Falcon MD20

[Goaty]

Hiya All,,,Can anyone say which version of the Falcon MD20 is a proven working version,,I've read the thread several times but have been confused because of the different versions. If someone can point me to the best one to make I'd really appreciate it. Thanks,,Marty

Hi Marty
After many weeks of reading this thread (probably a dozen times)I’ve built up two versions of the Falcon. They are both finally working but not yet tuned for optimum performance.
I built Walkman’s version of Friimans design (?) and also Dom44’s version which uses a 4093 instead of the 555 to generate the audio.
Until I work out how to tune them I can not comment on which is better but both seem comparable.


All of the board layouts by Dom, Friiman and Walkman seem to have some minor errors which were fairly easily corrected. I posted a correction to Dom’s board a couple of posts back, Friiman’s needs a track cut between the top pins of C7 and C6 , as does Walkman’s which also needs some alteration to the diode and capacitor circuit connected to the collector of transistor 3 ( I will try to post a pic soon). Also, all of the 100n capacitors should be swapped for 10n to match the earlier circuits published (although 100n may work) 1N4148 diodes work fine even though they are Si.


My biggest headache was the 2222 transistor pinouts. The orientation of these things seems to be totally random so do not go by the silk screen patterns on the boards – I always use my cheap little Chinese component tester to check which pins are E, B, and C.


The coil proved to be fairly straightforward. I bought a 10 (9?) mm x 100mm ferrite rod off Ebay and cut it into 2 X 50mm lengths. I then glued a couple of pcb disks to one end to form a bobbin and wound around 50 turns of .25mm enamelled copper wire (ex plug pack transformer primary).
I adjusted the number of turns once I had the board oscillating to give 300kHz.


I duplicated the original Falcon probe design using 25mm pvc electrical conduit, an end cap and a joiner. The copper ring was just a circle of adhesive copper tape wrapped around the slider and the join soldered.


The Falcon 8 circuit is probably the simplest and I may give this a try too if I can find a suitable board pattern.
All of the designs seem to use very similar ‘front-ends’ with just the audio differing.

[MartyJ1963]

Thanks Goaty,,,really appreciate the reply. The best Pinpointer I've made up until now is the SMD Version of the Gnom-m. Andy (Silverdog) was kind enough to send me some SMD PCB's to practise my SMD soldering on and I had no trouble making a coil for it,,it worked straght away and I was really pleased. The only pain I had was programming the Attiny15,,it took me awhile to work the pins out with it being an SMD IC. Regards,,,Marty

[Goaty]

Friiman’s needs a track cut between the top pins of C7 and C6 , as does Walkman’s which also needs some alteration to the diode and capacitor circuit connected to the collector of transistor 3 .

Cut tracks at X and add jumper.

[MartyJ1963]

@Goaty,,,,Thanks for the Falcon MD20 correction you posted,,,I may have a go at that. I've redrawn the Gnom-M pcb,,,I haven't made it yet, but if it works ok I'll post the lay. Regards,,,Marty

[Goaty]

I've redrawn the Gnom-M pcb,,,I haven't made it yet, but if it works ok I'll post the lay. Regards,,,Marty

I think I made the pcb for that but never got around to getting the components together to finish it - I will be interested to see what you come up with as it may inspire me to finish it.

At the moment I am playing with the MD8 version of the Falcon based on Dom's front end and just driving a piezo for the audio. I'm slowly gaining an understanding of what the circuit does so that I can tune it but am finding it heavy going with so many adjustments all interacting with each other.
It would be nice if someone could come up with a defintiive original circuit diagram to start from as all of the modded circuits presented here get rather confusing.

