Barracuda Legend Kit Problems

[raygdunn]

Hi Qiaozhi,

One oddity on schemas you could quickly answer, I suspect. The feed capacitor to audio via. the Q5 MPF102 base. The closest to board schema, which I believe you are using, shows it as 100pF. Elsewhere this is 100nF and on the board it is actually 1nF.

Is it just that the value is not critical.

Ray

[jladre]

I also dont have issue with rev2 board except for one time that i used a cheap fake part. Bara works great with that board.
using a plastic 2N2222.. Me too..its more stable with a plastic type.

[Koala]

Interesting board looks the same as mine rev2 which works fine without modifications. Runs okay from 8 AA batteries. Only difference I can see is I am using a plastic 2N2222. I am also using a TIP127 and no second POT on the delay

[kt315]

on my board 74HC14AP chip. try it too.

[raygdunn]

Hi Qiaozhi,

That is great progress! I have five or so 40106 chips displaying spikes in output. Plus one complete failure. I've replaced all the three power supply chips and I've just found the 7660 overheating for the second time. I see now why mine ran perfectly with a 9V current limited bench supply, then ran into problems on a 3 cell LiPo. The latter would have virtually unlimited current on startup and really enhance the negative voltage effect!

I didn't see an improvement on applying the mod to connect the three 40106 resistors to earth, but I don't think I've seen the same detection range reduction. I can get a hint of detection at about 12" on a Victorian penny, with full beep at around half that distance.

Unlike your test unit, my original pulse widths seemed to match at 40us. (I suspect this varies with the accuracy of the 79L05 and probably should be 45us) As you have seen, there is something odd reducing the first sample pulse width, when the delay is reduced through optimum point. Pulse eventually disappearing completely. I found the timing on this varied a bit with coils.

One point in your report. As supplied R27 and R29 are one 20K preset. No external control. Impulse must have added this, which is a most useful mod.

I look forward with interest to your analysis of the audio. (I'd love to understand how it works actually). I'm sure mine often goes unstable. There is what sounds like a beat frequency that drifts through a zero point after it's had power for a while.

Brief tests in the field, I initially experienced a lot of extra noise. The main component seemed to be when accelerating and decelerating the coil. (The end of swing effect). Swing it round 360 degrees at constant speed and no sound. I traced this to having the PCB to coil cable wiring connected the wrong was round. Though there is still some sensitivity in this area. This fix has been mentioned before, so not sure if it is the real answer.

Keep up the good work☺

Ray

[Polymer]

Qiaozhi, thank you so much for digging into this ... you've been burning the midnite oil too , it also answers some questions I had about a year or so ago ...

Since you're at it:
This may also be part of the problem or compounding it (if I remember right - many were having issues with the 7660)
See Number SEVEN & possibly SIX ( I don't have Osci, so can't measure that) of datasheet info:



Could you please also check why most schematics for the Barracuda have a 2M2 resistor going to pin 7 of the 7660.
Some older schematics have a 1k resistor which works fine and is also shown on the data sheets. Anyway, the whole diode/cap/resistor combo going to pin 7 of the 7660 defies my understanding.
Perhaps you have an explanation of how it works.

Hope I haven't stretched this to far, but this looks like a great opportunity to get some more things about the Bara clarified,

In appreciation, Polymer

[6666]

Mine is like this.

[6666]

what a difference! Detection of a Victorian penny was now easily achievable at 12"

Q Could you also do any testing with an English pound coin as well thanks, and what revision PCB do you have ?
Which schematic are you useing? on my REV2 pcb there are no caps on output of 78l05

[Michaelo]

Qiaozhi, on the Main sample pulse delay.BMP, can you give me the pin numbers for both traces and which pin is the Earth field sample?

I made changes to both the 220uF caps and the noise on the +5 volts has disappeared, -5 has a little noise still but it's better...
Mike

[6666]

Thanks Q.

[Qiaozhi]

Update on Baracuda from Impulse

I found some time over the weekend to look at the Baracuda that Impulse sent to me.

Firstly, referring to post #414:

I have the Baracuda here, and gave it a cursory glance a few nights ago. The unit was very unstable and with poor detection depth. The first thing I noticed was that the MOSFET was very hot, so I detached the coil and found it to be 166uH which is far too low, and the reason for the excess heat. Power supplies were all in the correct range, but the TX oscillator output was distorted and the main sample pulse was false triggering and giving two separate pulses. During further testing the CD40106 went kaput, with the result that the -5V supply was being pulled down to -3.84V. Literally, a few minutes ago. the postman arrived with a bag of parts including the CD40106 replacement.

After replacing the CD40106, the detector started working with some improved results. This meant that the detection depth for a Victorian penny was around 6" instead of the previous 2 to 3". I've noticed that 6" appears to be the type of results being achieved by many Baracuda builders, but this [to me] seems quite poor performance. After some further probing with the scope, it was clear that the TX oscillator output and sample pulses / delays are corrupted in some way. Unfortunately, the replacement CD40106 then decided that it was time to kick the bucket, with the result that detection distances for a coin were again reduced to a few inches. During these tests I had been using a 12V lead-acid pack, so I replaced the CD40106 for the second time and changed over to a bench power supply with current limit (just in case there was surge problem at switch-on).

