Let's make a closely MXT like detector!

[Altra]

On the missing transistors. Search the core number "5087 transistor " then look for surface mount versions. FMMT5087 will be the same as a MMBT5087

https://www.onsemi.com/pdf/datasheet/mmbt5087lt1-d.pdf

The BC849 probably has a common replacement. If you study the datasheet

[Carl-NC]

Are you wanting to do this SMT, thru-hole, or a mixture? Some part numbers are specific to a manufacturer but have generic equivalents. For example, FMMT4403 is the same as MMBT4403 (SMT) and 2N4403 (thru-hole). Parts that are No Load were not used.

[Carl-NC]

Pretty much. The 74HCT4053 demod switches will happily run on 3V logic. Where you might need 5V (say, the display) you can use open-collector GPIOs with 5V pull-up resistors. Most STM GPIOs are 5V tolerant.

Just noticed the two 74HC4351's. The 3V logic-compatible part would be 74HCT4351. The 'T' means TTL-compatible which will run on 3V.

[KRinAZ]

On the missing transistors. Search the core number "5087 transistor " then look for surface mount versions. FMMT5087 will be the same as a MMBT5087

https://www.onsemi.com/pdf/datasheet/mmbt5087lt1-d.pdf

The BC849 probably has a common replacement. If you study the datasheet

A search found: "You can replace the BC849 with the 2SC3912, 2SC3913, 2SC3914, 2SC3915, BC847, BC848, BC850, FMMT619, FMMTA05, KST05, MMBTA05or SMBTA05."
I will search on availability of these, then for available ones will compare specs and look for good match.

[Carl-NC]

For general purpose bipolar transistors, I use the 3904 (NPN) and 3906 (PNP), either thru-hole (2N) or SMT (MMBT). It's rare in a metal detector that you need more than these.

[KRinAZ]

Are you wanting to do this SMT, thru-hole, or a mixture?...

Short answer: I would choose all thru-hole if possible and use a (thru-hole) Nucleo instead of bare STM32. In the event there is a component that is smd only with no thru-hole alternative then that would be the exception. I think more folks would be able to build this project if smd were at a minimum.

Long answer, I am usually a quick study in picking up new skills, but currently have no experience soldering smd components in place, especially complex ones. I do have a stereo microscope for viewing smd soldering and a probably low end smd soldering station. I have a nice traditional Weller soldering station for thru-hole only. I have practiced unsoldering smd components from junk boards with mixed success. I would need to greatly improve my skills before trying to solder a smd DAC or ADC or MUX or especially the STM32 in place.

Some part numbers are specific to a manufacturer but have generic equivalents. For example, FMMT4403 is the same as MMBT4403 (SMT) and 2N4403 (thru-hole).

Thx, I will widen my search for replacements of not available components.

...Parts that are No Load were not used.

That is good news! and makes sense as it appears the No Load components are often in parallel to components with actual values. I will mark up a copy of my printed schematic that removes all the No Load components.

[KRinAZ]

Just noticed the two 74HC4351's. The 3V logic-compatible part would be 74HCT4351. The 'T' means TTL-compatible which will run on 3V.

Great, will update the components list - which will ultimately will become the BOM.

[KRinAZ]

For general purpose bipolar transistors, I use the 3904 (NPN) and 3906 (PNP), either thru-hole (2N) or SMT (MMBT). It's rare in a metal detector that you need more than these.

Oh, ok great, the ubiquitous 3904 and 3906. I guess I was getting too literal about "...I've already mentioned that the resonance-boosted TX circuit should be preserved..." in post 68.

Would you be ok with us just using a 2N3904 for each NPN and a 2N3906 for each PNP - throughout - then?

[KRinAZ]

...For example, FMMT4403 is the same as MMBT4403 (SMT) and 2N4403 (thru-hole)...

Ok that just got my attention - didn't know to just strip off the prefix and search for the number or more the generic mmbt / 2N versions - that's very good to know.

[Carl-NC]

Although all my work projects are pure SMT, I still do a lot of thru-hole designs, especially smaller test circuits where I want to easily swap components. Even with a bad soldering iron (and my Weller fits in that category) I can usually solder 0805 passives and SOIC. My good soldering iron is a Metcal MXPS-5200 and I can do 0402 passives and most QFPs. In the past I've replaced QFP-80 STM32's but it's painful. There are other tricks like using solder paste and a heat pencil, I've used that to install QFN parts. (I work remotely from FTP so if anything needs to be done, I'm the only one to do it.)

In the MXT the biggest problem chips will be the micro and the ADC, and maybe any switching regulators. A Nucleo board solves the micro but it can be big. You can usually find eval boards for ADCs but they are often not all that friendly for incorporating into a design and are often expensive. So, most likely, you will be stuck with an SMT ADC. Same with switching regs, but fortunately it's easy to find one in 5- or 6-pin SMT that's easy to solder.

Another potential solution is to have the board house install the difficult parts. I've ordered fully built boards from JLC; their minimum order is 2 and their part pricing is quite good. You still have to pay a set-up and build fee and depending on how many parts you install it can get pricey. The last board I built was about 2x more complex than the MXT and 5 boards cost $840.

[Carl-NC]

Would you be ok with us just using a 2N3904 for each NPN and a 2N3906 for each PNP - throughout - then?

Offhand I don't see any problem with that.

[KRinAZ]

Regarding:

U5 ADS8320 ADC AVAIL SMT ONLY Mouser DigiKey - I will search for a thru-hole replacement.

U11 MC33077 Dual Op Amp NOT avail Mouser DigiKey Obsolete
U12 MC33178 Dual Op Amp AVAIL SMT ONLY Mouser DigiKey

Back when I built George's MiniPulse Plus PI project, I did some mods to it including cleaner opamps. I'm going to go back to my notes and remember the succession of opamps I tried and see if I can find suitable replacements for the MC33xxx's. These will require close scrutiny to get right as they are crucial parts of the Receive chain.

More soon with replacements and feedback on all of the pros and cons...remembering we are not wanting to mod the analog circuits, just complete the parts list with thru-hole, and available, parts.

[KRinAZ]

...In the MXT the biggest problem chips will be the micro and the ADC, and maybe any switching regulators. A Nucleo board solves the micro but it can be big. You can usually find eval boards for ADCs but they are often not all that friendly for incorporating into a design and are often expensive. So, most likely, you will be stuck with an SMT ADC. Same with switching regs, but fortunately it's easy to find one in 5- or 6-pin SMT that's easy to solder.

Another potential solution is to have the board house install the difficult parts. I've ordered fully built boards from JLC; their minimum order is 2 and their part pricing is quite good...

Something I have in the back of my mind also. If SMT can't be avoided, this would be an option for folks like me, and there are certainly those of you who would be quite comfortable soldering the SMT components.

But first I'm going to explore thru-hole possibilities...more soon...probably tomorrow...

[KRinAZ]

...My good soldering iron is a Metcal MXPS-5200...

Hmmm...I'll have to watch for a deal on a Metcal MXPS-5200...

[Carl-NC]

Opamps are not a problem, lots to choose from even in thru-hole.