Another update on the live feed from WiSense node
The live feed has been continuously running for 7 seven days now. The node on the terrace is solar powered. The solar cell trickle charges a NiMH button battery during the day. The node is sending temperature and battery voltage data to the gateway node every 5 seconds. You can see the live feed at https://xively.com/feeds/84250669.
The graph above shows the temperature variation over 7 days. There are some gaps in the data because of frequent power cuts in our area.
The solar cell is a “KXOB22-04X3” from IXYS.
- Size – 22 mm x 7 mm
- Voltage (open circuit) – 1.89 V
- Current (short circuit) – 15 mA
- Voltage (at max power output) – 1.5 V
- Current (at max power output) – 13.38 mA
This solar cell produces a lot of power given it’s small size. As a result it does not come cheap.
The rechargeable NiMH battery is a “V80H” from Varta.
- Nominal output voltage – 1.2 V
- Usable capacity – 80 mAh
- Weight – 4 grams
- Diameter – 15.5 mm
- Height – 6 mm
- Normal – 7 mA for 14 – 16 h
- Accelerated – 14 mA for 7 – 8h
- Trickle – 2.1 mA
The battery output is around 1.2 V – 1.3 V which is then boosted by a DC-DC converter to around 2.5 V which is good enough for all the components on the node. The MSP430 and the radio (cc2520) have a minimum voltage requirement of 1.8 V. The temperature sensor needs at least 1.4 V. The DC-DC converter is a TPS61200 from TI. It has programmable under voltage lockout threshold function prevents the output from being supplied if the input voltage falls below the configured threshold value. This allows the solar cell to charge the NiMH battery if the latter is deeply discharged.
I will go into the details of the node’s power consumption / energy budget in another post.
The picture below shows the solar powered node on the terrace. I have removed the cardboard box which covers all the components leaving only the solar cell exposed to sunlight. We will be building a pluggable board for the solar cell, battery and the DC-DC converter.