The intial design tried to use a limit switch to sense when we had acquired a power cube. We need to sense because the arm gets a lot heavier when we have a cube, and because we don't want the wheels to keep spinning rapidly against the cube.

The intial design tried to use a limit switch to sense when we had acquired a power cube, but mounting and actuation was unreliable. Then we found these:

https://www.adafruit.com/product/2168

we found them in stock at Mouser.

what's great about these sensors:

• they just work
• obvious color code to the wires: just hook them up to a DIO.
• pretty cheap. The 3mm ones are really cheap, but a little flaky at the distance we need for the cube intake.

Not perfect:

• Out of stock at lots of sources
• sensitive to stray IR light which could cause false operation

Since the sensor is facing towards the power cube, the cube would have to be reflecting a lot of IR for this to be a problem. A shroud over the sensor might help.

We brought the emitter and sensor wires together on a little piece of perfboard. The wires connect to a 3-pin socket that mates with a common “PWM” cable that runs back to RoboRIO DIO.

Also on the board is a red LED that lights up when the sensor is pulling down the signal wire. This provides instant feedback of whether the emitter and sensor are aligned with each other and working properly. The LED was very important for arm assembly and was observed at every pre-match check.