Detecting open wiring/coils in sprinkler system

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I have an AVR based timer that I use with a 24VAC sprinkler system. Some vandals decided to cut several of the wires at the valves last summer, resulting in the loss of one tree, possibly more. I would like to update the timer to detect an open on any of the valve lines, but not sure what the best way would be. Currently I'm using triacs to drive the valves. If it was a DC circuit I'd consider a shunt resistor that I'd measure the voltage drop with. Will that work with AC?

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There are some nice Hall-effect AC current sensor ICs (Allegro, and others). They might work.

Jim

 

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A shunt or HE sensor will work on AC, but you will have to look for peaks in the waveform unless there are ones that provide a logic level that indicates presence of absence rather then analogue level.

An HE will amplify as well, if using a shunt you'll have to do that yourself with an opamp or comparator.

Either way you could rectify and filter the output to give you a nice DC logic level to read.

Or just square it up and run into an interrupt pin.

An HE sensor has the added advantage of being isolated from the 24v.

_____
Rob

Scattered showers my arse -- Noah, 2348BC.
Rob Gray, old fart, nature photographer, embedded hardware/software designer, and serial motorhome builder, www.robgray.com

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If the valves are not paralleled the common return or the driver supply could be monitored for driven valve current.

An opto-isolator could monitor valve current on/off - needs about 1.5V dropped in series with one of the valve leads. Three diodes in series could provide the 1.5V drop, while a fourth reversed across the three for the other half AC cycle.

Stan

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How about just measure the DC resistance of one of em, then you know what the parallel combination of 10 of them should read. Then you just need to have a relay that is 24V on one throw and regulated 5V on the other throw and a series R, and read the volts across it, and see if it makes sense for 10 sprinklers hooked up?

Imagecraft compiler user

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Simply measure the voltage drop across A1-A2 of each triac.When triac is off state and the load is connected the voltage would be 24vAc,for mcu sensing rectify this voltage and divide it with resistors.
When load is not connected when triac is on,or triac is off the voltage will be zero.
More simple way is to connect a led with resistor across each of triac,in off state if all the loads are connected all leds should glow.

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Ok I'll look into the current sensing IC's.
Sbennet, not quite getting the picture there, are you saying hook the opto across one of the diodes that would be inline with the return path of the valves?
Bob, I don't want to add 10 relays, lot's of bulk there.
geolec, making any ADC measurement would require a +/- connection to a circuit that has 24VAC running through it. I would be concerned about sneak paths since the 24AC also creates the VCC.

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In reference to Bob's idea, why not use 11 miniature pcb relays? Ten for the zones, the 11th to isolate the common. Then using the avr and a decimal decoder to fire the relays, inject a voltage through a resistor to the solenoid and check the voltage with a window comparator. The avr can see based on the level if the line is shorted, open or ok. If there is a fault the avr will not attempt to turn that zone on and as such fire an alarm to let you know there is a problem.

JIm

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well there's only 8 stations so 9 relays would be required. I will consider that, I guess my concern is if a relay welds shut. The timer in use now is over 10 years old, so how long will the relays last? I like the idea of doing something with the single common return since it would require fewer components, just don't know what.

Not sure what I would do with those Hall-effect AC current sensors, do I wrap the lead going to a valve around a sensor so it senses like an inductive current probe would?

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HE sensors like the ACS712 have the current pass through the chip, so you put them in series with the line. As has been mentioned you can use a single one on the return line then if you only ever run one station at a time it's easy, you don't really need to know the current amplitude, just that it exists.

If you run more than one at a time then divide the current by N and apply a sanity test to the result.

All that applies equally if you use a shunt.

_____
Rob

Scattered showers my arse -- Noah, 2348BC.
Rob Gray, old fart, nature photographer, embedded hardware/software designer, and serial motorhome builder, www.robgray.com

Last Edited: Mon. Nov 28, 2011 - 06:03 AM
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Yes, that ASC712 looks like something I can work with. I don't believe we have more then one valve on at a time, but I'll have to see what is programmed in. I could always do something like measure the current before and after I turn on a valve and look for a difference.

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Current transformers. Well isolated, survive all kind of short circuit, no drop voltage.
You can connect secondaries in series, and rectify the result, measure with only one ADC input. As you know how many are ON at any time, then you will expect a certain voltage. If not...

George.

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Example to monitor individually controlled solenoid valves.
Diodes - 1N4002, resistor sized to meet opto-isolator specs for three diode drop.

Stan

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Ok, that's kinda what I thought you were describing. So the optocoupler will cycle with the sinewave of the AC power signal. Could capture that with an interrupt pin. Another interesting solution!

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Living in Arizona, if you swapped the trees out for cactus (Pleural ?) then you wouldn't have to water them...

I'd put one of the Hall current sensors suggested by Jim on the 24 V line common to all of the valves.

I'd have the micro run a test routine once a day, (or however often you desire), and turn each station on individually while you measure that station's current.

Current level below some low threshold = wires were cut, and you display the damaged station number on your display and beep a piezo beeper to get attention.

You can easily tied the test routine into your clock routine.

Running each station for a couple of cycles will hardly be enough to even charge the line(s), i.e. no big deal.

Short circuit detection is easy. You DO have a fuse in the 24 V line... Right?!

You don't have to have the micro "detect" this condition, the fuse will pop, and the circuit will be dead.

How you attract the User's attention to a dead system is part of the overall design, and I don't see much built in to the system in this regard. (Dual supply with supply monitoring, Battery back up for the micro, piezo beeper, etc.).

I know the micro doesn't draw much current, but your display backlight might... Keep an eye on the power dissapated by your linear regulator, (P approx =(Vin-Vout)*I). You might need a heat sink on it.

JC

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The scheduled valve check is what I have in mind. The existing case is in the garage and has an LED that blinks if the memory gets corrupt after a power failure (EEPROM). Same LED could alert me to a valve problem. There's no backlight on the display. If I decide to build a new PWB I'll certainly update the vintage AT90S8515 to something a little newer!

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I just did a similar thing by putting a 1M resistor across the driver. With the driver off, there should be 24v across the resistor. If it's open, there will be 0v. Of course, this won't help if your valves are wired in parallel off one output.

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Residential sprinkler controllers I've owned detect and display open and shorted conditions. They have some cheap way to do so, but I've not looked into how.