine the number of windings required.

That leads to my second question. What is the best way to wind an L1 coil for this BFO. Should it be a small air coil with a single layer of windings (30 AWG maybe). Would less turns of a 2 or 3 layer coil be better. How about a small air coil with a ferrite rod inserted into it to "tune" it the the proper uH?

Simply jumble-wind the coil. In other words, find a saucepan (or something similar) of the correct diameter and wind the required number of turns around the bottom. There is no need to neatness. Then remove the coil and apply some insulation tape to hold it secure. You may also need to apply an electrostatic shield to prevent coupling the ground. After that, fix the coil to a suitable base.

[Jerry]

OK, thanks for that!

.......but I want to make the search coil larger then 4 inches because my steps are longer then 4 inches.

I would suggest building it according to the article and get that working first so you have a known reference point for when you want to make changes like a larger coil.

As to the length of you step, that has nothing to do with metal detecting. To have success, you need to go slow and overlap your swings a lot. Remember the diameter of the area in which you can detect an object 6 inches down is much smaller than the diameter of the coil at the surface.

Jerry

[der_fisherman]

I would suggest building it according to the article and get that working first so you have a known reference point for when you want to make changes like a larger coil.

As to the length of you step, that has nothing to do with metal detecting. To have success, you need to go slow and overlap your swings a lot. Remember the diameter of the area in which you can detect an object 6 inches down is much smaller than the diameter of the coil at the surface.

Jerry

Good valid advice from you Jeremy.....

[mikebg]

Hi Beach
The circuit diagram contains two absolutely incompetent designed oscillators. The author should learn what means operating point of transistor amplifier and and how an oscillator works. Please connect the following circuit diagram and measure voltage between collector and emitter.

And SPICE shows the incompetent designed operating point:

[Beach]

And SPICE shows the incompetent designed operating point:

OK, now I am a bit confused. Does this mean the circuit is good or not good?

Thanks!

[Beach]

I would suggest building it according to the article and get that working first so you have a known reference point for when you want to make changes like a larger coil.

As to the length of you step, that has nothing to do with metal detecting. To have success, you need to go slow and overlap your swings a lot. Remember the diameter of the area in which you can detect an object 6 inches down is much smaller than the diameter of the coil at the surface.

Jerry

Thanks Jerry! This is what I will try first. Kind of like baby steps before running.

[Beach]

OK, as advised I am proceeding as per the Hageman schematic. Once it is finished then I will try to make what ever adjustments or corrections I can.

Today I wound my first search coil!!!

As advised I looked around my kitchen and found a souffle bowl that was almost 4 inches. The coil ended up to be 4.25 inches. It is 16 winds of 22 AWG magnet wire.



Once wound, I wrapped it with electrical (insulating) tape.
I then wrapped it in aluminum foil tape almost all the way around leaving about a 1 inch gap.

I then applied a single strand of copper wire to the top of the coil and gave it another cover of electrical tape.

I then checked for continuity between the foil and the end of the copper lead and it was fine.

So now on to the second coil.

As advised by Carl, I will endeavor to wind my next coil to 95 uH to match the search coil, but here is where I need help.



What is the best way to wind and mount this coil? Below are some samples of what types of coils and ferrite I have to work with. Any help would be deeply appreciated!





Thanks again!

[Qiaozhi]

What is the best way to wind and mount this coil? Below are some samples of what types of coils and ferrite I have to work with. Any help would be deeply appreciated!

If you use the ferrite rod, you can adjust the inductance by sliding the coil along the length of the rod. Maximum inductance should occur in the middle.

[Beach]

If you use the ferrite rod, you can adjust the inductance by sliding the coil along the length of the rod. Maximum inductance should occur in the middle.

OK, please let me know if this sounds like it would work.

If I wrap a pencil with a few wraps of paper. Wind 30 AWG wire around the paper. Remove the pencil and then insert the small ferrite rod from the picture into the paper and wire cylinder until the uH matches the 95 uH search coil.

Does that sound like it will work?

Thanks!

[Beach]

This could be a bit easier to build no pesky varicaps

http://freecircuitdiagram.com/2008/1...scillator-bfo/

Thanks for this! I will look at it closely!

[Carl-NC]

And SPICE shows the incompetent designed operating point:

Hi Mike,

It's not uncommon for oscillators to have a strange (or unsolvable) DC operating point.

- Carl

[mikebg]

Hi Mike,

It's not uncommon for oscillators to have a strange (or unsolvable) DC operating point.

- Carl

Hi Carl,
I know that you (and most participants of this forum) are not radio amateur designers. Ham designers know that the transistor oscillator is an amplifier with enough high gain. It is absolutely incopetent to design operating point of such amplifier near to transistor saturation, because the gain is minimall and (if oscillation starts) the circuit can not deliver enough amplitude.
However you are correct for the opposite operating point. Then the self-bias makes transistor to amplify in class C and the amplitude of oscillation is largest.

[Qiaozhi]

OK, please let me know if this sounds like it would work.

If I wrap a pencil with a few wraps of paper. Wind 30 AWG wire around the paper. Remove the pencil and then insert the small ferrite rod from the picture into the paper and wire cylinder until the uH matches the 95 uH search coil.

Does that sound like it will work?

Thanks!

The technique you described would work, but the problem is going to be determining the number of turns required. This will need some experimentation.

The designer probably used a standard RF choke, such as these -> http://uk.farnell.com/jsp/search/bro...D2031%2B202745

[mikebg]

OK, now I am a bit confused. Does this mean the circuit is good or not good?

Thanks!

Murphy’s rule when paraphrased for oscillators reads:
“Amplifiers will oscillate but oscillators won’t.”
The following link is for a paper
"Analysis of Common-Collector Colpitts Oscillator"

http://seit.unsw.adfa.edu.au/staff/sites/hrp/teaching/Electronics4/docs/PLL/colpitts.pdf

Note that there is no information how to design the operating point of transistor for reliable start of oscillation and for large amplitude (because the author is not radio amateur :-).