This is a remix of the original but this one uses the Sainsmart Hall-Effect board.

you can find that here:
https://www.sainsmart.com/sainsmart-hall-effect-sensor-switch-magnetic-detector-module-for-arduino-motor.html

cheep and good...

for the rod I use a rod from a CD/DVD player / Rom, they seem to be all the same size.
The magnets are 3/16" cube.

You may need to ream out the hole as well as add a spring to make sure it comes back out...

some setup details

1. make sure your magnets are orientated so they are in attraction.
2. Bend the sensor down so its closer to the magnet on the rod. you want the sensor to trigger when the magnet gets close it.

3. If its not triggering when it gets close but rather after it get past the sensor. Flip the magnets over. they need to be the correct orientation.

4. Use the potentiometer ( that small things with a slot in it on the board) to adjust the sensitivity. This will adjust the triggering of the sensor.
   You want it to trigger with out the going to far where the rod goes up.

you can then set up your printer using G commands so that when it is done probing it goes down past the sensor height and then the rod will stay up.

if you get all the rod the right height this will be out of the way when printing. then when your ready to use it again just pull it out.

please leave a comment if you need more help.

~Russ