Falcon MD20

[dom44]

The diodes are garden verity 1N4148 they are there to steer a positive voltage to the base of T1. the output speaker drive is from tesoro designs the way it works is T1 changes the frequency when positive voltage applied and T2 controls the volume when positive is applied, the back of the diode is facing the base of T2 so will not conduct but the moment you apply a positive voltage to the base the square wave from the oscillator modulates the base and the volume is proportional to the voltage as well as the frequency,
I have the circuit working but am not happy with the frequency swing not enough voltage swing from the detector.

[dom44]

This is where i am now quick clip using my ring as the copper ring the metal piece is a 2.5 mm component wire distance is about 5mm from tip of rod, you cant see it but the metal piece dances to the magnetic field when you get close.

[homefire]

Sounds like a plan.

[homefire]

VIDEO UNAVAILABLE Emmmm

[dom44]

https://www.youtube.com/watch?v=Ccld...ature=youtu.be

[dom44]

Try it now forgot to hit publish button

[homefire]

Looking GooD Sir.

[dom44]

some small improvements to circuit help stability

[WM6]

some small improvements to circuit help stability

Nice, thanks.
Do we really need 12V supply? Is 9V not enough?

[homefire]

9V is what the Original uses. Nothing in the circuit demands much in the line of current.

[dom44]

9V is ok i like to use 12V AA battery pack will last a long time specially if you are running from one battery and not two the audio section and LED use most of the power. I added a 5 volt reg to the oscillator as the unit becomes unstable at high gain settings. It goes into a a kind of feedback and the sens pot is hair touch sensitive just a fraction ether way makes a big difference to the distance it picks up small metal pieces .
helps but doesn't totally fix it. I am working on it as well as a radically different design but be a while have to test the various elements of the circuit.

[dom44]

I was messing arround on the bench today made another improvement or two if you add a 1M resistor at the base of T3 to ground helps a lot with stability and add a 100k pot in series with R14 at T4 collector gives you the fine adjustment you need to get best sensitivity.

the way to use it is turn the ground adjust R10 till LED comes on then back off till goes out, adjust R14 till LED comes on then back off, adjust fine tune 100K pot then just back off again to a position that is stable you have more movement to play with the 100k this gives excellent sensitivity, but the R6 trimmer is important set for most stable with sensitivity as this adjust the amplitude of the oscillator signal.

[Tim]

Great stuff dom !! Anyone going for a pan or a crack-scratching needs one of these beauties. My [bought] one, I set up on a pvc cane-probe pressed into the base, cord up the middle and box mounted at the top for probing around mullock heaps and creekbeds.

Keep up the good work- good to see the renewed interest in this !!!

[dom44]

I am familiarizing my self with this circuit right now and experimenting I have built one circuit that is more sensitive picks up a grain of solder the size of a pin head at 10mm easy but needs a lot of work, the threshold gives problems with stability.
I still think a VCO is the way to go on the MD20, the short fall in the design is the basic on off sounder or 555 timer sounder and the fact they use transistors for amplification when op amps do a better job.

what most people don't realize is the sensitive adjustment of the sens pot I was watching a youtube video where one guy was only just picking up good size nuggets touching the tip, where on the bench i get from 5mm-20mm for the same size it shows bad adjustment
and makes you wonder how many where missed.

[dom44]

I was asked to share the circuit for 4093 VCO version here you go have some more work to do on it but i will post as is the MD20-fixed.RAR is the one to use had some values missing and small amendments to make.




md20-fixed.rar