EAA Sport Aviation - November 2010

HANDS ON

BUILD-IT-YOURSELF

Flap Position Indicator

Simple, inexpensive addition

BY J.L. RIFFEL

I LIKE SIMPLE. THE flap indicator on my RV-7A was four pieces of tape on the flap showing how many degrees are dialed in. There was nothing to break, and it was almost foolproof. I pushed the flap toggle switch down for a couple seconds, looked out the window, and verified the tape marks—done.

But a recent night flight got me thinking: Can I see that tape if it’s a really dark night? If I’m slogging through a night instrument approach, will I be distracted by fumbling around for a flashlight during the landing sequence? What if I forget to raise the flaps? (Not that I’d ever forget to raise the flaps, mind you.) Clearly my simple flap indicator needed some rethinking.

There are sensors and indicators on the market that do exactly what I want (and more), but they’re sort of pricey for me. Because I like frugal solutions, I decided to see what I could engineer.

After discarding a number of approaches and spending a number of days searching the Internet, I finally discovered a simple electronic circuit that drives an LED bar graph based on voltage levels. I am not an electronics guy—far from it—but when you build an airplane, you learn about all sorts of topics. On my trips to a local surplus electronics store while building my panel, I learned about potentiometers. They can be used as variable resistors (for dimming LEDs) and also as voltage dividers.

Eureka! I could use a slide potentiometer to sense flap position and the LED bar graph circuit as a display. It’s still pretty simple, only has a couple inexpensive components, and is easy to build.

Figure 1 shows the circuit diagram, and Figure 2 shows my assembled circuit board. The only places to go wrong (assuming you are reasonably careful when you solder) are connecting to the wrong integrated circuit (IC) pins and getting the LED’s polarity reversed.

First, I’d recommend you scan through the connection diagrams in the datasheet (visit www.SportAviation.org for a PDF) to see how the IC pins are identified. As you can see in Figure 2, pin 1 is located on the lower right when the “notch” at the end of the IC is pointed down.