Help with a RACHEL AND STEVE HAGEMAN, BFO

[Beach]

Hello, I am a new to metal detecting. A few months ago I bought a relatively inexpensive Shaper Image Express metal detector for using on my local beaches. Mostly I like walking along the sea shore and thought that picking up a few coins along the way might be fun. As it turns out, I have already found 1 gold wedding ring, 2 silver rings and enough change to pretty-much pay for the units purchase price.

Needless to say I am hooked!

Anyway I decided I wanted to learn better how metal detectors work so I decided to build one. I looked around the internet and found the Hageman BFO schematic and thought it was simple enough that even I could build one.

http://www.geotech1.com/pages/metdet/projects/edn/edn.pdf

But I am having trouble getting it to do anything.

My question is this, has anyone on this forum ever build one of these BFO’s and if so, is there any tricks to getting it working?

Thanks!

[der_fisherman]

Hello, I am a new to metal detecting. A few months ago I bought a relatively inexpensive Shaper Image Express metal detector for using on my local beaches. Mostly I like walking along the sea shore and thought that picking up a few coins along the way might be fun. As it turns out, I have already found 1 gold wedding ring, 2 silver rings and enough change to pretty-much pay for the units purchase price.

Needless to say I am hooked!

Anyway I decided I wanted to learn better how metal detectors work so I decided to build one. I looked around the internet and found the Hageman BFO schematic and thought it was simple enough that even I could build one.

http://www.geotech1.com/pages/metdet/projects/edn/edn.pdf

But I am having trouble getting it to do anything.

My question is this, has anyone on this forum ever build one of these BFO’s and if so, is there any tricks to getting it working?

Thanks!

What test equipment do you have access to?

regards

Andy

[Beach]

Andy thanks for getting back to me:

The only test equipment I have is a multi meter (volts, amps, ohms and hFE) and an LCR meter (capacitance, ohms, microhenerys and hFE)

I have no access to an oscilloscope.



I know that the audio portion of the circuit works because I just jumpered it with an audio input and it sounded fine.




Thanks Again!

Beach

[der_fisherman]

GrammarA scope is really what you need to see if the oscillator for the coil is working. Then you need to check if the second oscillator is working as well.

Then you need to get them to be only slightly dissimilar, so that you can hear the "difference". They should be within less that 1 KHz of each other.

The only way I know to do that is with a 'scope.......

Did you measure each component before soldering it in? A wrong component can move one oscillator so far away, frequency wise, that you will hear nothing......remove all suspect ones, at least one end and remeasure...

Anyone who ever builds MDs, needs a scope as well, buy a cheap dual trace on ebay, new or second hand......

Regards

Andy

[Beach]

Andy thanks for getting back to me:

Looks like an inexpensive dual trace oscilloscope is in my future.

I do have a couple more questions about this circuit though.

On the top of this circuit are the resistor and capacitor soldered together? ( note the red circle.

Also, does the NTE618 actually go into the circuit this way. It looks like the Anode and Cathode are reversed.







If the image doesn’t work please follow this link.

http://s328.photobucket.com/albums/l...BFOCircuit.jpg


Thanks Again!

Beach

[Tepco]

Resistors 1K, 20K and C 10uF are connected together at point indicated by red circle. NTE618 is properly oriented, varicaps work whit reverse bias, if you have trouble to get NTE618, try to cannibalize one (or something similar, check datasheets) from, say old analog SAT receiver audio demodulator stage, this is high capacitance diode, typical general purpose varicap will not work here, and this part is a bit obsolete.

And don't expect miracle from this thing, there are many more good detector designs on this forum

[Beach]

Tepco:

Thanks for the info. As suggested, I won't wait for a miracle, I just want to get it to work. Mostly to learn. Once I can get this one to work then I will feel better about trying something a bit more challenging.

Thanks again!

[Jerry]

You could try this simple circuit along with you multi=meter to check for presence of oscillator activity:

http://www.google.com/imgres?imgurl=...QEwAA&dur=8535

Also you could try listening to the second or third harmonic on a radio receiver tuned to the AM broadcast band. Simple transistor radio should work, just sweep across the band and you should hear it. If you can pick up both oscillators, find out which one changes with the tuning control and move it towards the other. The actual frequency does not matter so much as having them within audio range of each other. If you hear a squeal out of the radio, they are pretty close.

Not a substitute for a scope but sometimes we have to improvise

Good luck.

Jerry

[Beach]

Thanks Jerry!

Looks like a cheap AM radio for me tomorrow.

[der_fisherman]

I feel that someone forgot to put a dot in the middle of the circle, there should be a connection there. Try this first.

I am not an expert on Capacitive/Varacter diodes (I have never used one at all!!), but it appears to be making a resonant circuit with the second coil. EG. Its tuning the frequency of the 2nd oscillator.......you could try turning it around as from the data sheet, I could not even identify the pins, maybe you cannot either, so it may be inserted wrongly......

If only you had a scope.....

Regards

Andy

[der_fisherman]

The varactor is placed electrically correct as far as I can tell, you need to look around and measure resistances and voltages to see if everything is as it should be.

You do have a 10 turn pot for tuning I trust?

You could possibly completely modify the circuit and replace the varactor with a 4-500pF tuning capacitor......old AM radio tuning cap for example....

Regards

Andy

[Carl-NC]

Beach,

Since you have an LCR meter measure the inductances of the coil and L1 and make sure they are very close. If they are not, then you are likely outside the tuning range of the varactor; adjust the coil windings until they are within a percent or so. Also measure the 2.2n and 22n caps in both tanks, you may want to match them as well.

- Carl

[Tepco]

AM radio vs. scope

Thanks Jerry!

Looks like a cheap AM radio for me tomorrow.

If you go to AM radio, find inexpensive one, manual tuning, without display and synthesizer. And you don't even need rest of this circuit anymore. Just build part around Q1 (use any small signal general purpose NPN you have at hand) and the coil, 9 components only, AM receiver will take over all audio and second oscillator. This will work like basic "proof of concept" BFO, maybe even better compared to original ckt. no oscillator locking, stability will be an issue (will be anyway), going on harmonics may even increase sensitivity...good to find installations or steel bars in your wall, metal detecting, well, kind of...There are many better BFO designs, but limitations of BFO itself is something that cannot be beaten.

And only question here is how much and for how long you are interested in this, or any other aspect of electronics? If you are, scope is a MUST! Fortunately, Ebay is the miracle. Please don't take this as an advice for buying, consult others etc, but just an idea, just somewhat more expensive compared to AM radio." Old school" analog equipment like Tek 465 or HP 1740 series can be obtained at a price lower than shipment will usually cost. These tings are 30+ years old now (so what? space shuttle is too), last remembrance of times when something is really built in USA, at a price tag of two decent cars when this equipment is introduced. Whit Tek, you will have to change electrolytics occasionally, HP will probably work 'till end of days, and this is cheapest ticket in the town.

Electronics is nice for hobby, but otherwise doomed...Best regard

[Beach]

Beach,

Since you have an LCR meter measure the inductances of the coil and L1 and make sure they are very close. If they are not, then you are likely outside the tuning range of the varactor; adjust the coil windings until they are within a percent or so. Also measure the 2.2n and 22n caps in both tanks, you may want to match them as well.

- Carl

OK, thanks for that!

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.

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 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?

Any thoughts would be greatly appreciated!

Thanks again!

[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.