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====== History: WCP-0971 ======


Eastbots robots have used [[https://en.wikipedia.org/wiki/Hall_effect| hall effect]] magnetic sensors in at least 2015, 2016, and 2019.   All of those have been the [[http://www.wcproducts.net/wcp-sensors| West Coast Products WCP--0971]], which until recently looked like this:

{{ :wcp-971_hall_effect_front_2.jpg?400 |}}

Two new ones ordered during competition season 2019 look a bit different:

{{ :wcp971-2019-cropped.jpg?400 |}}

These arrive with no connector, and only tiny surface pads for wires or a connector to be attached.
Surface-soldering a three pin receptacle as show makes them plug-compatible with the original WCP-0971s,
but that solder joint isn't very sturdy.  One ripped off during a match at Guilford.  Another problem with this batch: both LEDs, power and magnet-present, are green.  So one has to look really carefully to tell if the magnet is being sensed (both LEDs on), or if just the power LED is on.

======Alternative Hall Effect Sensor======

If we're going to need a special mounting scheme to protect the WCP sensors anyway, and they're harder to use because of the same-color LEDs can we do better with somthing different?

I noticed these at adafruit, and bought a few: 
https://www.adafruit.com/product/158
  * datasheet: https://www.adafruit.com/datasheets/US5881_rev007.pdf

With slightly creative lead bending, we can attach these to one of our DIO test boards:

{{ :hall-sensor-dio.jpg?600 |}}

Mounting the sensor with the part number down as shown makes it sensitive to the same pole of the magnet as the WCP sensors, allowing  this one to be dropped in without flipping a magnet over.

The LEDs on the DIO test board indicate power and magnet-present, just like the LEDs on the WCP board.
A big drop of glue would be a good idea to hold the sensor chip firmly to the board.  
We'd still have to figure out how to mount the DIO board.

These haven't been used on a robot yet, but it is an alternative.

====== Wiring ======

The three pin connector on the WCP-0971 (or our DIO board, of course) is ready to be plugged into a
RoboRIO DIO port.  It provides power, ground, and signal.  The signal goes LOW when the magnet is near the sensor.

It gets a little bit more interesting when we want to use one of these for a limit input to a motor controller.
Issues are:
  * most of the motor controller breakouts only have a two pin connector for a contact-closure limit switch
  * those two pins are usually "signal" and "ground", but which is which is seldom labeled
  * Power for the sensor has to come from somewhere

Here's one solution, sketched out (TODO schematic of this cable)
{{ :hall-effect-to-talonsrx-breakout.jpg?600 |}}
and a picture of the hookup:

{{ :hall-effect-to-srx-breakout-pic.jpg?300 |}}


=====Better Wiring=====

There are other things we don't like about all of the available Talon-SRX compatibile motor controller breakout boards, so we're talking about doing our own.  One feature it can easily have is three-pin limit switch connectors.