Nuts and Volts - June 2014

details of how to install the necessary jumpers. Figure 5 is a photo of my completed UPS board; you may want to refer to it as you construct this project. (Before we continue, I need to mention something about the photos in this article. My digital camera recently “died,” and I have been too busy to replace it yet, so I used a webcam for all the photos. As a result, they aren’t as clear as I would like — sorry about that!)

Testing the Stripboard Circuit

After you’ve built the UPS, it would be a good idea to check and double-check your completed stripboard circuit for accidental shorts and other wiring problems before applying power. When you’re ready to test and apply power, you will need a way to support it in an inverted position so that you can use a multimeter to check for the correct voltage (+5V) at the USB connector. I used a small piece of

1.5” square scrap wood for this purpose (see Figure 6), but you can use any arrangement that holds the completed stripboard in a stable inverted position.

To do the testing, connect the main power supply and the battery pack to the board, and carefully invert it so that it sits reliably, and is in the same orientation that’s shown in the layout of Figure 3. Locate points J13 (+5V) and M13 (ground), and carefully (because the board is powered) determine that a voltage very close to +5V is being supplied to those two points. If it is, remove the main supply connection and test those two points again. The same voltage should be seen because the battery pack is now supplying the circuit. If your board does not pass these two tests, you need to do some troubleshooting.

When you’re sure the board is functioning correctly, we’re ready to move on to final testing of the UPS when it’s actually powering the Pi. However, before doing that, you may want to secure the battery holders, batteries, and the UPS stripboard on some sort of mounting board, as well. I used a piece of 1/2” plywood for this purpose (see Figure 7).

In the photo, the odd-looking eighth “battery” is actually a wooden dowel with a bolt running through its center. You can just solder a wire in its place, if you prefer.