By MIKE BENDER

EAA 238526 45550 Pickford Ave. Lancaster, CA 93534

The purpose of this device is to alert the pilot should the alternator or regulator (or pilot) malfunction. Two Zener diodes fix reference voltages that define the limits of the normal voltage range of the electrical system. These reference voltages and main bus voltage are input to an integrated circuit dual operational amplifier. Any time the op-amp determines that main bus voltage is outside the normal voltage range, a warning light will illuminate. For my RV-4, equipped with a 55-amp alternator regulated at 14.6 volts to keep the Gel Cell battery happy, I established the normal range

as 13 to 15.5 volts. If the engine is not running, the light will illuminate when the master switch is ON - a gentle reminder. If the engine is running, the warning light will illuminate under the folowing conditions: 1. Bus voltage exceeds 15.5 volts. This could occur if there were a fault in the voltage regulator. It might occur during charging right after a battery grinding, hard start but that would be a temporary condition and the light should soon extinguish. 2. Bus voltage less than 13 volts. This would occur if alternator output dropped as a result of a malfunction, or it was inadvertently turned OFF. It could also occur if electrical demand exceeded the capacity of the alternator. That's another temporary condition; otherwise, you need a big-

+V from main bus 22 AWG

470 a

1/4 watt

92 MARCH 1994

1000 ii

1/4 watt —

ger alternator - or, as Tony Bingelis would say, "You need a bigger alternator, amigo." Anyone with the ability to strip wire and solder can build this device. All of the components were obtained from Radio Shack, including a printed circuit board (Cat. No. 276-159A) and a small plastic box (Cat. No. 270-230). The cost of one device was approximately $13.50. A second device can be built for only $5 additional because most of the components come in quantities greater than needed for just one. A note about using the LED instead of an incandescent lamp - 1 recommend you place it as near to directly in front of you as possible. Viewing even this bright LED from other than straight-on, especially in sunlight, can significantly reduce its apparent brightness. Lastly, if you are (like me) a baby boomer or older, you may be somewhat intimidated by words such as integrated circuit, op-amp, Zener diode, and CMOS 12volt phase-shift oscillalamp tor. Radio Shack has a book titled "Getting Started in Electronics" and a 2n2222 series of booklets NPN called "Engineer's transistor Mini-Notebooks" that are excellent primers on electronics - in other words, thin, cheap and well-written. Not an electron nerd myself, I developed this device from example circuits in the Basic LED instead of 12 volt lamp Semiconductor and Op-Amp notebooks. Didn't even to +V AAA2n2222 have to learn what Bright LED 470 £1 a CMOS phase1/4 watt Cat. No. 276-088 shift oscillator is.

By MELVIN L. YANDA EAA 374286 100 E. Kingswood Dr. Enterprise, AL 36330

While building my Lancair 320 I noticed that after removing the fiberglass engine cowling several times the screw holes were rapidly becoming oversize. To eliminate this problem and avoid using exposed washers I used the following solution. Step 1 - Remove the gelcoat from the cowling in the area around the screw holes. Step 2 - Countersink the fiberglass cowling to accept the bevel of a tinnerman washer. Step 3 - Bevel the top of all of your tinnerman washers. Do this by m o u n t i n g the washer on a screw then turning it with a drill and simultaneously placing the top against a grinder. By grinding a bevel you will have a small lip that remains

under the fiberglass when your work is sanded smooth. Your washers should look similar to the washer in Figure 1. Step 4 - Bond each washer in place with epoxy adhesive. Step 5 - Apply 2 layers of fiberglass cloth and resin over all washers. L Step 6 - Sand fiberglass cloth until the top of each washer is just exposed. A cross section of your completed work should look like the drawing in Figure 2. Using standard fiberglass f i n i s h i n g techniques you should be able to install these washers and have them remain completely invisible. This technique

should work on most any fiberglass component that needs additional reinforcement around the screw holes.

A Before

After Tinnerman Washer

Side View Fig. 1

Fig. 2 2 Layers of Tinnerman Washer

Fiberglass

Gelcoat

Fiberglass

Cowling

Readers are invited to submit entries to EAA's Hints For Homebuilders, Att: Golda Cox, P.O. Box 3086, Oshkosh, Wl 549033086. Entries will be reviewed by a panel of EAA judges. Readers whose hints are published in any EAA magazine will be awarded a 3/8" Drive Socket Wrench Set from Snapon Tools. Aircraft Spruce & Specialty will award a $25 gift certificate plus a current catalog, and American Saw & Mfg. Co. will award a Lenox 4012 Hacksaw Frame. Members are also invited to submit hints of an electrical nature. Any hint used will receive a Fluke Model 23-2 Multimeter with Holster from the John Fluke Mfg. Co., Inc. The contest will run from August through July of each year with a Grand Prize being presented by Snap-on Tools (KR657 Roll Cab and KR637 Top Chest), Aircraft Spruce & Specialty ($250 gift certificate), and American Saw & Mfg. Co. (Lenox VBKMA-6 Vari-Bit Kit). A Grand Prize will also be awarded by the John Fluke Mfg. Co. These awards will be presented during the EAA Convention. Our thanks go to our sponsors for these awards. SPORT AVIATION 93