LED Power Meter

For one of the kids playdates, my lovely wife requested I make a power meter (the playdate was electricity themed). Well, this was my first foray into electronics, so it took me a while to pick up the basics (maybe that should be a separate post), then I had to learn how to solder, buy some components (mainly from RadioShack and eBay) and finally design the circuit and put it all together... phew.

Well that was a great journey, which I heartily recommend!

I have to admit that I didn't come up with the design from scratch. I googled for ideas and settled on a simple chained diode approach. With this design you connect a set of diodes in series with LEDs forking off to from the anode of each. Each diode creates a voltage drop of ~1V (depending on your choice of diode), so, as you increase the input voltage, you light up more LEDs. Genius ;)

I put together a couple of prototypes on a breadboard (bough from eBay) to make sure I was getting it right and then soldered everything together on a prototype PCB board (another eBay purchase).

To make it more interesting for the kids, I decided to use 3 different colour LEDs: red for "DANGER - low power!"; orange for "medium power"; and green for "WOOT!".

The result? Voila!

The extra diodes at the base create a bridge-rectifier; I needed this as I was powering the circuit from a re-purposed hand-crank torch which I'd bought for $2 from savers.

The "who can light up the most LEDs" game turned out to be quite popular :) Having just two green LEDs at the high end made for quite a challenge; only the "big kids" managed to light them up. Below is a couple of pictures of the finished product in action:



Yes, that's a "Stand Back! I'm going to try Science!" t-shirt, from xkcd.