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#258 ClapSwitchKit2

Practice some Boldport-style soldering on yet-another clap switch kit.



I’ve had this clap-switch (voice activated) kit sculling around for a while (another “stocking stuffer” bargain on aliexpress).

The circuit is very similar to a previous ClapSwitchKit project. The operating principle is identical, but it varies in two main ways:

  • no output stage included
  • component selection and values are different

So I pulled this kit out primarily to practice some Saar Drimer/Boldport-style soldering. The objective?

  • beautiful solder joints
  • smooth to handle

See the Making beautiful solder points video to find out more on the technique.

Comparing Solder Techniques

I used different techniques for the three solder points in the picture:


  • A (left) 2-pass boldport style: solder; clip down to the board; re-flow and make the dome
  • B (middle) the cheats’ boldport 1-pass style: blu-tack the part and clip the lead first, then solder with dome on the first pass
  • C (right) normal technique - lead clipped after soldering
  • Note the joint in the upper right - now that’s more like it! … though still not very shiny

A and B are clearly superior to C - which is asymmetrical and not finger-safe.

In practice, I can’t tell much difference between the results of A and B. I think for now my preferred style is B:

  • place the parts - I use blu-tack to hold them in place if necessary
  • clip the leads as close to trim with the board as possible
  • solder and make the dome in one pass
  • side benefit is you get to harvest the clipped leads free of solder and flux (if you care - I keep some around for ad-hoc protoboard wiring)

Making the “Dome”

At first, I had real trouble making smooth symmetrical domes. I’d always end up with “meringue tips” when I drew the iron away. The technique that seems to work best for me is:

  • make sure the iron isn’t overloaded with solder
  • withdraw the iron by first pulling down and around the nearside of the solder joint, nudge in a little then withdraw smoothly

This sounds more like icing a cake!

Making Shiny Joints

So now I’m getting decent domes (most of the time) that are electrically and mechanically sound. But they are rarely shiny.

“Shiny” is not usaully a QA issue for solder points (advice abounds on how it is not important). This is not to be confused with dry joints, that will be both dull and flakey/crystaline.

Google delivers many opinions, for example:

The main advice for why solder points are not shiny seems to boil down to one or more of the following:

  • unleaded solder “rarely produces shiny solder joints”
  • no Silver (Ag) in the solder - especially lead free
  • iron is too hot
  • iron is too cold (but may be confused with dry joint issues)
  • cooling too fast/airconditioning/cold-board
  • contamination
  • not enough flux
  • disturbed before cooled

In other words, lots of theories!

So I ran some tests:

  • I’m using WL-0510 B-1 0.5mm 63Sn/37Pb
  • I tested with a range of iron temperatures from 250˚C to 325˚C
  • with A/C (24˚C) and without (probably heading to 27˚C)
  • with and without additional flux

… and I get pretty much the same result all the time: a nice shiny ball when I remove the iron, then as it cools, it gets to a point when the ball instantly frosts over and I end up with a “dull” solder point.

It seems pretty clear that it is oxidizing at the phase-change from liquid to solid. Not sure what to try next to control this…

… except elbow grease;-) Turns out that with a bit of polish (using solder-station copper wool), I get some of the shine back.

So after a bit more research, I am suspecting the dullness created during the phase change from liquid to solid is due to the eutectic property of 63Sn/37Pb. As you can see in the phase diagram below, 63/37 is the alloy with almost perfect eutectic properties (meaning it goes straight from liquid to solid without passing an intermediate plastic phase).


REALLY Shiny Domes

Since everything I tried so far hadn’t made much difference (temperature, technique, aircon), the only thing left to try is different solder.

So I got hold of some Pro’sKit 9S002 Solder (62% Sn, 37% Pb, 2% Ag) … and the difference is amazing:

  • it creates “domes” with hardly any effort
  • and they are all nice and shiny

Here’s a side-by-side comparison using the same iron and tip, and both at 275˚C.

  • on the left: 62Sn/37Pb/2Ag. Soldering is a breeze, and the results are perfectly formed and shiny
  • on the right: 63Sn/37Pb. Making domes is “hard work” and they rarely turn out shiny


I’ll need to do some more tests to determine if the problems I was originally seeing are typical of a 63Sn/37Pb formulation, or if perhaps I just have some very poorly manufactured 63Sn/37Pb.

First Pass on the Board

Here is the first pass. Electrically/mechanically sound but not really perfect “Boldport” quality:

  • my dome sizes/shapes are a bit uneven
  • a few meringue tips
  • dull (see above) but brightened up with a bit of buffing


Circuit Performance

This is a trace of the “clap-on” transition

  • CH1 (yellow) is the output signal
  • CH2 (blue) is the piezo speaker input to the base of Q1
  • no vertical offset


The board under test…






Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.