Open Valve Circuit Determination

[Jim Brunette]

Hello,

I am programming my OSPi (v1.42+) with a RPi (model B+ v1.2).

Since, I want to prevent short circuit damage from solenoids, I will follow Ray’s advise:

There is a fuse in line with the COM wire. If you want to protect each individual valve line, you can add additional fuse in line from each station port to the station wire. 500 mA /250 V fuse should be fine but is a bit low on the amperage. Each valve draws about 400 to 500mA impulse current and 250 to 350mA holding current. (reply #39639)

My question is can I determine that a short circuit has occurred (indicated by an open valve circuit due to a blown fuse) via a signal from the OSPi to the RPi? If so, which pin(s) should I monitor, etc?

Thanks,

Jim

[JimS]

The signals needed don’t exist on the OSPi. You would need to add capability to monitor current or output voltage. Output voltage would require sensing each output. If you only have one zone on at a time you could peak detect by using a diode to each output and monitor the single combined signal.

A better way (IMHO) is to monitor total current for the system. One sensor is needed. Then you can detect additional things such as open valve coils, bad wiring to valves, etc and the additional wiring is simplified as well. You would need to measure what normal current is for each combination of zones that you will use. Then apply some tolerance to get a window of acceptable values. This calibration step could be automated. The current sensor would need to be added and read with an A/D.

Here’s a link with various suggestions including this one:
http://www.avrfreaks.net/forum/detecting-open-wiringcoils-sprinkler-system

[Ray]

This generally requires current sensing support. At the moment OpenSprinkler DC has this support — it’s done by simply connecting a 0.2 ohm power resistor on the ground return wire from all stations, and wire that to an analog pin on the microcontroller. So it monitors the total current draw from all stations. By turning on each station individually, and monitor the current draw, you can detect each station’s status individually and tell if there is a shorting.

Sensing AC current is a bit more complex, but doable. One solution is to use a precision rectifier:
https://en.wikipedia.org/wiki/Precision_rectifier
to sense the peak voltage of the current sensing resistor, and that’s roughly proportional to the RMS current. This involves using an op-amp, but it’s pretty cheap to add.

[bryanmee]

Using a flow meter would let you know that the valve is working and that it is watering

[Author]

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