This was a weekend project that I did so I can use a breadboard and get power to it. Some of my projects involve using 5V for a logic supply, while for others I may need a higher voltage. In this instructable I'll show how you can use a fixed voltage regulator as a adjustable one. The advantage of this is method is that your minimum output voltage isnt 2V, such as in the LM317T, but your minimum is what ever voltage regulator you used. For example, in this project I use a LM7805 as a adjustable Regulator, It gave an adjustable Voltage from (Vin-2) to 5V. I say Vin-2, since there is about a 2V drop across the regulator.
The Main reason for making this is because I wanted to make a portable breadboard with a Power supply in case I was bored at someones house, had a circuit with me, and wanted to play with stuff. This orignally stemmed from me salvaging a BUNCH of Mosfets/Transistors and not being able to test them when I was away for a weekend. Thus, this thing is powered from a wallwart or anything that has a 2.5 mm Plug
Step 1: The Design
I needed a PSU that gave both 5V and was adjustable in case I needed 12V. Granted, I could have used a LM317 AND an LM7805, but because Of board space, I couldn't use both. I could have used Just a LM317 Adjustable Voltage regulator, But I didnt feel like breaking out my volt meter if I needed Just 5V.
Then I looked around the web and found that a Fixed voltage regulator can be used as an adjustable version. The reason why this works is that it makes ground "float". By adjusting the pot, your adjusting the ground point seen by the regulator.
The switch shown in the schematic is optional. I removed it from the final design (but its still on the PCB). I later figured I can just remove the power Plug and use that as my switch.