Falcon MD20

[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.

[homefire]

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

[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.

[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.

[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.

[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.

[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.

[Wailedflame]

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

[dom44]

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

[homefire]

Get Crazy and Use Darlingtons. LOL

[dom44]

that's cool I have had some time playing with this thing it is dead simple really and has lots of room for improvement, I am playing with a few different oscillator designs to find something more stable than this one, at the amount of amplification going on here a few Hz makes the tuning difficult to keep on track and as i said thermal insulation of the oscillator transistor is important.
maybe when it is in its case completed its enough to keep it stable but on the bench just air movements can effect it.

[homefire]

Thank you . I was thinking it was some positive feed back.

[dom44]

sorry homefire T1 shuts off the oscillator and acts as an auto tuning stage T3 is just a high gain amplifier stage with DC rectification. T4 acts like a comparator stage to set the trip point to activate T5.

[dom44]

R5 the 10k trimmer is critical where it is set as it effects the wave form shape and amplitude at some setting will stop the oscillator completely from working that's as it should be. the correct setting is just as the sign wave becomes symmetrical and and just before it shuts off is the sweet spot and the the most sensitive setting for it. I used 2N2222 but any NPN will work.
If you watch the scope and bring a piece of metal close to the coil the wave form will totally co laps and it will stop oscillating when it is set right.
From the second version of this detector I am working on I can tell you the real secret to this detector is only that second stage amplifier marked T3 with out it its just a pin pointer, and the auto tuning T1 which resets the oscillator dependent on the output level, the
the oscillator is prone to drift and temperature sensitive the auto tune circuit keeps it in Check to a degree but you still get some drift so it is important to keep the oscillator transistor thermally insulated when you set the threshold if you blow cold air over the transistor will significantly shift the oscillator.

and no homefire T3 is not feedback it just grounds the base of the oscillator shutting it off dependent on the DC level after the dc rectifier stage as set by the R10.

[homefire]

Centered the Two Pot's ? 3904's are better at singing . Reduce R4 and R15 a bit and see if it wakes up. Try it without the Ring first.