Hi
A few weeks ago I posted on making a non immersed level dector, using capacitance.
I could have purchased a metal detector, but foolishly with my (very) limited knowledge, I decided to have a go at making my own. Well, what a nightmare,
The story so far.( im treading a well worn path as apparently Amps oscillate- oscillators dont)
The difficulties started with the sense oscillator, this is usually done with a colpitts,unfortunately this relies on a split cap which I didnt really want, so, I decided to go for a Hartley and split the coil.
Secondly the capacitance on the actual plate sensor is only about 5 to10 pf, so the a tuning inductor would need to be around 100mh at the few kilohertz these things normally operate at, unfortunately the reactance impedance would be horrendous at low frequencies, also there always seems to be a residual 10p of parasitic,
Fortunately there dosent seem to be any set frequency for this kind of thing, apart from I suppose, keeping out of the broadcast spectrum, so up to a few hundred khertz should be ok, and the high value inductors are readily available from Cricklewood.
Tried a couple of bfo circuits,
Tried a 555 using the sense cap as part of the frequency timer, this didnt work.
(Dosent have to be a sine wave)
I then realized that the conventional detector type circuit had a fatal flaw in that they rely on the user making constant adjustment each use to compensate for drift, temperature, stray capacitance etc, this would need to be be a permanant fixture
So back to the Hartley,see pic. I had a 10 meg xtal handy, put this in the circuit and amazingly (for me) it works, I then hoped that in operation, any variation in the tuned circuit capacitance would kill the oscillation and trigger an output stage.
So now ive got a nice steady 10 mhz output from the oscillator,with a 9 to 11 mhz tunable tuned circuit (a rectifier/ comparator final output stage that triggers at .8 volts, ready to go.)snag is , I would have expected the oscillations to cease when I significantly vary the tuned circuit, away from the xtal frequency, unfortunately the output frequency moves from 10 mhz and just continues to happily follow the tuned circuit frequency right down to 7 mhz .
Its possible that the amplitude drops, though, that would still work, but I dont have an oscilloscope, and ive found simulators (mine anyway) struggle with oscillators over a few khz and tricky to start.
Interestingly, in real life they rely on noise to initiate which I suppose is difficult to simulate.
Im not too concerned about the outcome, this is proving to be a fascinating and enjoyable subject in its own right.
Any and all thoughts most welcome.
im a bit puzzled on beat frequency oscillators.when they mix they cancel each other out. how? as there is no mechanism to set the phase position.
When I next have a play, I think I will leave the hartley as it is and put a LC tank from output to ground on the output and use this as the sense capacitor.
Regards
dougal
A few weeks ago I posted on making a non immersed level dector, using capacitance.
I could have purchased a metal detector, but foolishly with my (very) limited knowledge, I decided to have a go at making my own. Well, what a nightmare,
The story so far.( im treading a well worn path as apparently Amps oscillate- oscillators dont)
The difficulties started with the sense oscillator, this is usually done with a colpitts,unfortunately this relies on a split cap which I didnt really want, so, I decided to go for a Hartley and split the coil.
Secondly the capacitance on the actual plate sensor is only about 5 to10 pf, so the a tuning inductor would need to be around 100mh at the few kilohertz these things normally operate at, unfortunately the reactance impedance would be horrendous at low frequencies, also there always seems to be a residual 10p of parasitic,
Fortunately there dosent seem to be any set frequency for this kind of thing, apart from I suppose, keeping out of the broadcast spectrum, so up to a few hundred khertz should be ok, and the high value inductors are readily available from Cricklewood.
Tried a couple of bfo circuits,
Tried a 555 using the sense cap as part of the frequency timer, this didnt work.
(Dosent have to be a sine wave)
I then realized that the conventional detector type circuit had a fatal flaw in that they rely on the user making constant adjustment each use to compensate for drift, temperature, stray capacitance etc, this would need to be be a permanant fixture
So back to the Hartley,see pic. I had a 10 meg xtal handy, put this in the circuit and amazingly (for me) it works, I then hoped that in operation, any variation in the tuned circuit capacitance would kill the oscillation and trigger an output stage.
So now ive got a nice steady 10 mhz output from the oscillator,with a 9 to 11 mhz tunable tuned circuit (a rectifier/ comparator final output stage that triggers at .8 volts, ready to go.)snag is , I would have expected the oscillations to cease when I significantly vary the tuned circuit, away from the xtal frequency, unfortunately the output frequency moves from 10 mhz and just continues to happily follow the tuned circuit frequency right down to 7 mhz .
Its possible that the amplitude drops, though, that would still work, but I dont have an oscilloscope, and ive found simulators (mine anyway) struggle with oscillators over a few khz and tricky to start.
Interestingly, in real life they rely on noise to initiate which I suppose is difficult to simulate.
Im not too concerned about the outcome, this is proving to be a fascinating and enjoyable subject in its own right.
Any and all thoughts most welcome.
im a bit puzzled on beat frequency oscillators.when they mix they cancel each other out. how? as there is no mechanism to set the phase position.
When I next have a play, I think I will leave the hartley as it is and put a LC tank from output to ground on the output and use this as the sense capacitor.
Regards
dougal
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