Add SPDT Switch for CVBS / PPUdigitizer Luma Selection on Merlin Shaw

[azagramac]

Board used: Merlin Shaw USB-Powered-RCA-Classic v1.4
Purpose: Allow switching between the original CVBS video output and the PPUdigitizer Luma output using the same RCA connector.

Note: The board already has an unused footprint labeled SW1 on model Merlin Shaw USB-Powered-RCA-Classic v1.4, intended for an SPDT switch. This could be reused to implement the feature without additional modifications.

Connection Diagram

                   +-------------------+
                   |                   |
CVBS               |                   |
-------------------|                   | 
                   |    SW1 (SPDT)     |----> RCA Video (positive) 📺
PPU Luma           |                   |
-------------------|                   |
                   |                   |
                   +-------------------+

RCA Video (shield) ------------------------> GND (always connected)

Technical Notes:

• The RCA center pin connects to the middle pole of the SPDT switch (SW1).
• One side of the switch receives CVBS, the other Luma.
• The RCA shield/ground remains always connected to GND.
• This allows switching between the original video and the Luma signal without permanently modifying the PCB.
• Use a switch with low resistance and proper impedance (~75 Ω) to avoid degrading the video signal.

Benefits:

• Enables using either the original CVBS video output or the PPUdigitizer Luma output as needed.
• Avoids permanent cuts or modifications to the Merlin Shaw PCB.
• Reuses the existing SW1 footprint, making the upgrade cleaner and fully integrated.
• Compatible with TVs or RGB-to-HDMI converters.

Regards