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**Qiaozhi** · 12-11-2008, 11:45 PM

I think silent search just means that there is no detectable threshold tone. So you need to adjust the threshold to make it so. This also relies on the use of comparators at the output of the two channels, because essentially a comparator is either on or off, and there is no gradual change in audio amplitude with target depth.

**Max** · 12-12-2008, 07:23 AM

Originally posted by porkluvr View Post

I've been jumping around between various Tesoro IB models trying to decide on one that I'll actually build, but I am hung up on one common feature. I can't understand out how it is possible for the threshold adjustment to allow for a steady tone in All Metal (AM) mode, and then go silent when DISC is selected. I can't see how that's possible, being as how the same amplifier is used as Threshold in either mode.

As I recollect, the operator manuals that I've read state that there is a threshold tone in AM mode, and DISC mode is silent. But I don't see how the various models found in these forums could accomplish that, without operator adjustment.

What am I missing? Perhaps it could be explained because the various schematics here are for older models and the operator manuals are for later versions? And, maybe I shouldn't throw all the IB models into one basket, so to speak. (I'll pull out some more models' schematics and maybe I'll get a clue.)

I have been simulating target response and audio circuits and have been unable to find a way to set up a threshold tone in AM mode that goes silent when DISC is selected, unless I throw in a seperate amplifier for the two different modes.

I just don't get it. What is the explanation? Anybody, please?

Sometimes I think I'm living in a different universe where physics laws aren't the same - until suddenly it dawns on me what I've been doing wrong.

Hi,
if you read bandidoII manual you'll see exactly what you described: all-metal is with threshold (motion or not motion), disc is silent (always motion)

Now, just look at bandidoII circuit: you'll see 3 channels, 2 used for disc (disc and geb channel) and last one lower on the page is the all metal channel.

Look at switch where you select disc/all-metal: there you'll see (as Qiaozhi stated) that in disc mode the comparators jump in and thus provide the 2 discrete levels required for to achieve sound/no-sound only behaviour : it's the so called "silent search".

But what happens when switch is at middle (all-metal) ?

Simple: comparators are out... and you have a direct link between output of all-metal channel and the last amplifier (the one with threshold control).

That way you can set a particular threshold level using the pot and then start the all-metal search, that's thus not-silent.

The way the ground balance work in all metal (cause the upper geb channel is disconnected from output signal path) is little tricky: it's controlled by same gate control (controlled by geb pot) by the resistor/cap stuff at left... that way device is STILL ground balanced as in disc mode, but act like threshold all-metal cause don't use anymore comparators' discrete levels.

In stuff like TGS (instead) that is a pure "silent search/ motion" device you'll see just 2 channels and not 3: the 2 channels act both for all-metal when in all-metal mode BUT it's silent search cause , always, there are comparators at the end, so again 2 discrete levels that control output stage.

Kind regards,
Max

Attached Files

Bandido-Revised-Schematic.jpg (137.7 KB, 1000 views)

**porkluvr** · 12-13-2008, 02:50 AM

the truth is out there

Thanks for your replies, Qiaozhi and Max. I should work with the Bandito II uMax until I beat this thing because we have a good schematic here on geotech AND the correct manual is available from Tesoro's website. Instructions AND a diagram, almost too good to be true!!

I can't get my simulations to work correctly and I'm ALMOST too much a scairdy cat (but, almost definately too lazy) to go to the trouble of building the thing without verifying circuit performance. I think that is why God invented LTspice.

After setting up a threshold tone in AM mode I can't get the tone to dissapear when I "switch" to DISC. See here, I am looking at making some pretty severe modifications in several areas of the design, so I would like to have high confidence in the ORIGINAL schematic before it gets butchered, do you know what I mean?

I believe I have found a way to have the circuit behave correctly, but it requires two additional opamps in the signal chain. It gives flexibility and should be easy to setup, but I do not like fixing something that ain't broke (and doing it the hard way). Max, you say that it ain't broke and I trust you, so maybe I'll go ahead without preliminary spice verification - but I'm not giving up yet. I'm going to keep pulling my hair out for a few more days, and resist the temptation to optimize my extra opamps, - instead, hammering on the original design. (grrrrrrrrrr)

I don't have a pleasant environment (or the tools) to do any breadboarding, so progress is sloow. I'll look at the schematic and re-read what's writ, and maybe I'll find enlightenment. There must be something I am overlooking that I just need to let sink in.
I feel so, ummh, dense.

**Merc** · 12-13-2008, 03:48 AM

Motion Disc.

Please remember that the disc. stages are capacitively coupled and the all-metal stage is direct coupled. The discriminate/motion mode requires a quickly changing signal to exceed the threshold. The all-metal/non-motion mode has a very slow near DC response.

Merc

**porkluvr** · 12-13-2008, 05:38 AM

The gain equations following are ballpark accuracy but should be close enough to make a point (I hate to resort to math but this is necessary):

(Disregarding the 0.1u capacitor on IC3B threshold opamp) the threshold pot voltage (on the (-) input) sees a gain of 100/200= (-)0.5. Meanwhile the (+) signal input sees a gain of approximately 102.2/2.2=(+)46.

Observe that TLC2262 is a rail-rail output device. In the case where All Metal channel amplifier is driving negative, this would put almost -5V on the (+) terminal of IC3B, driving IC3B-7 output to the negative rail. To offset this in a positive direction would suggest the need to turn the wiper of the threshold adjust pot toward -5V. But the pot's voltage gain is only (-)0.5, while the signal gain is (+)46. This suggests impossibility of ever obtaining anything higher than -5V from IC3B-7 so long as the signal input is -5V.

I have experimented replacing IC10A and IC8A with TL062 or similar, so that output swing is limited to approx +/- 3.5V. But this is not a solution because IC3B still hangs on the negative rail even with the threshold pot at -5V.
Everything else seems OK, but assuming that the All Metal channel could drive to the negative rail stifles my attempt to setup a threshold audio output.

Is there something that I am overlooking?

I hope I haven't made any stupid math errors - I've been going since 4:30AM yesterday so I'm going to call it a night.

OOPS. Now it occurs to me that I need to rethink this situation , but this time set the threshold audio with the signal input more closer to ZERO volts (geees, now, I can call it a day.) What a waste of time.

**porkluvr** · 12-13-2008, 04:46 PM

problem solved

Ahhh, that's better. I was trying to calibrate a threshold tone with a negative signal output coming from the AM channel. That was stupid. I can be a real blockhead sometimes.

Thanks to all who tried to help. Now, hopefully I'll concentrate on real issues, instead of made-up ones.
... moving right along.

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Silent search vs threshold tone

Silent search vs threshold tone

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