Thursday, 21 March 2013

RFM12 water meter

The latest use I have for the RFM12 mini-node is to monitor the water I'm using on keeping my garden alive.

Where I live, there is no public water supply, ALL of my water comes (directly) from rain. This is collected on my house and shed roof areas and stored in several tanks.
At long last I fitted an irrigation system that waters the garden around my house, but then I wondered, how much water am I actually using?


I purchased a 3/4" flow meter from littlebird electronics (and available from several other suppliers).

It is spec'ed at 5V but runs fine at 3.3V with clean edge transitions and 0-3.2V switching on the sensor o/p (using the internal pin pullup)

Without the sensor connected, the node board (in active mode) consumes about 0.5~0.6 mA. This is at 1Mhz.
With the meter connected this jumps up to a whopping 3.2mA if the sensor is giving a 'zero' or 2.6mA if its a 'one'.
Its a bit high for comfort.
When paired with the solar charger this should actually be fine.
The solar panels are supposed to give 170mA (I measured just over 150mA).
If we round up the consumption to 4mA and then have 18hrs of no charging per day, (18 x 4), that's only 72mA/hrs. This takes less than 30-min of good charging to replace the used energy.
I have a stack of 1000mA/hr single-cell Lipos as that is what is used in the garden lights so one of then should let it run for (theoretically) about 250 hrs in the dark, just over 10 days.

The 1st attempt software is done. The board keeps a running total of water through since powered up and also the water through in the last minute.
From the data-sheet, if I got my sums right, it gives a pulse for each 3ml of water through the meter. This will only be so within the specified flow-rates (see here) but I'm not needing a precision device, just an approximation. If its within 5~8 % it will be good enough for my purposes


Another possibility I have not tried yet is to tie the 4 free port pins together and see if I can power it from them directly.
The trouble with that is I would then have to find another way to detect the water staring to flow.
If I managed to use three pins, that leaves an analog i/p free.
Perhaps a piezo sensor on the pipe. Or a micro hydro generator that gives power from the water flow so no batteries required.
Or fit a micro-switch over the anti siphon vacuum breaker that is already installed on the pipework. Its a simple little valve that when the pipe is pressured, it pop's up, pushed by the internal water pressure, and closes a seal. When the pump is off and the pressure drains out of the system it releases and lets air into the pipework so my water tank does not siphon out and empty itself.

I think I'll make one for the shower next. Its probably the largest water consumption item I have. If you have ever had teenagers in the house then you know what I mean....


1 comment:

  1. This looks like an interesting project. I'm thinking about using one of those flow sensors on the post war communal water system my village here in Japan uses to detect when flow from the water sources cease (which frequently happens). Most of the village is also connected to the municipal grid, so the older water system is auxiliary, except for my house, and a couple of farmers also use copious amounts, so we frequently have to trek into the deep woods to fix the PVC pipes as often as once or twice per month. The problem is that the system has 4 huge sedimentation tanks and a holding tank, and unless we hike over to the tank area every day, we usually only find out there is a breakage long after it occurs (in other words when the tanks run dry).

    Your project looks like a great reference because I'll have to face the same power issues.

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