Build your own transponder for simulation cockpit cessna 172.
Format for 6.25" stack
Electronics are design with EasyEDA.
3D parts are design with SolidWorks.
PCB are sold by JLCPCB, you can use directly Gerber files --> JLCPCB
Only electronics, you can choose the microcontroller of your choice, the base design have addtionnal card for Mega2560R3. For mine i use Mega2560R3 and Mobiflight on MSFS2020--> MOBIFLIGHT
For others controllers you need to design the support for the card, or add wires to link them.

Capacitor C3 to C18 are optionnals, they just do debounce for switch, but work fine wihtout on mobiflight.

Parts list:

MAX7219 x1 --> Aliexpress
Selector 2P6T x1 --> Aliexpress
Screen 7 Seg 4 digits x2 --> Aliexpress
Switch 8x8m x11 --> Aliexpress
LED red 3mm x2 --> Aliexpress
Capacitor 10nF x16--> Aliexpress Aliexpress
High Frequency Low ESR Aluminum Capacitor 22µF 50V x1 --> Aliexpress
Capacitor 0.15µF x1 --> Aliexpress
Resistor 1/4W 10kΩ x1
Resistor 1/4W 10kΩ x2
Header male angle 2x15pin x1 --> Aliexpress
Header female 2x15pin x1
Header female 1x6pin x4 (for 7 segment) --> aliexpress