For this part of the project, you can decide in which direction you go. In the end we want to install a Raspberry Pi in an old Leitz Prado Retro LED-Beamer. The best way to do so is to mount it inside the cage below. The Raspberry Pi fits rather well.

We need to do/consider the following:

• print a case that can be mounted with screws and penny washers (I used this one: https://www.thingiverse.com/thing:922740/files)
• build/buy a 4-pin-audio jack to FBAS/RCA adapter (special pin layout explained here: https://www.raspberrypi-spy.co.uk/2014/07/raspberry-pi-model-b-3-5mm-audiovideo-jack/)
• buying is difficult as they are not many adapters that fit (special pin layout) and most are too long
•  if you want to build one, you have to deal with very precise soldering
• decide how to get audio out
• you can use the analog audio output of the Rasperry Pi - then you need to do even more precise soldering as your adapter 
• you can use some Raspberry Pi hats for sound (I don't know if they interfere with the pins needed for temperature monitoring)
  • you can use Bluetooth audio (didn't get that to work)
  • or you can use an USB audio device (used this method with a Bose sound station which turned out to be a good decision)
• check your Raspi skills
• if you are good at running Raspberry Pis headless, you can put everything together from the beginning
• if you are not very experienced with running Raspis headless, you should configure everything on the flly with a keyboard attached and a larger display. You won't be able to work with the small 2.0" display at 320x240 resolution.

Download Raspbian Lite (Buster) and write it to a decent SD card (with Etcher, for example).

Then go to the config directory (in Windows it is shown as an accessible drive) and create a file named wpa_supplicant.conf. Edit this file and add your WiFi credentials to connect to your WiFi.

country=US # Your 2-digit country code
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
network={
    ssid="YOUR_NETWORK_NAME"
    psk="YOUR_PASSWORD"
    key_mgmt=WPA-PSK
}

 Add a file names ssh in the directory to enable ssh access. 

If you came here from the Retro LED-Beamer Project, you want to do this step first. It's about mounting the DS18B20 to the heat spreader/LED unit to measure the LED temperature.

Don't buy the transistor alone but rather buy the complete probe consisting of a metal probe and a short cable. You will need a 4,7k-resistor, too. Follow this instructions to solder the resistor to a the correct cables and connect the pins to the Raspberry Pi: https://raspberrypi.stackexchange.com/a/48360

Now get a 6mm drill and drill a hole just through the heat spreader. The hole must be at least as deep as the probe's length.

Put some thermal grease onto the probe and slide it into the heat spreader as deep as you can. You might need to slice the heat shrink tubing a little.

It is possible to install everything headless, so you can just plug the SD card into your Raspberry Pi and plug in the power. The Raspberry should boot up and after some seconds you should be able to ssh into it with the hostname raspberrypi. Username is pi, password is raspberry.

Type

sudo raspi-config

and configure it to your needs. You should configure

• Change User Password
• Localisation Options/Change Locale
• Localisation Options/Change Wi-fi Country
• Localisation Options/Change Keyboard Layout
• Interfacing Options/Camera (Enable)
• Interfacing Options/VNC (Enable)
• Interfacing Options/I2C (Enable if you want to use a display)
• Interfacing Options/1-Wire (Enable if you want to use a temperature sensor) 
• Advanced Option/Expand Filesystem

The system will ask for a restart, do it.

Now do a 

 sudo apt-get update && sudo apt-get upgrade 

to upgrade your system.

Do a

sudo apt-get install xscreensaver

 to install the screensaver (which we will set to disabled later).

Edit your autostart file /etc/xdg/lxsession/LXDE-pi/autostart and the following lines

@xset s off
@xset -dpms
@xset s noblank

This should disable all Power Management settings.

Download RealVNC Viewer to connect to your Raspberry Pi. Use the username and password of the Pi to connect to the desktop. Set everything to your needs and don't forget to got to Settings/Screensaver and turn it off.

There are different ways to monitor your Beamer's temperature. You can use a text file for temporary storage, MQTT if you have a smart home or InfluxDB and Grafana to display it remotely. I decided for InfluxDB and Grafana.

Install InfluxDB via

sudo apt-get install influxdb influxdb-client

Connect to your database

influx

and create a database temperature

CREATE DATABASE temperature

Press Ctrl-C to exit the session.

Warning! The database is unprotected. That's fine if you don't put sensitve data in that database and no one can access it from the outside world. Refrain from opening the database port to the internet until you have a proper authentication enabled.

Follow these steps to connect a DS18B20 temperature sensor to your Pi. No magic is involved. http://tuxgraphics.org/npa/raspberry-pi-ds18s20-temperature-sensor/

At first, get your DS18B20 address

ls /sys/bus/w1/devices

The sensor id begins with a 28-, write it down or copy it, you'll need it. If you don't see a device, check your cabling.

 Now create a folder ~/opt and put the following files into it: getTemperature.sh, logIt.sh and logger_simple.sh.

logIt.sh needs to be edited if you chose a different database name, but tat's optional.

You need to edit logger_simple.sh and put the sensor id into the mySensors variable. 

Credits for the bash files go to Magnus Lübeck, https://gist.github.com/maglub/34de79be0a79b9d88cea942766c293d0, I derived my files from him.

Now do a 

sudo crontab -e

sudo crontab -e

 and add

* * * * * /bin/bash -c /home/pi/opt/logger_simple.sh

 Save the file, your job should now run every minute and collect temperature data.

