how does the bandido IIµ max work

Originally posted by Stijn View Post

hi everyone,

im making a tesoro bandido IIµ max for school and i need too know how the elektronics in the detector work .
could someone help me with the schematic cuase i dont understand how it works.
i have fiured out wich part is the oscilator for the Tx coil but the rest i dont understand

thank,
stijn

ps sorry if u have some bad english but im from belgium

Hi Stijn,
I've not much time at the moment but try to explain what I understand of it.
I think that a serious study of this circuit cover too much things...that maybe also a simple
university electronic course can't treat in details.
Many things are for "advanced" ...

Is a VLF/T-R detector that uses IB coils. RX coil signal must be nulled to give good response and
avoid saturation on RX preamp stage.

I refer to Carl's schematic and to active components involved only. It can be studied by blocks:
0. is main oscillator (tx) that drive tx coil, built around T1

1. IC7a (1/2 mc3317

is rx preamp in diff. amplifier configuration ac coupled to other
blocks

2. disc control block ic7b, ic5b is a phase shifter used to control first analog gate
then to give samples to upper disc channel input signal

3. gnd control block ic5a is again phase shifter too used to drive two other analog gates
to feed right samples to second disc channel and all-metal channel

4. and 5. are two disc channels (ic9b, ic10b and ic9a, ic10a) formed by integrators ac coupled
and "filters"...well there are much things to describe here...not so simple

6. is "sum-of-effects-network" (of two channels) stage that uses two comparators (ic8a/b)

7. is main all-metal amplifier/integrator ic6b

8. is feedback block ic6a, used in retune operations and also in motion modes

9. is audio response driver/controller (ic3b)

10. is battery checker (ic3a)

11. is internal clock and sync frequencies generator (ic1, 4024 ripple counter); frequencies are 5KHz, 2.5KHz,1.25KHz, 625Hz

12. is audio stage built around t5 and t6...simply a small power amplifier

13. is voltage regulator for +5volt (+V) (lm2931a)

14. is negative voltage converter built by t3,t4,t7 (not -5volts but say, maximum, -4volts, actually less...)

To be brief (hard to do...I'll don't enter in details in disc/gnd operations) :
Before anything else battery check is performed, during about 3 or 4 seconds...
when 5volt present on +v line stage 0 creates oscillation using colpitts
schema oscillator, then tx coil is tuned to about 10khz with that components.
Part of the sine wave signal generated is feed to IC1 that creates clock/audio signals and
then 5KHz are feed to negative voltage converter, that is a charge pump.
This way two voltages are present +5 and -3 or -4 volts...not so important, so op amps have
differential supply (not symm but it's not important here) referred to GND.
So stage 1 amplifies rx signal...10KHz too (coil is an air core transformer...) and feed
other blocks through analog gates.
Gates control is made by stages 2. and 3. but too long to explain this, just remember that
gate control is based on phase shifts.
DISC pot and GND pot changes phase shifts in their outputs thus giving the opportunity to
cancel out gnd signals (phase shifts due to ground signal) and to discriminate between ferrous
and non ferrous metals --> controlling two channels input signals sampling strategy

You end up with no signal at pin5 ic3, then no sound, with ferrous items...(ideally) and signal, then sound,
for non ferrous. This way it discriminates non-ferrous from ferrous.

In all-metal stage 7 is involved too, as also in disc mode, but what's happen in disc channels means nothing.
If an increase of rx signal is detected you have sound. In manual mode you have dc coupling and so drift exist.
When retune or auto set is performed you have stage 8. came in place , thus negative feedback zeroes
output of main all-metal amplifier...you gain the threshold level.

But here in auto...only motion behaviour is present because feedback tend to cancel out imbalances.
The same happens in disc mode...where only motion type of detection is performed.

This all is because there will be drift if non-motion only...giving not stable threshold
so losing much performance. Bandido uses manual-all-metal (non-motion) just for pinpointing.

Battery checker is really a easy thing...based on charge of c7...comparator...2/3of battery voltage
as "fixed" reference...and is due by note duration and ear of abrupt tone stop...wow
Why they used such a thing ???? I don't know. Maybe to avoid visual indication of battery status.
But it's really boring when you switch on to ear this note every time.

Negative converter is hi-inefficient. Can't give -5...and also -4volts is too optimistic.
For sure...in real bought ones V- is something like -3volts, -3.2, -3.3. But no problems here...op amps work fine
also with +2.5/-2.5 here, so it's enough for good operations with right components.

Anyway, Bandido is a very good circuit. But some aspects let me, still now, confused. Not about the circuit itself...

but why they used some design decisions...

Best regards,
Max

Announcement

Announcement

how does the bandido IIµ max work

Leave a comment:

Leave a comment:

Leave a comment:

how does the bandido IIµ max work