Reddit reviews DROK 090029 LM2596 Numerical Control Voltage Converter Board DC 5-32V 24v 32v to Adjustable 0-30V 12 v 5 v Switching Regulator Module 1.5A Volt Transformer with Red LED Voltage Tester

You don't want to use a linear regulator for this big of a voltage drop - you're going to waste a LOT of power (and you're going to need to dissipate that power loss as waste heat).

Easiest solution here is to use a Buck Converter - you can easily achieve >90% efficiency with this. You can either build one yourself, or if you look on amazon there's tons of them available (e.g. this one for <$10)

For programming, I had to provide the 3.3v power to the sonoff from a separate power source. My USB ports couldn't provide enough juice for programming so I used on of these: https://www.amazon.com/gp/product/B00BYTEHQO/ref=s9_dcacsd_dcoop_bw_c_x_1_w

I've been using this as a speed controller on my stir plate and have been really happy with it. It costs a little more than buying components and building your own circuit but it's worth it to me.

http://www.amazon.com/gp/product/B00BYTEHQO

Have a look at good old EEVBlog's post on building an electronics lab. If you're young then you're probably going to have a hard time affording stuff like a new oscilloscope and you won't need stuff like function generators for your project, but that video should give you some ideas about tools and parts. Since you're just getting started I recommend buying them as you go along.

The bare essentials though are IMO:

• Breadboard, as big and as many as you can afford. Also the dupont wires that go with it, with all three combinations (male to male, male to female, female to female).
  
• A variable power supply. For the very basics, these do a pretty decent job and are way cheaper than a real lab supply. Just find a 12V wall adapter and cut the plug off and feed the wires into the leads on the converter and boom, decent enough power supply.
  
• Soldering station, like mentioned in the video try to get one with a base and adjustable temperature. To go with it, a solder pump (aka solder sucker) is invaluable and they're cheap.
  
• A multimeter. A cheap auto-ranging model is sufficient for starting out, but if you can manage to buy only one nice thing this is a good investment. Again shout out to EEVBlog, their branded multimeter is a great deal for a nicer meter.
  
• Some flush cutters. These are not to be confused with diagonal cutters which are also useful, but the flush cutters IMO are more versatile. I also use these for stripping wire and they tend to do a better job than most special purpose wire stripping tools.
  
• Some sort of case/box to keep tools and parts in. The little plastic cases with compartment dividers are great for storing parts and any old toolbox is handy for umm... keeping tools in duh.
  
  Now here's the tough part - an oscilloscope. For what you want to do (working with the speakers and audio) as well as in general, an oscilloscope would be an absolute gift from the gods and would also teach you a ton, but it's going to be pretty much impossible to find one new for a reasonable price. You can get a surprisingly decent digital scope these days for ~$300, but I know that would have been way too much for me to afford when I was younger. Instead, you might want to try asking around any local electronics factories/repair shops/universities/etc to see if they have any old scopes they want to get rid of. Tell them you're young and trying to get started and that you'd be really happy to take an old scope off their hands (analog is perfectly fine too). Even if it's broken, if you can fix it that'll teach you a lot as well. If you can't find one though I wouldn't worry about it, it's not something you must have and would merely be useful.

They are both 5v cell-phone recharge batteries, got them off amazon on deal for around $10 each. Fully charged at around 4.6v, which under-powered the pi, and shows their (lack of) quality. discharges / recharges of the battery would decrease the amount, so I was scared of it damaging the Pi.

I hooked the batteries into series, hooked the (now ~9v) battery to a $15.00 step-down voltage converter I got from Amazon, and I set the output to 5.2v

The pi pulls the amperage it needs, and it's variable depending on the power use.

I consulted here for pi current draw specs

Raspberry Pi Model B+ 1.8A 600mA/1.2A (switchable) 330mA

I gave it 2A of available current to draw.

The rainbow box in the top right (the under-power symbol from the past) doesn't show up anymore, and it's properly powered.

The batteries are no longer shielded, so it's a possible concern of leakage over long time, and I'm aware of it and not using this for more than a hobby cam :)

Just two. Simple enough really. Someone posted this voltage converter a while ago, which will make adjusting the stir plate speed much, much simpler than figuring out a potentiometer/resistor combo to find the ideal speed (college circuits courses are failing me). Add in a fuse and an on/off switch, bolt it all together, and it should work out just fine.

Today I got a configurable regulator, wireless adapter, and wireless pigtails so I could untether my Galileo.

Now it's operating off a 2s 7.6v lipo and ready to be the new brain of my good old tracked robot. The antennas came from an old router.

you are going to be pushing it with this unit. Without no load, the 24V tranformer of an HVAC system can output almost 29V. It is designed to drop down to the 24V range under full load. You could see spikes of almost 39Vdc after rectification in some cases. That might be too much for this guy. I had the same dilemma as you and went with this unit that can accept up to 40V Dc instead. I have had 0 problems with these in my circuit.

http://www.amazon.com/gp/product/B00BYTEHQO/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1

This unit is nice because it will show you the input voltage and output voltage without having to break out the volt meter.

This is the one I have. Avoid if possible. http://www.amazon.com/gp/product/B00BYTEHQO/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

https://www.amazon.com/dp/B00BYTEHQO/ref=cm_sw_r_other_apa_IOzKxbCB2Y7KS

https://www.amazon.com/dp/B00C93Z8JY/ref=cm_sw_r_other_apa_KPzKxbWHW3VRC

I built my own setup. https://scontent-b-ord.xx.fbcdn.net/hphotos-xpf1/t1.0-9/10469048_802616879762394_7511436800829330098_n.jpg

I have since set it up so that I can use anything 4-40v and charges at 3 amps max.

buck power converter http://www.amazon.com/gp/product/B00BYTEHQO/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1
solar panels
http://www.amazon.com/gp/product/B00CBT8A14/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1
rechargeable batteries
5 of these
http://www.amazon.com/Exell-5000mAh-Rechargeable-Battery-Button/dp/B00JPPLJTM/ref=sr_1_1?s=electronics&ie=UTF8&qid=1405218205&sr=1-1&keywords=1.2v+rechargeable+d+batteries
http://www.ebay.com/itm/Battery-D-Cell-Linkable-Battery-Holder-D-Set-of-5-/181205540311?pt=LH_DefaultDomain_0&hash=item2a30b11dd7

This setup has allowed me to go all day. I'm sure there are areas where this could be improved and i'm open to anything someone smarter knows. I have walked and biked around all day with no issues. Normally I will see a 3% drop every 1hr of play. This also actively charges the battery pack. I'm still trying to up my amperage so if someone can tell me how to do that let me know