Thursday, 21 March 2013

Solar charger for RF nodes

In my remote nodes, they currently use alkaline batteries. These will run a node for perhaps 12~18-months. It depends on how much they send and how often and what type of heat cycling they are exposed to.

Typically on a temperature node, I send the info every minute. This comprises of temp and battery voltage.
The duration that the RFM12 is actually powered-up and transmitting is a large proportion of the power drain so if the string that is sent is longer, ie it goes from 15ms to 30ms, thats a doubling of power consumption. If you send more, batteries last for less. Not really a surprise.

I wanted a solar option as one of my nodes in on the roof monitoring the solar hot water system. It has a Dallas 1-wire sensor globbed on the on the end of the tank, under the cover, with heat-sink paste.
I dont measure the tank internal water temp, but its close enough so I can see whats happening. The manufacturer has the electric thermostat mounted in a similar location. I reckon where I measure its about 3~4 Deg cooler than the internal temp.
This is a Thingspeak live graph of the solar hot-water system. Its logging most of the time but off now and again if I'm messing with it.
I figure if I put a solar one up there, I can just forget about it.

The board is pretty small, about 23 x 33 mm with a couple of mounting holes. It uses the same single cell Lipo charger IC as the garden lights, the CN3083.  I got them from Ebay

This is the circuit. Nothing fancy, pretty much off the data sheet with the monitoring dividers added.

I use it wired-in and as a plug-in.

The PCB on the left is my first 'real' version, a general purpose board that I use for assorted purposes.
The software files are on AVRfreaks but quite old now, Its evolved slightly but essentially the same. The core RFM12 software is almost unchanged.

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