EAA Sport Aviation - November 2010

resistor between the other LED lead and the negative battery post. Don’t forget the resistor, or the LED will be ruined. If the LED doesn’t illuminate, reverse the leads. When you get it right, mark the positive side. Double-check the LED polarity and IC pin orientation before you solder and you shouldn’t have any trouble.

A couple notes: The datasheet shows that the LM3914N integrated circuit will drive up to 10 LEDs, so you could add LEDs to pins 18, 17, 15, 14, 12, and 11. I chose to use only four (for 10-40 degrees of flap). Also, the resistor on pin 7 controls the LED brightness. A 1K-ohm resistor gave me an acceptable brightness, but you can experiment with others. While you could replace the fixed 1K-ohm resistor with a variable resistor to dim the LEDs, it seemed like overkill to me. They aren’t (or shouldn’t be) on for more than a few minutes each flight. I found a nice four-LED assembly at my surplus electronics store—but any LEDs seem to work.

The PC board (see parts list) that I chose
gave me a convenient set of connections,
so I didn’t have to solder in a lot of extra
wires. I laid out the parts on the board and
trimmed it to size with my jigsaw. By
soldering in a retention socket for the IC,
I could just plug in the IC rather than risk
overheating it by soldering it directly onto
the board. Remember to protect the
soldered side of the completed board
from shorting out when you mount it in
your panel.

PARTS:

For links to suppliers for some of these parts and a PDF of the datasheet, visit www.SportAviation.org.

● One integrated circuit LM3914N Dot/Bar Driver

● Datasheet

● Four (up to 10) LEDs

● One 18-pin retention contact (Radio Shack 276-1992)

● One PC board (Radio Shack 276-168)

● One 1K-ohm resistor

● One 10K-ohm linear slide potentiometer, 60 millimeters of travel

● Two Dubro heavy-duty 4-40 ball links (Dubro 497)

● One model aircraft 4-40 rod

● Connectors (your choice)

Jerry Riffel, EAA 703052, is a retired IBM software engineer/manager. He’s a private pilot with an instrument rating. His RV-7A (night/instrument flight rules) took him four years to build, and he flies it weekly.