February 1, 2015



I’ve been meaning to build a benchtop power supply for some time. A while back I came across an old calculator that had 3 rechargeable (dead, leaky) AAs soldered in. That finally kicked me into gear, and I set about building something.

Requirements included 3.3 and 5 volt outputs, as well as a variable output. I wanted some kind of flash, so a microcontroller driving an LCD displaying the voltage of each rail had to go in too. I also wanted it to fit underneath my plastic parts/tools bins, and have a kinda retro look to it. Finally, I wanted to build it using as many parts on hand as possible.

When it was all said and done I ended up buying a PCB, 2200uF capacitor, 2 2k pots, 2 pair of binding posts, and a 2.1mm barrel jack and plug. All were acquired at the Radio Shack 50% off store closing sale (We actually bought a lot of stuff at half off, SWMBO approved!) Everything else was already in the stash.

A 15V former laptop power brick supplies the juice. From there it goes into a 5V, 3.5A switching regulator and an LM350 variable linear reg. The 3.3V 0.5A linear reg is powered by the 5V rail. An old ATMega168 (yes, with the arduino bootloader) drives the LCD and retrieves each rail’s voltage from a MCP3208 SPI 12 bit A/D converter.

The name, YRTL-T1, is because it’s the first tool built under the YRTLBots name. I’ll draw up a proper schematic and post it sometime later. A full writeup and photos will be posted at yrtlbots.com, and I’ll end up putting the code up (probably on github) as well.

Oh, and the old calculator lit up its old VFD and worked great. I’ll put up a pic soon.