2021.10.29
- dramatic revision of the design of the greenhouse Pi HAT

  My plan is to deploy two of these, one in each greenhouse, perched on Pine64
  boards which have a Pi header.  

  The current design supports 

	4 one-wire temperature sensors with screw terminals
		- at minimum, want an outdoor temp and inside temp on each
		  greenhouse.  could be interesting to measure different 
		  places in greenhouse, or slab temp in new greenhouse?

	4 solid state relay outputs with screw terminals
		- could be used for environment controls or whatever, not
		  used in our current greenhouses since heat and vent are
		  run by "dumb" thermostats

	2 clip-on AC current sensors with 1/8" stereo jacks
		- the point of these is to be able to see when heat and vent
		  are active, and indirectly to see what it's costing us to
		  run the greenhouse
		- one side is biased to 1/2 VCC, the other side is
		  connected to ADC input.  measurements will require sampling
		  at least one half-cycle of 60 Hz to get height of AC voltage
		  peak.  sensors bought output 1V is 30A.

	on-board MS8607 for temperature, pressure, and humidity
		- this sensor does not want to be in direct sunlight

	on-board photocell for light level
		- if the board is packaged, this sensor needs to "see light"

	on-board eeprom for HAT identification
		- will load the parts, not sure if I really care if it works
		  or not, but this would allow auto-detection by kernel and
		  loading of appropriate drivers?

2021.10.31
- Karen asks about adding rain gauge support

  RainWise makes a wired "tipping bucket" rain gauge, the "RAINEW 111", that
  sells for $72.95 and comes with 60' cable and a little display.  Clams to
  meet NWS accuracy specs, and measure 0.01" per bucket tip.

  Doing a little web searching turned up this article on hooking one to a
  weather station design from Slovakia:

    https://www.baranidesign.com/faq-articles/2019/9/3/how-to-connect-rainwise-rain-gauges-to-meteohelix-personal-weather-stations

  The gist is that they say the cable from the sensor is 2 wires and provides
  a normally open switch contact that pulses once per "tip".  They also assert
  this is a common interface.

  So, to read this, we'd want to have another GPIO input with a pull-up 
  resistor.  Might as well support two such on a 4-screw strip since I have
  those?