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
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.
Second, identify the LED’s polarity.
Connect one LED lead to the positive post of
a 12-volt battery. Then connect the 1K
Circuit diagram from the
document available on
6 7 8
RHI REF OUT REF ADJ