Run

bash logger_simple.sh

 and you should get a temperature and no error messages. This is the temperature probe's temperature, we will work with it from now on.

Grafana is used to have a temperature history on the device - it's optional, but a good visualisation of what you are doing.

To install Grafana, you must not use the version in Rasbpian (it does not work) but install a new package.

wget -q -O - https://packages.grafana.com/gpg.key | sudo apt-key add

echo "deb https://packages.grafana.com/oss/deb stable main" | sudo tee /etc/apt/sources.list.d/grafana.list

sudo apt-get update && sudo apt-get install -y grafana

 will do the trick (make sure you don't break the line while copying).

The source is here: https://simonhearne.com/2020/pi-influx-grafana/#step-3-install-grafana

sudo /bin/systemctl enable grafana-server
sudo /bin/systemctl start grafana-server

 will enable Grafana and start the server.

Warning: we a

Open a web browser, go to http://[your Pi's adress]:3000 and login with admin/admin.

• change your password
• select IinfluxDB
• create a Dashboard - the only thing you have to select is the FROM in the SQL statement. Set it to sensor_data. You can ignore the WHERE clause if there is only one sensor.
• Set the Min time interval to 1m (lower left)

• Configure the Dashboard

You will now be able to review the temperature of the LED and the history. Some words about the sensor temp. The DS18B20 measures the temperature of the heat spreader. This temperature is lower than the temperature we are interested in, the LED temperature.

This temperature is significantly higher than the temperature of the heat spreader as the LED is mounted on a COB and glued to the heat spreader. All these components have a thermal resistance which is calculated as Kelvin per Watt (K/W). This thermal resistance depends on the material. This thread https://www.ledhilfe.de/viewtopic.php?t=23448 (German, see last comment) gives some values. It ends with the conclusion that you have to add like 53K to the measured temperature. If your sensor reads 50°C, the LED temperature is 103°C. That's hot, but still ok. The temperature of the LED should not go over 120°C.

To check if your cooling system is suffiscent, open the Grafana Dashboard and wait for at least an hour to see if the temperature stops increasing.
 
If the temperature does not stop increasing, you will damage something, either the LED or the soldering. Soldering connections melt at 150°C LED temperature (means 97°C sensor temp).

If you want to display the temperature data to an LCD display at the beamer, you can use a 128x64 wide SD1306 display I2C for this. You can print a holder for the LCD screen.

Follow these instructions on how to attach the display to the Raspberry Pi. You need to use 3,3V pin for power (if your display works with 3,3V) and use another ground as the one in the display is taken by the DS18B20: https://indibit.de/raspberry-pi-oled-display-128x64-mit-python-ansteuern-i2c/#Bibliothek_einbinden

In the end I connected the display to 3,3V and the temperature sensor to 5V. Be careful, I fried one of my Raspberry Pi's i2c-Ports.

To check if you did everything correct, type

i2cdetect -y 1

 The output should look like this:

pi@raspicast:~ $ i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- 3c -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

 If not, check if you mixed up SDA and SCl.

Now get the lib_oled96-drivers from Github.

cd ~/opt
git clone https://github.com/BLavery/lib_oled96
mv lib_oled96 display
cd display

and copy the file displayTemp.py to the directory /home/pi/opt/display 

I'm not a good python coder (beginner level) and got the idea from here: https://indibit.de/raspberry-pi-oled-display-128x64-mit-python-ansteuern-i2c/

No need to to edit this file.

Create a file displayTemp.sh in the same folder. This file needs to be edited: insert your sensor id into the WHERE clause

WHERE \"sensor\"='sensor-id'  

 Do a

sudo crontab -e

  to edit your crontab and add the script to run every minute. 

* * * * * /bin/bash -c /home/pi/opt/display/displayTemp.sh

 Now your display should update regularly and show the current temperature and a diagram with the temperature up to 25 minutes in the past.

 Warning: if you set this up for the first time, there are not enough data rows to let the script run successfully. You need to wait for 25 minutes until it works.

You should then be able to see a histogram and the current LED temperature on the screen:

If you have been working with HDMI and the Raspberry Pi not connected to the RCA/FBAS display, you should now put the Raspberry Pi inside the Retro LED-Beamer and connect the RCA/FBAS output to the display.

Start everything up, you should see a small vision of LightDM on your wall, albeit having a lot of overscan. To adjust the screen to the Beamer, use this small software:

https://github.com/ukscone/set_overscan

Install it with

cd ~/opt
git clone https://github.com/ukscone/set_overscan
cd set_overscan/
make
sudo bash set-overscan

You might need to reboot as overscan is most propably disabled in the config.txt. Run the script again and  follow the instructions. I ended up with something around TOP=43, LEFT=87, BOTTOM=67, RIGHT=90

Do a reboot again, you should not see any parts of the screen being clipped.

Get a RealVNC viewer and connect to your Rasperry Pi's hostname/IP. Password is your user/password combination from Pi. Don't try other VNC programs, they won't work.

Edit the following (no screenshots)

o    go to the start menu, go to settings, choose screen saver and set the mode of the screensaver to "deactivate screensaver"

o    right click on Desktop, goto settings and disable the Wastebasket and Mounted Disks, so your desktop looks clean

o    right click on the task bar, go to settings and go the Enhanced (?) settings tab - click the minimize the task bar checkbox

If you want to use a Bluetooth speaker (not recommended), VNC can be used to enable the connection or change the background. Congratulations, you have your new Beamers background image set.