🛒⚙️ You can get the items you need here: https://kit.co/a_make_designs/ender-3

If you want to test your electronics skills with an inexpensive DIY bed leveling sensor, you have come to the right place!
The second part of the video with the explanation on how to prepare Marlin Firmware, set the slicer and configure the printer is here:    • Ender 3: DIY inexpensive Bed Level Se...  

The boooooring explanation:

The core of the system is an optocoupler that is a diode coupled with a light activated transistor.
When the diode is powered up, it will in turn open the transistor gate that will act as a switch.
The diode of the optocoupler has a minimum activation current of 5mA, 20mA is common. The power transmitted depends on the current through the diode, but the absorption from the digital input of the board is so small that any current should do the trick.
I decided to use a little less current to stay on the safe side since I don’t know the previous history of my opto that could be already worn out.
Since the 24V comes from the power supply of the printer, any resistance up to 4.8kOhm should work. I got 12mA from the 2k I’m using.
Please note that during the first few millimeters of print the sensor will still be on so there will be current dissipated through the resistor.

NOTE before starting: this modification requires fidgeting with the electronics of the control board. You can fry it, trust me, I did. It also requires opening the connection box of the power supply where deadly voltage is present. DISCONNECT your mains plug from the printer before starting.
If you are not comfortable, do your research, post a comment before working with this mod.
DO THIS AT YOUR OWN RISK I’m not going to accept any responsibility if you do something wrong and you damage your printer or hurt yourself.

BARE MINIMUM TOOLS AND MATERIALS:
- basic soldering skills, a soldering iron
- a bootloader-enabled printer. We need to make firmware changes to activate the bed leveling
- Arduino IDE software and a Marlin branch of the firmware
- any resistor between 1k and 4k Ohm
- one optocoupler (also known as photocoupler or optoisolator)
- one NPN normally open sensor

If you want to support my work with a small donation you can do it through PayPal here:
🍩 https://paypal.me/amakedesigns

🔗 LINKS:
🛒 My recommended items: https://kit.co/a_make_designs
💾 Thingiverse: https://www.thingiverse.com/A-make-de...
🐦Twitter:   / a_makedesigns  
The bracket on Thingiverse: https://www.thingiverse.com/thing:336...
The Breadboard I used: https://hobbyking.com/en_us/diy-pcb-b...
I used a 4 pin optocoupler FL817. There are plenty of sources. Mine was salvaged from a scrap board I had lying around.