Re-using MacBook Screen I have a MacBook Pro 15" around 2009, that isn't working. Is it possible to take its screen and turn it into an external monitor? Maybe buy some controller and do a little soldering?
You will need an LCD Controller board kit that works with your LCD.
Find the model number of the LCD, make sure the controller will work with it.

You're basically looking for an LVDS controller.

The Mid-2009 15" MacBook Pro utilizes a LTN154MT07-G01 made by Samsung, it has a standard 40-pin LVDS connector.

https://www.ebay.com/itm/HDMI-DVI-VGA-LCD-Controller-Driver-Lvds-Board-Monitor-Diy-Kit-for-LTN141W1-L01-/123128016280?oid=280634929240#ht_6675wt_1141

Or

https://www.ebay.com/itm/RTD2281-LCD-Controller-Board-Kit-For-DIY-Apple-A1286-LED-Screen-LG-LP154WP4-TLA1-/152696810140

If you need to find what model LCD is in a different computer, you can easily find out using eBay to search for the LCD for a specific model, find that LCD's model number, then search to find out the connector type.

... coincidently (or not) Hackaday just posted this article:

AN HDMI INPUT FOR A LAPTOP SCREEN, MINUS LAPTOP