I have been looking into autonomous persistent dataloggers for a JeeNode based sensor network. The requirements include a Real Time Clock, SD-card storage and battery backup operation. the performance of a standard Arduino / JeeNode based solution was a bit risky, in view of the SD-card operation. Maybe libraries have improved by now, but during SD-card operations, interrupts may be blocked for maybe over a tenth of a second, and that can lead to missing messages from the wireless RFM12B sensor network that I want to collect data from. I considered small ARM-soc computers, like the RaspberryPi. This could also be interfaced with a RFM12B module. It has sufficient processing power, but for me there were two downsides: 1. It consumes 2.5W, so there are higher demands on battery backup. 2. Sometimes it requires maintenance, and may require a reboot once in a while. I just do not want to miss data. So I started looking into more powerfull Arduino-like solutions, and stumbled into a Maple (leaflabs) clone.
The Olimexino-STM32 is a very interesting board to build a SD-card datalogger on. It has the SD-card slot on board, next to the possibility to feed the circuit with a LIPO battery, which automatically takes over when the USB-bus power supply stops. The board contains a LIPO charger and there is a integrated RTC which can be tuned to drift only 10 seconds per year.
Given the power supply and charger options this board is ideal for a datalogger that should work 24/7 365 days per year. Besides power outages, it also allow for maintenance activities on the server collection all data. When power gets disconnected, it can proceed for about 24 hours on the 1400mAh LIPO battery I connected to it, and when power is up again, it recharges the LIPO cell.
I use a 2GB micro-SD-card (yes, the smallest I had). With my current data inflow, it can buffer data for over a year. The power consumption is about 0.5W from memory. (TODO: measure it again).
To the board I connected a RFM12B 868MHz RF module to communicate with standard JeeNodes. This module is soldered to a RFM12B carrier board obtained from the JeeShop. It provides a pinout compatible with arduino shield connectors, which happen also to be implemented on the STM32 board. This results in very clean wiring.
The Maple-clone is a 3.3V circuit, as well as the RFM12B, so I could leave out the voltage conversion stuff from the board.
At reasonable low cost of around 40 euro’s this datalogger could be build.
- Olimexino-STM32 E.19.95
- Lipo 1400mAH (Olimex) E.5.95
- RFM12B (JeeLabs) E.6.00
- RFM12B board (JeeLabs) E.3.50
- box E.3.00