Selfmade (modified) variable voltage AC using step down converter

Let's suppose that you need to plug something in that requires less voltage than any of your spare ACs give out. I had to make a thingy work that needed 3.5-4V max, but I only had 5,9 and 12 volts ACs at my disposal. What do you do in that case? Well of course you could order voltage-variable AC on ebay, that comes with a lot of plugs too! But then you'd have to wait for shipping and pay somewhat 10$, but you already have a lot of ACs, why not upgrade them? Besides, the majority of variable ACs are not really adjustable, they have 3-5-9-12 switch, and that's it. So you can't really use thouse if you need exactly 4.2V output.

The things you'll need:
1) ~30 minutes of your time
2) Soldering Iron
3) Working AC adapter (I chose to upgrade my 12V and 9V adapters)
4) Step-down converter module, you can get those on ebay for 1.5$ (And yes, you'll still have to wait for these to be shipped): DC-DC Buck Converter Step Down Module LM2596S Power Supply Output 1.5-35V
5) Hot glue (Not necessarily, but recommend using it)
6) Multimeter

Cheap step-down converter module

Now I would recommend ordering those in bulk. I made a mistake of ordering just 3 of those, and now I'm all out ):

What you have to do - is just solder 2 wires on In, and 2 wires on Out. You can see thouse marked on the picture above. You can modify the output voltage by turning that little thingy on the top of blue thingy with a nail or a screwdriver. It obviously doesn't have an indicator to show the voltage output, so you'll have to use a multimeter. I assume you have one. Everybody has one!

Almost finished voltage-variable AC

It would be nice to put hot glue all over the thing, to make it stick on the AC and to make sure nothing falls off. Just remember not not glue the regulator thingy (: There you go, simple, but very efficient.
I'm now thinking about upgrading all the ACs I own...