I also decided to modify the PCB to connect the pull-up resistors to the 0V line rather than +5V. Wow ... what a difference! Detection of a Victorian penny was now easily achievable at 12" (and maybe even 13") with a 300uH 9" diameter mono coil. There was also a dramatic increase in stability regarding the audio threshold.

Now, let's backtrack a little:
The original CD40106 was kaput, and upon replacing it I took some measurements with the scope. It was possible to adjust the main sample delay between 11us and 38us with R29 (sample delay) pot. The lower limit is restricted by R27 (delay trim) to 11us because the sample pulse disappears at any setting below this value. On occasion it was noticed that the TX oscillator would fail to start.
TX pulse rate = 645 pps
TX pulse width = 95us
Main sample pulse width = 17us
Earth field pulse width = 34us
Earth field pulse delay = 182us
From the above results it is clear that there's a problem. The EF pulse is twice the width of the main sample, and hence the Earth field is not being eliminated correctly. This is something that's been noted by some people, and in fact (on this particular detector) it was responding to a whiteboard magnet several inches from the coil.

After the modification (shown in the attached Modification.jpg file) - which consists of cutting a track and soldering in a short jumper wire - the TX-on pulse was approximately the same at 94us, but with an amplitude of -5V (this was previously at -4V), and a pulse rate of 644 pps.
Main sample pulse width = 98us
Earth field sample pulse width = 120us
Earth field sample pulse delay = 440us

In practice it was found that the lowest main sample delay was 27us (and not 11us) as the main sample pulse gradually reduces in width below this setting. You can readily see that the EF pulse width still does not match the main sample pulse width exactly, but it did improve the EF elimination. Clearly there is still room for improvement in this area.

However ... after saving the scope shots and taking the two photos, I thought it might be a good idea to try the detector outside to check for any instability in the coil winding, especially as this appears to be a recurring issue. To carry out the test I disconnected the PCB from the bench power supply and reconnected to the battery pack. Would you believe it? ..... there was an immediate reduction in detection depth, and it turned out that the CD40106 was [yet again] damaged in some way.

Anyway, this is what I think is happening:
If you examine the schematic in the area of the power supply (top right), you will see that there is a 220uF electrolytic capacitor connected between VSS (-5V) and VDD (+5V). At power-on, the 78L05 becomes active first (since it's connected to the battery input) whereas the 79L05 is driven from the output of an ICL7660. This charge pump takes a finite time to get started, with the result that the -5V supply lags behind the +5V supply when the battery is connected. Unfortunately there's a 220uF electrolytic capacitor connected between the +5V and -5V supply lines. This is wrong, and there should be two separate capacitors connected from each of the power supplies to 0V (not to each other). When the battery is connected, the 220uF capacitor looks like a short-circuit, which means that the +5V supply gets connected momentarily to the -5V supply. The result is that the CD40106 has its power line connected in reverse when the detector is switched on. When I was using the bench power supply, there is an additional delay while the PSU output ramps up, and so the circuit survives. But as soon as you connect a battery pack, then things can go horribly wrong.

I'm going to have to order some replacement CD40106 ICs, and then modify the power supply section. Hopefully the 12" detection range can then be restored. Finally, note that there's a second 220uF electrolytic capacitor across the regulator inputs as well, which needs attention.

In the meantime, it might be a good idea for others to make the same scope measurements and compare to my "good" readings below.

[Michaelo]

@ivconic, Many thanks appreciate the reply...

I guess I need to make a better coil though, I like to achieve better distances...
Note, I've also changed R17,18 and 19 (40106) from +5 volts to Ground (to operate the chip within specs), didn't appear to affect the operation all that much...

Mike

[ivconic]

I lost the iron reject on my Barra build. Those videos have given me an incentive to re visit my build.

With better coil and more precise delay adjustments you will be satisfied with your Barracuda.

[ivconic]

I'm getting similar detection distances to those posted in ivconik's last couple of posts but in other youtube videos the detection distances appear to be much greater...

I'm testing using Euro coins... I can detect 50 cent coins at about 6" but 2€ coins need to be at 4" or 5"... A small aluminium heatsink (1.5" x 1.5" x 0.5") can be detected at about 10" or more...

I test by suspending a string over the coil, attaching a coin and swinging it like a pendulum, that way I can try tweak the delay pot for best response...
Next test will be outside to see if I get any improvement but... can someone give me a reference to what I can expect so I know if my detector is up to scratch?

Mike

Usually distances are greater. Coil from video is only 7" diameter.
So i would say those are decent distances on coins with such coil and considering it is a PI detector.
Though i didn't insisted on air distances when i did those videos, i wanted only to show rejection.
Barracuda is even slightly better than MPP on small coins.
A bit larger coil (up to 10") will give better distances, for sure.

[Koala]

I lost the iron reject on my Barra build. Those videos have given me an incentive to re visit my build.