I was planning on mentioning these in my PolarGraph build thread but thought there might be someone with a similar application that could find this useful, so I am creating a separate topic. Not a good idea in an environment with sawdust, wood chips or metal filings flying around, but in a clean environment where it is difficult to mount physical limit switches, they fit the bill. I am using them for homing only. They are very inexpensive to make. All you need is a photodiode and a cheap laser pointer, and a 3d printer of course. I am using a 3DU5C diode and a cheapo laser led. They are a bit of a mismatch since the diode is designed to operate in the IR band and the laser pointer is red, but they seem to work well regardless. They were in my parts bin, so that’s why I am using them. Here is a pic of the laser pointers. Available on Amazon and eBay for less than $1 apiece but you have to buy a few.
Here are some pics:
Each laser led needs 3.3-5v, already has an internal resistor and draws about 20mA. I use the switched 5v out pins on the jackpot to turn them on for homing, then off again when homing is complete. Since the 5v out pins on the Jackpot are only rated for 25mA, you shouldn’t drive more than one from the same pin. It would be a better idea to pull the 5v from the 5v rail on the jackpot, but you would need an additional resistor and transistor to switch them on/off from a gpio. To start, I am using a small reflective strip on the “tool head” which is just a bit of white paper under some mylar tape. Concept is simple - when the beam gets reflected back to the photodiode, the diode pulls the limit switch to ground and turns it ON.
Connections:
- Switched 5v to led pointer (+)
- limit switch pin to photodiode collector
- GND to led pointer (-) and photodiode emitter
Limitations:
- Clean environment only
- Alignment can be tricky. I use mine inside a 300mm distance.
- Normally Open configuration, so only useful for homing