MagicTK made this mini adjustable voltage regulator, instructables here:
The minimum output is based on the voltage regulator’s internal reference voltage (look at the datasheet formula), and the maximum output is based on the batteries connected and how much energy they have left (as well as the maximum specified in the datasheet, in case you connected much higher voltage).
Okay for short periods of use, e.g. for bench testing work. Select a 3A regulator for two 9V batteries in series? How much juice did the author thought he can supply from that… heh heh, ever tried loading a 9V battery heavily? Also 9V battery have lowish capacity, better to stick to a power brick whenever possible — it will be expensive to upkeep this thing with two 9V in series. Most things that need >5V that I can think of involve motors etc and would not be a good match with this thing. AA batteries are a better match for most MCU and sensor circuits. I don’t think most hobbyists need this thingy.
Before I forget, a warning to anyone who wants to adapt this project: When using a high-current input supply, e.g. from a power brick, a bare TO-220 (as shown in the article) will hit its thermal limits very quickly for high Vin, even if the regulator is rated for 3A. Say a 10V drop at 3A, that’s 30W!
For high voltage drops, even a 1A TO-220 7805 will need a heat sink. In such situations, some thermal performance calculations is useful, one must at least get a feel of the power or heat dissipation involved. Remember folks, a 3A rated chip doesn’t mean you can use the bare package at that rating!
Never trust instructables completely!
For example, LM350 TO-220 has junction to ambient of 50C/W typ, its max rating is 125C, so a bare package can only dissipate about 2W in a 25C ambient. Say we are using 18V input, and drop 10V to get 8V output, then the 3A-rated LM350 can only supply around 0.2A, even then, nobody sane runs their chip at 125C…