TITLE:  Re-Volting

ISSUE:  October '99

By Jim Weir
Humble Tinker, RST


GENESIS

In the beginning was the battery, and without keeping a decent charge on the battery begat the problems of battery life.  For the last two months, we've been attempting to figure out a way of keeping that beast charged during extended periods of non-use.  This month we'll take a stab at a simple and cheap way of measuring the charge on the battery, and at the same time, a simple way of doing elementary troubleshooting through the airplane.


SOME PRELIMINARY THOUGHTS

Charge on the battery can really only be measured two ways, by measuring the specific gravity of the battery acid or the voltage under a specified heavy load.  This simple little voltmeter we are going to make isn't going to do either of these two, but it will give us the voltage of the battery by a relatively inexpensive readout mechanism.  While we could use a standard voltmeter (digital or analog) to make these measurements, trying to see a meter scale while you are hanging upside down under the instrument panel isn't all that easy.  Not only that, but for a lot of our measurements, all we care is that we found the correct battery bus terminal on the barrier strip.  It would be nice if we could have both a visual and an aural indication of battery voltage -- if the eyes are busy looking for terminals, the ear can tell when we hit the right one.


LET'S BUILD IT

For the optical readout, the least expensive option in today's market are light-emitting diodes (LED).  From your Mouser catalog, you can find some fairly inexpensive LEDS that sell in small quantities for about 15 cents each.  An even less expensive way to go is down at "Surplus Sam's" in most major cities, where the average price of a surplus LED is about a nickel.  Since we are using ten of them in this design, the more we can save on the LEDs the cheaper the resulting project.

The audible readout is nothing more than a "Shack" buzzer with a switch to turn it off when you don't want the sound indication.

The real star of this gadget is an IC that lights up one LED for each half a volt increment from 10 to 15 volts.  This IC (LM3914) was very common about twenty years ago, but is getting a little hard to come by these days.  "The Shack" doesn't have them, and Mouser only has them as an "NTE" replacement part (NTE-1508) for about $9.  For those of you who have access to Digi-Key or any other National Semiconductor distributor, they are still in the pipeline at about $4.  At any rate, I used ten LEDs to visually indicate the output of this IC, and I made them red for the bottom 3 voltages, yellow for the top 3, and green for the 4 "normal"  or mid-range indicators.  Even hanging upside down under the panel, you can easily detect what color of LED is lit out of the corner of your eye and get a very good indication that it is indeed the battery terminal you are on.


USING THE THING

In my airplane, this little rascal is plumbed in between the charger and the little auxiliary battery connector we installed in the July issue.  It gives me a continuous readout of my battery's state of charge every time I open the hangar door.  Of course, I turn the buzzer switch off so that the fool thing isn't buzzing all the time, and I've made a patch cord to make the box into a test voltmeter when I'm working on the airplane.  All in all, for less than $10, we've made a pretty nifty little gadget.


COMING DOWN THE ROAD

In the next two issues, we'll tackle a couple of problems that have popped up on the newsgroups in the last month or so.  The first design we'll consider is a landing light flasher.  Second, we'll see if we can gin up a quick and dirty magneto timer buzz-box.  When we're all done with those two, let's see if we can patch together a headset and microphone tester, and from there...who knows?  Most of my ideas come from problems that folks have discussed on the Internet aviation newsgroups (rec.aviation.homebuilt is my home turf) so if you have an idea or something you want designed, come on up on the newsgroup and let's talk about it.

Jim


Author's Note:  Jim Weir is the chief avioniker at RST Engineering.  He can be reached by email at jim@rst-engr.com  but prefers to answer questions in the newsgroup rec.aviation.homebuilt.

