Can pinpointer detecting thin chain?

[Nupi]

You have a lot of hours in there, thank you for that.
In total, it seems like a decent not great circuit.


Maybe if the lf198 would work better, by aliexpress.com you can buy for 1,78 dollar.
I'm impatient as a child. I'm so curious how it will work. A SAH + HEX Inverter + switch,
Interestingly. Just out of curiosity,can you show us something as a schematic?

[Thomas]

Now, to do it properly I'd need a cheap rail-to-rail FET input single op amp

I don't know what if this is what your looking for, but I have some AD8532 CMOS RRIO, if you want. They are dual OPs and SO8. I know I have others but I'll have look through my collection.

[Davor]

I think I'll go for TLC271, simply because I can get them here. They go down to the lower rail (input and output), and I can put the other rail to 9V so that the positive rail is not problematic. Real rail-to-rail are still best to avoid.
What I got so far is this:



Only 2 gates of 4069 are used, so the remaining 4 may be used as a tweeter driver. It was not important for establishing the automatic circuitry, so I simply omitted them to speed up simulation.
FET can be just about any FET with pinch voltage not exceeding -2V.
10k trimmer sets sensitivity threshold, and 20k trimmer is to set the working conditions right. With its correct setting the op amp output is at about 2.5V after the pinpointer sets itself.

[Nupi]

So Davor, that looks ingeniously.
Do you have use examples or Is this entirely your own work?
Anyway, great job.
I order this week the tlc271 then I can go try out.
Is there anything I should pay special attention when building?

First I'll build on my beloved breadboard.
Iam looking forward!


BTW, when are you going to try to build it, do you think?
Now that I'm still ask, My fourth and final question:
Do you know where I can find on the web a calculator thats a parallel RC network or series RC network to frequency calculating.?
I get a little tired of all this calculation.


Sorry for all the questions, but I am today in asking mood.
thanks!

[Davor]

Hi,
let me explain how this automatic thing works.
When a pinpointer is powered on by depressing a pushbutton, a timer capacitor starts charging, and the voltage drifts towards the 4069 toggle point. Because of the synchronisation circuitry it will toggle somewhat earlier.
Simultaneously the integrator output voltage goes down towards a value at which the device starts beeping. It is the most sensitive at the very onset of a beep, and it is the point to which the timer is synchronised to. When the timer toggles the integrator becomes a SAH in a "hold" function, so that voltage at op amp output will remain the same for quite some time.
Because such a voltage would cause a pinpointer to beep perpetually in the air, there is a back off mechanism set by a 10k trimmer and a 220k resistor connected to a timer. When a timer toggles the control voltage is raised a little, and it thus sets the sensitivity threshold. At 1k5 at the trimmer it would continue beeping, but slowly, and at 2k2 beeping in the air vanishes, but most of the sensitivity for targets remains.
For this mechanism to work properly the timer must expire after the onset of beeps, and it may be set by the 20k trimmer. Ideally it will be 3-4 beeps before the pinpointer enters the calm phase.
I did not put any circuitry to discharge a timer capacitor faster between uses to speed up repetitive use, but it will be simple enough. A diode can do it.

Choice of FETs is critical as the VGS(off) must be below 2V. Popular types that fit the bill are 2N4393, BF245A (maybe), 2SK170 or J201. In schematic I used 2N4117, but these may be a little bit difficult to find.

I gave a thought to the choice of op amps, and it seem JFET input types without rail-to-rail capability could do if the op amp is powered from 9V. Inputs are at about 4V, and if set properly the output will never reach a lower rail, so the choice may be a bit relaxed.

[Davor]

@Nupi, this is all my work, I have no idea when I'll be able to make it, but I'll try to make it before summer. As for calculator, I often use the simple one that comes with Windows. For any more complicated network I use spice.

[Nupi]

Nicely explained, thanks.
Really understand I do when I go play with the values and for that is a breadboard an ideal
tool.
Probably Would in practice what values need to be adjusted, I think.
Also the arrangement of the components will perhaps play a role, we'll see.
BTW I now know what you look like, and I like that when I communicate with someone.
regards

[Davor]

I realised that repetitive use may easily lead the above design to just latch, and it will need a few small corrections that involve Reset pin of CD4024, an inverter here and an inverter there. Hopefully I have a few inverters to spare, so it is going to remain simple. The automation part will remain the same.

[Nupi]

When you think you have the schematic with the new changes?
I wait to build and experiment with everything together.

[Davor]

Soon. This is uncharted territory. Let it pickle a bit more - you never know if anything else pops up.
I have this corrected version in Spice and it is completely clear to me, but hardly to anyone else. I'll transfer it to ExpressSCH when I'm done.
For the time being, this is what I got:

[Nupi]

OK, I'm going to work with as soon as I have the parts.
Do not worry that you are so far the only one that really understands.
"....but hardly to anyone else". If so, it is often a very good sign,
and would often say that you brains work in higher spheres than the rest.
I was, according to many people totally crazy, (maybe I am too ) and said that my PP would never work with the CD4024.
silly trolls.

[Davor]

A bit crazy is almost always a good thing

[Davor]

Yesterday I transferred a LTspice solution to ExpressSCH. I did not check it for errors yet, and I did not make it in hardware yet, but here it goes anyway.

I think the best hardware solution would be a narrow PCB for placing everything into a tube, so that it can go under water. A pushbutton can be replaced with a reed relay and it can be operated with a magnet from outside. I expect a single 9V cube to last for a season, so - why not hermetising it even with a glue. I could take it for a dive.

[Nupi]

Thanks Davor but I can't open the file!

Eagle light
Error: line 1, column 1: Start tag expected.

[Thomas]

Davor, I found this interesting, could you tell more, I think it's for input stability, but not sure.

Nupi, here you go.