Replies to Minipulse Plus

[Qiaozhi]

With connection of scope on TP3, the image is identical to the one I posted earlier - although there is more deflection when my mini side-cutters are presented.

Here's what the waveform should look like when measured across the damping resistor (R1).
Note how the decay happens over a period of about 5us (even with the original [slow] Minipulse coil). Note the use of a x10 probe. The flyback voltage is measured as 316V.

[greylourie]

I cant get an image like that. What could be wrong ?

[mickstv]

Hi greylourie, if you have heaps of cat5 cable, spend the time and remove the outer insulation and split into the 4 pairs then get one length and undo it so
you have a single strand then wind your coil with it ( I have done this before, but be aware it could take 1/2h or so to do).

[Qiaozhi]

I cant get an image like that. What could be wrong ?

The problem definitely appears to be associated with the coil.

BUT - just to be certain, let's do a simple experiment to test if the circuit is working ok.
Disconnect the coil, and replace it with a 10nF capacitor. Connect your scope across the damping resistor (R1) as before, and look at the waveform. Set your scope probe to x1, as the flyback will not be present.
Here's the result:

[greylourie]

Hi Mick, thanks for the tip. A continuous length probably my best bet. I only have about 1.2 m of cat 5 left - don't want to be joining lengths up. Will go see what the local hardware store has.

Those images I posted in #255, are bothering me. I can't get the earlier part to display - like in Qiaozhis image. I am failing to get the scope to display the on-period and then where switch-off occurs.

Sry Qiaozhi, just seen your reply. Can I use the poly cap from the kit marked 10n ?

[mickstv]

Hi Mick, thanks for the tip. A continuous length probably my best bet. I only have about 1.2 m of cat 5 left - don't want to be joining lengths up. Will go see what the local hardware store has.

Those images I posted in #255, are bothering me. I can't get the earlier part to display - like in Qiaozhis image. I am failing to get the scope to display the on-period and then where switch-off occurs.

RE the CRO, connect a second CRO lead to the output of the 555 then set trigger source to CH2, then the full waveform should show up.

[greylourie]

With 10nF cap on in place of coil, i get the following image. Approximately 11.5Volts.

[greylourie]

RE the CRO, connect a second CRO lead to the output of the 555 then set trigger source to CH2, then the full waveform should show up.

Hi mick, all these knobs are doing my head in. I have a port for connecting an external trigger, haven't learnt how to use it yet. When I try running both channels at the same time, I find I don't have enough space on screen to display all information.

When I mentioned my images in #255 - not looking like the one posted by Qiaozhi, I meant mine does not show all the signal (TX off and start of flyback) when scope connected across R1. All I can see in my images is what looks like the pinnacle of flyback, and the drop back down, and the small oscillations on the tail. Counting squares get 275V, but that is only what is shown on the drop down, and I cant count from start and use as a proper baseline because it does not show.

[Qiaozhi]

With 10nF cap on in place of coil, i get the following image. Approximately 11.5Volts.

That looks good to me.
Conclusion: your coil is the problem.

I would suggest that you go to your local electronics/electrical store and get a small reel of hookup wire. Then wind 20 turns on a 10" diameter former, leaving the tails long enough (about 12") so that you can connect it to the PCB. Simply wrap the wound coil with electrical tape, and don't bother about shielding. Also, do not twist the wires that connect to the PCB, and make sure you keep the coil away from any large metal objects.

[greylourie]

Thanks Qiaozhi. Will do.

[Davor]

Darn! I couldn't obtain 79L05 so I'll have to cram in a "normal" 7905.
Tomorrow I'll go for a vacation without any chance to get parts that I miss, so 7905 that is. With a view of Adriatic...

[greylourie]

Darn! I couldn't obtain 79L05 so I'll have to cram in a "normal" 7905.
Tomorrow I'll go for a vacation without any chance to get parts that I miss, so 7905 that is. With a view of Adriatic...

Off to the beach ! Have you built the ground balance plugin to take with you ?

[Davor]

No, but I packed all the parts that I need to make it. I also bought some extra DIL sockets that I'll violate to make FET sockets and facilitate transition. I also bought a few FETs extra of various kinds to cross-compare in the sockets. I couldn't get J113, but I got some other types. Besides, J113 with capacitances that large is perhaps not the best component regarding charge injection.
I don't think I'll be able to do much with minipulse in the next few weeks as I don't have a proper coil yet, but I plan to go for deserted beaches in fall, so no pressure there.
I also packed parts for a modified Nupi pinpointer with automatic settling.
So much to do (finally ) so little time.

[joop]

Hahaha, first time for me that I read about someone that builds a detector while on holiday instead of going beachhunting....

[Davor]

My trouble is that the law in my silly country makes most of my detecting illegal, so unlike most of the people I'm keeping my chastity till fall when all the tourist-related hullabaloo subsides. I'll also have time to actually solder something that I simply can't afford with everything else at hand.

If you've never been to Croatian coast, it is mostly wild and sparsely populated. Tourism is the only thing that really grows there, aside from the prices in the markets. Unlike other places you can still get in touch with nature there and enjoy it, but no detecting.

You may even find places where even tourism is only a foreign expression with no true meaning. We call such places "vukojebina" [woo:koh:yebi:nah] - wolfs' procreation area (only largely without actual wolfs). If that is what you seek - you'll find it here. A stone house with open fire and all commodities 1700's style - no problem. A distant island - no problem at all. Diving? Paradise.

So who is detecting?