Thursday, 24 September 2020

Tinysensors Redux


Raspberry Pi Zero-W with Tinysensor Hat

It is often instructive to revisit an old project: you never know what you might see with fresh eyes. Such is the case with Tinysensors, which has featured here several times over the years. Since it lives in Github I use that platform's Issues feature to collect ideas which pop into my head when I am otherwise occupied. This provides a nice context from which to start revisiting.

Among the issues addressed since the last time we featured this project are:

  • Switching to the ATTinyCore. From my point of view, the most important feature of this great project is its support for Optiboot. This decreased the testing time for changes to the Tinysensor sketch by an order of magnitude. No more external programmers!
  • Removing the RF24Network library. This supports a very versatile model of organising nRF24L01+ radios into a fully-connected 5-way tree. However it necessarily limits the number of leaf sensors to 5, requiring bridges to interconnect different levels. A star topology removes this limitation, and also reduces the code footprint enough to support debugging via SoftwareSerial.
  • Reducing power-drain. With the above improvements in testability, it didn't take long before a gotcha in power consumption was spotted: a voltage divider made with a built-in pullup resistor consumes current even during SLEEP_MODE_PWR_DOWN!
  • Moving the hub to Raspberry Pi Zero W. This has more memory and more processor for smaller footprint; in addition to built-in WiFi and Bluetooth. Once that was complete, a proper Hat was designed to hold the nRF24L01+ radio and status LEDs... and a power button, just 'cos.
  • Outsourcing the RF24 library to nRF24. These guys have put a lot of work into improving the performance of Maniacbug's excellent original library, which means I don't have to!
Future work? A nice next step might be to redesign the sensor PCB for a coin-cell and surface-mount components. However I'm pleased enough with these changes to park it for another couple of years!