Bendix King KT76C

The Bendix King KT76C is a 4096 code mode C transponder with a design that is relatively simple to replicate.

I had originally intended to build a Garmin GTX 327 but sourcing an appropriate display proved challenging.

To simulate the KT76C I am using a MAX7219 8-digit 7-segment display that is easy to source and simple to integrate with Arduino.

Parts List

Faceplate, hardware mount, mounting brackets (Ebay) 3/16” bright white cast acrylic
Display 1 (Ebay) MAX7219 7-segment display
Mode switch 1 (Amazon) Panel Mount 2P5T Rotary Switch
Mode knob 2 (Ebay) 10pcs 6mm Shaft Hole Dia Plastic Threaded knurled Potentiometer Knobs Caps
Push buttons

These are a soft-feel push button with about 2mm of travel.

11 (Amazon) 7mmx7mm Momentary DPDT Mini Push Button Switch
Pan head screws

Used for attaching the hardware mount to the rear of the faceplate.

4 (Amazon) #6-32 x 1/2” Phillips Pan Head Screws
Flat head screws

For assembling the acrylic parts (holes should be countersunk before assembly).

6 #4 x 1/2” Flat Head Wood Screws

Note The Amazon and Ebay links above are affiliate links and help me pretend that my hobbies are self-sustaining.

Main Assembly

The front of the unit is assembled by sandwiching three layers of acrylic together: the faceplate, the spacer (also used for mounting the 7-segment display), and the hardware mount (for mounting the buttons).

The following image was taken after the three layers were assembled and painted.


The faceplate is laser cut from white acrylic, painted black, and then laser engraved.


File: faceplate.svg


The spacer provides a layer for mounting the 7-segment display. The thickness of the acrylic provided the perfect depth for mounting a thin layer of darkened plastic in front of the display (see Bezel below).


File: spacer.svg

Hardware Mount

The hardware mount provides a layer for mounting the push buttons. I used acrylic for mounting the push buttons to simplify construction and to avoid having to create a circuit board. The push buttons are glued into the hardware mount.


File: hardware_mount.svg

Frame Mounts

The frame mounts are used to attach the assembly to the frame.


File: left_frame_mount.svg


File: right_frame_mount.svg

Electronics Mount and Mounting Brackets

The electronics mount is used for attaching electronics and attaches the side mounts to the front assembly. The holes and slot are for attaching a multiplexer which I plan to convert to in the future to simplify wiring.


File: electronics_mount.svg

The 3d printed brackets are used to attach the components together as shown below.

File: front_bracket.stl (Thingiverse)
File: side_bracket.stl (Thingiverse)

Display Bezel

A 3d printed display bezel is inserted into the faceplate to hold a layer of darkened plastic in front of the display. The plastic layer hides the internals and provide the appearance of a larger display.

File: bezel.stl (Thingiverse)

For the plastic cover, I cut a rectangle from a CD jewel case and applied a layer of darkened film to the underside.

Mode Switch

The mode switch switches the transponder from between the off, standby, test, on, and altitude-reporting modes using a 5 position switch.


The buttons are 3d-printed from white PLA, painted, and laser-engraved.

All of the buttons are the same size and the svg file contains a layer for each of the labels.


File: button.svg
File: display_bezel.stl (Thingiverse)