For making up boards , I picked up one of these a while ago, they include a magnifier which is very handy.

 

I have been eyeing a soldering microscope lately and I though a decent unit would climb into the thousands of dollars. this one seemed to have little or no lag in movement under the microscope.

I use a Elmo document viewer instead of a microscope. It requires a laptop, but there's no lag. I bought mine used on Ebay for under $50.

 

For making up boards , I picked up one of these a while ago, they include a magnifier which is very handy.


View attachment 368506

I have one of those but it just does not magnify enough. in fact I just bought another one with a bigger base and 3* magnification on it but my glasses are 3.5 mag themselves and I still can't see past the end of my nose.

 

I used it because I decided on a 2.5V cutout voltage for the Li-ion battery. If you wanted it to be higher, you could just use a higher voltage zener. I think the next value in my stock would have been 3.3V.
I chose MOSFETs because they'd dissipate less power than a BJT would.

AO3400 is rated for about 5A (at a higher threshold voltage). RDS is less than 20mOhms, so it would dissipate around 40mW at 2A. A BJT would dissipate watts at that current.
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I started studying electronics in the mid-1970's and they didn't teach about MOSFETs. They were more curiosities at that time and didn't have anywhere near the capabilities they have now (in particular low threshold voltages, low on resistance, and high current capacities, and SMT wasn't that common back then). I learned about them pretty much on my own.
I tend to choose LM358 and LM393 (both around 50-year-old designs) when nothing better is required. LM358 is an opamp and LM393 is a comparator. Aside from the compensated gain stage of the LM358 and the open collector output on LM393, they're pretty much the same in functionality. That's why I suggested using a comparator for the opamp if you wanted to eliminate one of the ICs. Comparators are okay as slow opamps (due to the lack of compensation).

The opamp needed to allow a low input voltage and the input stage of LM358 is PNP, so ground is allowed. These days, users are more likely to opt for a rail-rail input device (and pay a buck or so for it vs $0.25 for LM358). The issue I have with rail-rail input opamps is that they usually use a complementary input. I worry about distortion in the crossover point. Early rail-rail opamps used charge pumps to operate the internal circuitry at more than the supply voltage. That could introduce noise. So I only use rail-rail input/output opamps when I need something better than LM358. It's hard to study modern opamps/comparators because manufacturers don't include detailed schematics. Also, you need to know the threshold voltage of the MOSFETs to analyze the circuit. You can't assume single Vt because multiple Vt's in the same device aren't uncommon these days. It just requires a different gate implant.

Similarly for LM393. I wanted the output to be able to go to the positive rail. Modern comparators usually have push pull outputs and can't quite get to the positive rail. The limitation on input voltage swing is addressed by using a 9V power supply LM393 allow around 7V, so getting to around 4.2V isn't a problem.
For home use, I think a hot air tool is better than a hotplate. With a hotplate, I'm concerned about exceeding the maximum allowed soldering times (like 10 seconds). For commercial applications, the equipment can control the temperature and time better.

For most of the SMT devices I use, I use a soldering iron and 0.015" solder wire. When I try using paste (without a stencil), I usually end up not using enough solder and have connectivity issues over time.

Thank you verry much you have been most helpful. Looks like I have to figure it out with hopefully through hole components. If not I figure out how to mount and heatsink an SMD chip fortunately pins and 3 chances each not to cook it.

 

If not I figure out how to mount and heatsink an SMD chip fortunately pins and 3 chances each not to cook it.

You can buy these adapters on AliExpress (used to be about a nickel each):


I use solder wire on them.

The leads (called leadframes) cost about 10 cents each in quantity 100 when you can find them (I have of couple types available for 0.06" thick boards barter in the barter thread). You can also use right angle male headers.

 

@robp1956

The N channel MOSFET in the circuit I posted needs to be a power device. When used in a current sink, it is being used as a variable resistor and at 2A with a fully charged battery (4.2V), it will dissipate over 8W. So, it needs to be something like IRLZ44N with a heatsink.

 

Thanks I was going to go to Mouser to look for something in the n and P channel mosfets that would at least be logic level. But I have a few IRLZ44N in one of my drawers. Thank you again for you help in this.

 

You can buy these adapters on AliExpress (used to be about a nickel each):
View attachment 368524

I use solder wire on them.

The leads (called leadframes) cost about 10 cents each in quantity 100 when you can find them (I have of couple types available for 0.06" thick boards barter in the barter thread). You can also use right angle male headers.

I have seen ones Similar to that but also had the 6 pin and 8 pint adaptors and a few I would not use loke BGA type chips the rebal type.