You have a few choices if you want to use a probe to level your 3D printer bed. Rarely, you’ll see optical or capacitive probes. More commonly, though, your probe will sense a metal print or uses a physical probe to touch the print bed. [Design Prototype Test] has long used a BLTouch which uses the latter method. However, putting it in a heated build chamber prevented it from working so he set out to make his own simple design using an Allen key.
We’ve seen Allen key sensors before, but usually, they use a microswitch. We’ve also seen microswitches used to directly probe the bed. But, in this case, a 3D printed fan shroud uses an optical sensor to note when the Allen key hits the bed.
What we really liked was the retraction mechanism which is sort of semi-manual. A magnet holds the wrench up out of the way during normal operation. Before you home or otherwise probe the bed, you release the wrench from the magnet. When you are done, you can park it again. No need for a servo motor or anything else electrical.
It looks pretty reasonable with a range over 10 measurements of less than 0.05 mm. Not great, but not bad for $3. We are waiting for the STLs which are forthcoming. However, you can easily adapt the idea to your specific printer setup pretty easily. In addition, there are plans to improve the accuracy with a small redesign.