[MartyJ1963]

Hi Goaty,,,,The Gnom-m pcb's that Silverdog(Andy) sent me work great but the trouble I had programming the Attiny15L was that it had no 5v pin on the pcb. It only had a place to solder 5 pins to so I had to 'lightly' solder a wire to the VCC leg of the Attiny15L and connect it to programmer properly. I just made a new little board for the Gnom-m,,all I've done to it is make it a little larger and added the VCC pin to it. I made 2 actually,,one is SMD just like the original but with the VCC pin added and the other is a Through-hole pcb but with SMD Attiny15l and SMD BC857. You can try it if you like,,,the coil is as the original,,I.E:- 8-10mm wide X 100mm ferrite rod/200 turns x 0.3mm centre tap, then 200 turns x 0.3mm. Not sure of the Inductance but I'll measure it and post what it is. I used 0.25mm and it works great,,no falsing at all. Regards,,,Marty

[MartyJ1963]

Hiya Goaty,,,,,Can you please check my layout drawing before you decide to build it or not,,just to make sure I have the Attiny15l pinout correct,,it seems correct to the Attiny15l SMD data sheet but best to make sure. Thanks,,,Regards,,, Marty.

[MartyJ1963]

@Goaty,,,,,It's not correct,,,I forgot to reverse it with it beng SMD,,,,I will correct it and and re-post,,,I was drawing it as if it was all Through-hole,,silly mistake I know,,I'm glad I didn't build it yet,,,lol,,,Sorry for posting the above Layout. I think the Layout just needs to 'Flipped' horizontally but I'll check before re-posting it again,,,Regards,,,Marty

[Goaty]

Found this info from "ElkGold" on the Canadian Gold Prospecting forum -


"I've had the Falcon MD-20 for about 3 months and can say it is a great unit! It will find the smallest of gold - as well as a very useful tool for detecting black sand deposits. I have had some longevity issues however. After using the unit for a month or so the RCA male connection going into the unit appeared to have a short. The unit would go off by itself and became very frustrating. The end solution was chopping off the original connector - and soldering on a new RCA plug to connect the wand to the main case. I am sure Falcon will repair this issue for you - but it should be designed a little better. When these units are being packed - and used in the field all day, they should be able to stand up too a little more abuse. It almost seemed as if the internal soldering had become compromised.

Also you can easily re-calibrate the unit by using the following instructions (From Falcon Support)
Make sure you only adjust R2.

1 Open up the box and locate R2 on the circuit board. It is
probably a blue square directly behind the ground balance knob. Scrape the wax seal off the top of R2 and using a small straight blade
screw driver turn R2 counter clock wise until it stops.
2. Turn the unit on and turn both knobs counter clock wise. Place
the copper ring just over the red line on the probe. You should hear
no sound at this time.
3. Turn R2 clockwise until the unit sounds. Slide the copper ring
back to the black line. The unit should stop sounding. Slide the
copper ring back toward the red line. The unit should sound just as
the copper ring covers the red line.

The MD20 should be ready for use.
You will not invalidate your warranty by turning R2. If this does not solve your problem, let me know. Thanks"

It may help with adjustments.

[Goaty]

Please do not laugh at me, just wiring test
In addition my laser printer is not too good, therefore routes are thicker diameter.
For bernte_one most likely only weekend I will have some time working[ATTACH]33718[/ATTACH]

Hi Noro Could you please post just the copper pattern as a .jpg or pdf as I can not open Sprint files in Linux
Thanks

[homefire]

Yes, I would appreciate a copy as well. Which version is this one ?

[Goaty]

Yes, I would appreciate a copy as well. Which version is this one ?

I am hoping it is this one -



I have marked the fixes required (Yellow - links, red - cut tracks) but will need re-checking.

There don't seem to be any component values around the 555 audio section but these could be gleened from other circuits in this topic.
This one has a LED indicator which is quite useful when tuning the front end.

It would appear that the original MD20 board had extra trimpots which were later replaced with fixed resistors - all adding to the confusion around which version to build!

[dom44]

I thought i would play around with the circuit a bit more been to busy with other things lately, just built this test circuit to improve the oscillator using current mirrors to give thermal stability to the circuit, so far looks good built a test circuit works as expected. any suggestion would be appreciated.

[dom44]

have been running the circuit on and off quite a while, mine is locked at 277.7khz with 60 turns on a 50mm fer-rite rod, and is very amplitude sensitive to metal objects.

[Goaty]

Nice work Dom - your efforts are appreciated!