Falcon MD20

[dom44]

that's all we need you wont have a hope in hell of keeping it stable

[Wailedflame]

Friiman; any chance you still have the .lay file for the pcb version you've printed with the ne555p timer?

[belax2018]

I like falcon sensitivity and want very much to understand its basic principles of operating.
I have experimented with monocoils (colpitts oscillators) a lot. When target approaches the coil - frequency grows, the amplitude gets smaller. How is discrimination of ferrous vs gold possible in Falcon MD20?


Could you please correct me in my thoughts:?
1) T2 is a common base colpitts oscillator. R5 sets the level of operation for T2 - so it works in a linear mode – almost on the edge of collapsing.
2) T3 stage is multiplying the AC signal from colpitts oscillator.
3) on the C8 there is a rectified voltage of the T3 output.
4) R10 is setting the tripping point of T1 – if C8 provides enough current to open T1 then T1 starts loading the base of T2 and bring the oscillation to halt.
5) All other stages are mostly indicator-responsible (sound and light).

Are you sure T1 is npn transistor and not pnp?
Looks like R10 voltage becomes closer to ground when there is a metal target.

[belax2018]

Ok. Ignore my last post.

I have build a ver-4.jpg schematic. But I have disconnected the T1. I could get the functionality behind T1.

My Results. The detector is very sensitive - but you need to struggle with thermal drift all the time.
It is very unstable in this sense.

How can we improve the stability?
Things like NP0 caps, tightly wound inductor are already in place in my version.

T4,T5 and T2 are all dependent on HFE of transistors - 100% HFE drift dependant.
T3 is biased like in electronics cook books - it has around 5% of HFE drift dependance.



Thank you.

[dom44]

I have been a busy lately the stability is much better when it is in the enclosure a bit of insulation around the oscillator wouldn't hurt as well. one of the Russian pinpointer circuits addressed the thermal instability in the oscillators I have to remember how they did it.
t1 is important it constantly re tunes the circuit it works fine on 3 versions of this unit i have made you may have some kind of error around your circuit.
if you apply a positive voltage to the base of T1 which is correct as a NPN it should kill the oscillator and the pot R10 sets the threshold.

[dom44]

this is the Russian pin pointer thermal stability circuit that i found not sure how well it works haven't built it yet.



shouldn't be to hard to adapt to this circuit it is everything around the NTC resistor on the positive supply line to the oscillator.

[belax2018]

I have been a busy lately the stability is much better when it is in the enclosure a bit of insulation around the oscillator wouldn't hurt as well. one of the Russian pinpointer circuits addressed the thermal instability in the oscillators I have to remember how they did it.
t1 is important it constantly re tunes the circuit it works fine on 3 versions of this unit i have made you may have some kind of error around your circuit.
if you apply a positive voltage to the base of T1 which is correct as a NPN it should kill the oscillator and the pot R10 sets the threshold.

Actually In your 4'th version T1 can be used as a negative feedback to counter the thermal drift.
For e.g, if temperate rises => HFE of T2 rises => amplitude of oscillation rises => rectified voltage on the top of R10 rises => current through T1 rises which means less current to the base of T2 and the main result of this: amplitude of oscillation is smaller. How smaller depends mostly on R10 and HFE of T1.

But why did the original version of Falcon 8 used PNP transistor in place of T1?

Ok I will look through my circuit again.

[homefire]

Been trying to follow this and my conclusion is I'll build the Original . LOL

[dom44]

ver 4 is the original circuit posted originally with only 2 minor changes a 78L05 regulator and a 1M T3 resistor at the base apart from that there is nothing much different apart from the sounder stage but yes belax2018 you are right it is a PNP transistor but seams to work with a NPN as well I will have to see if there it makes any difference on my unit.

[jtz54]

All good stuff here! Can anyone summarize the circuit operation by section/component? I know it's a pretty easy circuit but I'm just getting back into it. I'm sure the other members would like to see a circuit description more detailed than a Colpitts oscillator.
Thanks!

[karim1980]

Please help. What is in these places

[Tim]

Sorry my MD20 schem/lay has a 555 ic not 4093. I have not come across this before. Perhaps it is something else??

[karim1980]

Thank tim
Maybe there 's an error in this pcb

[dbanner]

Read through thread from start. You will find answers to all your questions.

[Tim]

Yes it is dom44 version 4093 is VCO (post #153 here). Sorry db/karim. My bought MD20 with 555 works just fine. Your query-below LED seems to be an end-standing resistor( see schem for value). Nothing connected at point above 2n2222 (?)