Archive

DOGE

I recently finished some boards for a 915MHz/2.4GHz network. Their design files will go up on github soon but until then here’s a summary.

Sensor Node

This was the first board and was meant to be flexible. It had two radios – 915MHz (MRF89XAM9A) and Bluetooth LE (BL600-SA) along with environment sensors and hot-swappable power supply circuitry. The sub-components passed initial testing but the BASIC programming language required on the BL600 chip proved to be less convenient than I had hoped. Along with a high BOM (~$40) I determined it would be better to break it up into smaller sections.

Sensor Subboard

Temperature, pressure, altitude (MPL3115A2)
Visible light, IR, IR proximity, UV index (SI1145)
Humidity, temperature (SI7020)
Sound (MAX4468 + electret mic)

Power Subboard

This board takes in up to three power supplies – USB micro, single-cell LiPo battery, and a coin cell battery – and outputs 3.3V. The supplies are hot-swappable and the ORing circuitry will connect the load to the largest supply automatically. For example, if you’re running off a coin cell then plug in USB, the coin cell will be disconnected and the load will use the USB power instead.

A voltage supervisor and load switch disconnect the LiPo when it reaches 3.5V and reconnects it when the voltage rises to 3.7V. A charging IC allows the USB supply charge the LiPo at 100mA (configurable by resistor).

Leaf Node

This board is meant to be the furthest out on the network yet hopefully capable enough of running a limited version of the Contiki OS. The LPC812 was picked for the cost ($1.58) and memory (16kB flash, 4kB RAM). The board has a temperature sensor but since the LPC812 doesn’t have an ADC I added some passive components to make a sigma-delta ADC. There’s also a header for a nRF24L01+ radio to plug in. The total BOM ended up being about $5.