standard voltage-divider resistors, R3 and R4, from a
pile of 1K and 110 ohm resistors. Then, I soldered them
on the PCB, along with all the other components and
the LCD connector. R1 was only used during
development so it may be omitted.
Testing
Next, I plugged the unmounted LCD into the PCB
and did a thorough electrical check of every function,
using clip leads to simulate the switches. When I was
confident that everything was working, I mounted the
LCD in the case, soldered the PCB to the switches, and
connected the wires from the battery and sense
resistor.
Finally, I briefly shorted TP1 to test the low battery
5 Hz fast rate, then closed the case and put in the four
screws. Done at last. Eagerly, I plugged a wall wart into
the appropriate input jack and the output plug into a
Parallax BOE Board. It read 7. 66 volts at 88 milliamps.
Success!
Final Word
I’ve been using the Wall Wart Power Monitor for
several months now, and it works great. It’s been a fun
and useful project for me. I think you’ll find it useful
too.
For the Next Model
If the input and output connectors were changed to
the board-edge mounted type, then all the connectors
and the 0.100 ohm resistor could be mounted on the
PCB, making a cleaner design.
Here’s an outside-of-the-box idea. How about using
solar power from the room lights to supply the 1 mA
for the LCD? NV
Looking for solar stuff
and kits?
www.nutsvolts.com
February 2011 47
