Flat CRT display and pico-zxspectrum

I'm not yet sure what role the pages fulfill, let's see. This is like a blog post.

I bought a doorbell CRT monitor. It's a flat CRT that was frequently found in intercoms and doorbells in the noughties. 4-inch, black and white and it's flat -- similar to what was used in Sinclair's pocket TV and maybe some other ancient portable television sets.

I'm not at all original in this, they were featured by many popular youtubers before. However there was nothing in my household to test it with, so naturally I started looking around for some projects that I could try with one of the dev boards that I have. Found some ESP32 options first, but they were fairly old and didn't build right away so I put them aside. I also had some pipicos lying around.

So the first thing that I found and built is this amazing pico-composite-video:

https://github.com/alanpreed/pico-composite-video

There's a project and a tutorial and a whole Pong game. I have built the pong and we enjoyed a couple of days of ponging with the family. But you begin to feel the limitations of a pong sooner or later...

The next thing I've discovered is fruit-bat's pico-zxspectrum:

https://github.com/fruit-bat/pico-zxspectrum

It seemed amazing and very portable, something you can use with literally just a couple of wires plugged into anything. Just the thing that I needed. 

Turns out it could be plugged into literally anything, except for a composite monitor.

Well, not a problem! After some experimentation. And then more experimentation and then some.. I've got this. A ZX Spectrum with a tiny, slightly geometry-challenged B&W display, with a fresnel lens to compensate for its minuscule size:

Hopefully my changes will be the beginning of composite video support for pico-zxspectrum.

In order to make grayscale from RGBI that the Spectrum produces, I used a circuit from this page

https://zxbyte.ru/byte_connection_to_tv_and_monitors.htm

I didn't have the amazing KT315. Instead I had to use a mundane SS8050. Maybe because of this, or maybe because the input levels are different, the transistor tends to saturate and it produced smears after longer stretches of bright colours. I helped it by adjusting R11 to something much lower than 1K. It's a mess of wires so I can't really tell the current value, but it's significantly lower than 1K. I'm also not sure if this circuit is really safe to use with 5V power, but it works for me. Maybe it can be improved to be powered from 3V3.

For the reference, the CRT connection diagram found somwhere on Aliexpress:

I did not have a potentiometers handy so I picked some semi-random fixed resistors. The display is very forgiving and most everything within given values will work. But to get the full dynamic range for a given input signal it's best to find some real trimpots.