A few years ago, I made a little light metering sensor using a light resistor and a NodeMCU (using analog read).
This would measure if a Led was turned on from my Alarm system. (I had no other way to read this state.
(I also had it connected to our washing machine for a while, to read the state.)

Now I wanted to read the state of the TV set-top box.
This Arcadyan hmb2260 has a green/led state LED, so measuring light is not enough.
(I can only toggle the device from Home Assistant, so there is no way to know the current state!)
Toggling on/off and portscanning the device gave me not another way to get the state (no extra open ports/web state pages) than using an RGB Sensor.

Flashing this with ESP Home and integrating it in HA was a breeze.

I wanted to turn the Illuminance LED also off, because the Set-top box already emits light.

Now I’m going to 3D print a holder which can be used to press the button manually when needed.

ESPHome Code

esphome:
  name: rgbsensor
  friendly_name: RgbSensor

esp8266:
  board: d1_mini

# Enable logging
logger:

# Enable Home Assistant API
api:
  encryption:
    key: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx="

ota:
  password: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

  # Enable fallback hotspot (captive portal) in case wifi connection fails
  ap:
    ssid: "Rgbsensor Fallback Hotspot"
    password: "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

captive_portal:

i2c:
  sda: 4
  scl: 5

sensor:
  - platform: tcs34725
    red_channel:
      name: "TCS34725 Red Channel"
    green_channel:
      name: "TCS34725 Green Channel"
    blue_channel:
      name: "TCS34725 Blue Channel"
    clear_channel:
      name: "TCS34725 Clear Channel"
    illuminance:
      name: "TCS34725 Illuminance"
    color_temperature:
      name: "TCS34725 Color Temperature"
    glass_attenuation_factor: 1.0
    address: 0x29
    update_interval: 5s

light:
  - platform: binary
    name: "RgbSensorVerlichting"
    output: light_output

output:
  - id: light_output
    platform: gpio
    pin: 2

Automation in HA:

If Red Channel > 60% then device is OFF.

Other ideas:

Measure the dishwasher machine LED.

Using color cards to (program) the wleds I’m using.

Using the Korg CA-20

On the side you see the bagpipe tuning.
In the past, I replaced one of the LEDs. But now it didn’t turn on any more.

So I took it apart, and what did I miss when replacing the LED?
This tuner has more functionality hidden.

This is more often the case with devices, because it keeps the costs down. Make one print and use in many.
(See the diskman story below)

When looking at the cover plate, you see only 4 buttons. But there are six!
Then I noticed some solder pads with the lettering Buzz.
So button 5 and 6 are for a metronome!
You see the little piezo buzzer I got from my component’s drawer.

A similar hack was the Diskman player in the 90s.

It was something like the model below.

There was only play/pause stop and skip whole track or back whole track.

When opening, I figured out which chips were doing what.
Saw that some signals were not connected to the control panel.
These were : Fast forward, and reverse. And skip 30 seconds.

So I glued some buttons on the side and presto.

A few years later, I took it apart and connected a DB-9 connector to it.

Now I could control the playing using my Amiga!

Some drawings I found

Will post chip info, and more when found.