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.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_mounted.jpg)
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_mounted_on.jpg)
Parts List
Component | Quantity | Source |
---|---|---|
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.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_layers.jpg)
Faceplate
The faceplate is laser cut from white acrylic, painted black, and then laser engraved.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_faceplate.jpg)
Spacer
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).
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_rear.jpg)
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.
Frame Mounts
The frame mounts are used to attach the assembly to the frame.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_side.jpg)
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.
The 3d printed brackets are used to attach the components together as shown below.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_assembly.jpg)
![front_bracket.stl](https://cdn.thingiverse.com/renders/67/5a/b3/0e/54/809f090a86a41fb306a07d19479f3988_preview_featured.jpg)
![side_bracket.stl](https://cdn.thingiverse.com/renders/8f/55/6f/31/b2/7cb270f7cf0b0fc12d74a7559b9151b3_preview_featured.jpg)
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.
![bezel.stl](https://cdn.thingiverse.com/renders/09/46/45/73/2a/6adf7d57c5af66891184265308b86544_preview_featured.jpg)
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_display.jpg)
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_display_bezel.jpg)
For the plastic cover, I cut a rectangle from a CD jewel case and applied a layer of darkened film to the underside.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_assembled.jpg)
Mode Switch
The mode switch switches the transponder from between the off, standby, test, on, and altitude-reporting modes using a 5 position switch.
![](https://res.cloudinary.com/allanglen/image/upload/f_auto,q_auto,w_2048,h_2048,c_limit/cessna172sim/images/bendix-king-kt76c/bendix_king_kt76c_mode_switch.jpg)
Buttons
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.
![display_bezel.stl](https://cdn.thingiverse.com/renders/6c/10/b4/38/b0/fd9e4b077ecdf45762ce9707d49869b7_preview_featured.jpg)