Here's the result up and running! Hmm... let's just say that I won't ever be taking this device into the terminal :)
The green LED indicates the status of the heater (on or off). The display counts down until the next server check-in and blinks the HTTP response code for a few seconds after a server-check in. The yellow LED illuminates while the controller is communicating with the server. And finally, the red LED illuminates when there is a problem (one that the controller knows about, which is a network failure or non-200 response from the API).
Final testing! A lamp is substituted for a heater, which is currently off, but scheduled to turn on.
Having passed the test, it's now up and running in the hangar.
Future Modifications
I'm already thinking about how to improve this version. The list includes:
Switch to a pay-as-you-go wireless plan and chop the expenses in half (or more).
A proper housing (and perhaps a smaller form-factor).
Replace seven-segment display and LEDs with a bigger/better display.
Add a thermometer to know how cold it is in the hangar.
