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Water meter sensor
Function

Mounted sensor
Analog water meters are equipped with a x0.0001 m3 dial which makes one revolution per liter. On top of this dial a half moon iron disc is attached. By sensing the revolutions of this disc and counting them the water flow can be logged in Domoticz.
This solution uses a proximity sensor available through dx.com described as K1208065 Metal Detection Sensor Module for Arduino at just USD 7,99. It has 3 wires: Brown (+12V), Blue (ground) and black. The black wire connects to ground when metal is detected within a range of 4mm from the sensor. Also a LED at the rear of the sensor will light up. Check the documentation on the device itself because the information at the dx.com website is different (and wrong) at the time of writing.
Mechanical
To mount the sensor firmly to the meter a large hose clamp can be used which fits around the meter. The sensor is mounted on a curtain rail wall mount with a 12mm hole drilled into it, this leaves the other dials readable for the water company. Play around a bit to find the best position for the sensor.
Electrical
As described above the PiFace inputs need some protection from the 12V used to supply the sensor. This 12V is derived from a wall plug supply. A small circuit provides this. When metal is near the sensor the relay will be switched and the contact pulls the input to ground. Without metal the switch is opened and the input will be high. A 12V reed relay like the SIL121A7272L can be used. When using a relay it could be possible that multiple counts are detected on a single event. However, so far this has not been observed.
Domoticz
In Domoticz define an input as counter by adding these lines to the piface.conf file found in the /domoticz directory.
piface.0.input.3.enabled=false
piface.0.input.3.pin_type=level
piface.0.input.3.count_enabled=true
piface.0.input.3.count_update_interval_sec=60
In this example the fourth input (input number 3) is configured as counter which reports its value to Domoticz once a minute. Restart Domoticz when the piface.conf file has been changed. Once the counter has reported its first counts an unknown device will appear in the device list. Assign a name and use the type "Water". Configure the RFXMeter/Couter setup value of Water to 1000 (pulses per cubic meter) in Application setup.