Replies to Minipulse Plus

[Qiaozhi]

Hi George,
I have just purchased the PCB and some other parts from Silverdog the remaining parts from Element14 here in Oz. The one part I could not purchase locally in DIP was the NE5532 so have purchased the MC33078 as a possible alternative..!

The MC33078 looks like a good alternative. Please let us know how it works.

[sinclairuser]

seems that my initial reaction was wrong, and that my usual sources were crap in this aspect, ordered the board today, gonna try to use what i have in, and source parts that i dont have separately.

[Dean Sarelius]

Hi George,
I have just purchased the PCB and some other parts from Silverdog the remaining parts from Element14 here in Oz. The one part I could not purchase locally in DIP was the NE5532 so have purchased the MC33078 as a possible alternative..!

[Qiaozhi]

Hi George

I guess a 2n5485 could be used as well.

As various constructors find that they don't have access to some of the specified parts, or just want to use stuff from the junk box, we can build up a separate document showing which alternative components have been tried successfully.

[6666]

Hi George

I guess a 2n5485 could be used as well.

[golfnut]

Once again looks fairly tasty George.

Great to see Version Nos on the silkscreen. S

[Qiaozhi]

I have ordered some real J113 fets, in the mean time is there a suitable replacement ?

Here's a list of equivalents:

http://alltransistors.com/mosfet/crsearch.php?&struct=FET&polarity=N&pd=0.35&uds=35&ugs=&id=0.05&tj=150&fr=&cd=3&rds=30&caps=TO92

However, I suspect just about any general purpose FET will work, as it's only being used as a switch.

The design is fairly forgiving as regards components tolerances. Although I've specified a 1% tolerance for the resistors, you should be able to use 5% without any issues. As a general rule, I only keep 1% tolerance resistors in stock. Unless you're a pile 'em high, sell 'em cheap sort of person, the odd fraction of a penny in component costs is neither here nor there. For the capacitor voltage ratings - these are also not fixed in stone. As long as the rating is greater than the maximum possible voltage across the component, then it will be ok. For caps on the 5V supplies (for example), simply choose the next higher voltage such as 10V. Of course, that doesn't mean you can't use one rated at 16V (or even 50V). As long as it will physically fit on the board, then it will be ok.

[Qiaozhi]

Although the Surf-PI and Baracuda are relatively simple designs, it appears that a great number of builders have trouble actually getting them to work.

This seems to be due to either lack of knowledge on how to logically debug a circuit, not checking component values properly and fitting the wrong parts in the wrong places, not having access to the correct equipment for setting up / tuning, or just generally not understanding how a PI works.

The Build Document is an attempt to make construction of the Minipulse Plus easier, and [hopefully] problem free. However, as they say: "Design something that any fool can use; and only a fool will use it". I hope we don't find the same issue here.

Clearly, everyone has to start their learning curve at the bottom and work up, and it's possible that building the MPP will help some people to do that. If so, then I'll be happy.

[golfnut]

great to see a ground or power plane on your board. should introduce more repeatability between everyones builds around the world at the very least. S

[Qiaozhi]

... i wondered about some of the component choices, some of these bits are not that common ...

Which parts do you consider "not that common"?

The bipolar transistors have been replaced by general purpose types (2N3904 and 2N3906), and all the other parts are pretty standard. The only thing you may not have in stock is the LT1054, but even so it's inexpensive and readily available.

[sinclairuser]

before any jerk starts saying i'm sore that the kits are not up yet(its not that).
but i wondered about some of the component choices, some of these bits are not that common, i'm also wondering about the cost of kits, some of the prices i have been quoted for some parts as singles have made me give up(i never normally buy singles always multiples), i intend to wait for the kit, may even wait until the next revision if i'm honest, when no doubt some of them will have been replaced

[6666]

I have ordered some real J113 fets, in the mean time is there a suitable replacement ?

I have also ordered some LT1054 chips, in the mean time will just use a 9V battery for U3.

[eogeo]

Thanks for the FET matching explanations. Good to know it is not a factor in this design.

[Davor]

Often neglected is a gate charge. In effect all half wave detectors with switches, and even more so with FETs suffer from gate charge injection. In balanced configurations, such as this one, FET gate charge injection is cancelled together with any offset and earth field.

[Qiaozhi]

There was a mention somewhere else on Geotech, Baracuda thread perhaps, that matching the J113 FET's may improve performance a bit. Is that also the case with this design?

Personally I don't think you would notice any difference.There are two samples taken: the main sample and the secondary sample. The secondary sample is taken much later after the eddy currents from any metal targets have more or less died away. However, both samples will contain signals that are received from an external source (in this case, the Earth field), so subtracting the second sample from the main sample will eliminate the unwanted external signal. There would be virtually no benefit from matching the FETs, as they are either turned on or off, and do not operate in the linear region.