Yup, you heard right, true TTL on a (1959) Canon P Rangefinder. TTL stands for Through The Lens, which would be kinda impossible for that manual-focus, manual-exposure camera. Yet, Kevin Kadooka (who also made the beautiful LUX TLR) managed to build one using some 3D printed parts and a Trinket Pro 3V microcontroller.
Here is a brief description of the project:
The goal of this project is to add a TSL235R light-to-frequency converter to the exposure chamber of a Canon P rangefinder to produce a TTL-off-the-frame exposure meter for convenient and accurate exposure metering in a wide range of lighting conditions #
The project uses a light-to-frequency converter to measure the light bouncing of the center of the frame inside the camera.
This involves taking the camera apart, drilling a few holes for the wires, and fitting the exposure chamber with a light sensor, so this is definitely not for the faint of heart.
The back side however is quite elegant and is encased in a 3D printed shell that only displays old school 7 segment LEDs, which shows the exposure.
Here is a video showing the functionality of the unit:
If you are brave enough to try this yourself, there is a step by step tutorial over at hackaday.