A simple power supply for a small gardening project

[jimjam]

Hi, I want to build a small controller with the following rough specs:

- I think I'll use the arduino mini / nano board - they're only < $3 for a complete system and they're very tiny (smaller than a USB memory stick)
- This device will act as a "slave", possibly connecting via some sort of wireless link to the master controller which will have some sort of user interface (web or LCD, or just plain cron job on a computer) 
- connect to solenoid garden irrigation valve to control watering - via a relay or some sort of fet. Relay is probably easier. The power supply needs to drive the solenoid (24V - not sure how many amps)
- receive input from a rain sensor (it comes as a stereo jack from the rain sensor).
- I am thinking of using some sort of plastic housing and figure out the water proofing for it - it would be nice if the housing has a rubber o-ring seal but I don't think I've seen one (from jaycar).
- This box will live outside, mounted on the wall, subject to sun and rain.
- Powering the controller - this my main question - got any ideas what I can use (circuit / diagram or a small inexpensive module?) There will be an outdoor power point nearby. Most small irrigation controllers seem to be powered by 2xAA or 1x9V batteries and can manage to work for 6 months+.

Options:
- Use a transformer / power supply / SMPS and take power from the outlet
- Use a 9V battery and have a step up to convert to 24V to drive the solenoid valve - will the 9V battery last? It'll only need to turn on the valve for maybe 10-20 minutes a day total.
- or find a valve that operates on 9V (non standard? might be harder to find)

I'd appreciate any tips / ideas. Thanks!

[cellularmitosis]

I think the key thing which will decide if you can get away with battery operation vs having to use a transformer off the mains will be measuring how much current the solenoid requires.

Solenoids are usually fairly power hungry (from a battery perspective).  Especially for 9V batteries.  This guy measured between 300 and 500mAh for a 9V at a discharge rate of 100mA (hopefully you will need less current than that).  At 15 minutes per day, that's 20 to 30ish days before you need to replace the 9V batteries.

http://www.powerstream.com/9V-Alkaline-tests.htm

Measure that current and report back 

[cellularmitosis]

Actually, a really fun twist on this project would be to attempt to make it solar powered.  8 hours of insolation is plenty of opportunity to store up enough charge to run a solenoid for 10 minutes, just a matter of sizing the solar panel right.

You might be able to buy some solar garden lights at the dollar store / harbor freight and rip the solar panels out of them.

[cellularmitosis]

Actually, this reminds me, I characterized 3 different types of solar cells which I had ripped out of various garden lights over the years.

https://www.dropbox.com/sh/509p59ssubpv0ar/AABsTbxGw-acXFeNKAWX48Dha

if I recall, the lamp being used was a 23 Watt compact fluorescent bulb (100W incandescent equivalent).

[22swg]

My greenhouse watering controller uses a 6v battery for the controller (PIC) and a 12v  3.2ah battery for a bilge pump (relay driven ),the controller monitors the 12v.  time obviously depends on use. my pump cycles about 10 sec every 3 hrs during a window 10:00 to 16:00 12v lasts about a month.  6v 4 months

[aroby]

I don't know about Australia, but here in the US irrigation controllers are 24v AC and have a battery backup for the programming and timer.  However, those controllers are designed to control multiple solenoid valves.  Sounds like what you need is a simple solenoid valve that screws onto a tap or hose.  Those are usually battery operated.  Maybe you could acquire one and hack the internals to get your connectivity to the Arduino?  Then you wouldn't need to mess around with 24v.

Anthony

[ajb]

If you drive the solenoid directly via a FET you may be able to shave off quite a bit of power consumption by doing peak-and-hold.  Once the valve is open it should take only a fraction of its nominal operating current to hold it there, so you would apply full power for some fraction of a second, and then switch to PWM at whatever duty cycle you find keeps the valve from chattering.  Be mindful of snubbing, as a solenoid of the size typically used in sprinkler valves will give a heck of a kick when turned off. 

[jimjam]

I don't know about Australia, but here in the US irrigation controllers are 24v AC and have a battery backup for the programming and timer.  However, those controllers are designed to control multiple solenoid valves.  Sounds like what you need is a simple solenoid valve that screws onto a tap or hose.  Those are usually battery operated.  Maybe you could acquire one and hack the internals to get your connectivity to the Arduino?  Then you wouldn't need to mess around with 24v.

Anthony

Actually, I just saw the "specs" for the solenoid saying 24V but I didn't know whether it was AC or DC. I guess I should go and buy one and play with it. The idea for hacking an existing one is nice, but they are usually more expensive than a simple solenoid valve, and at the moment I want to have two valves, and once it's working I want to build one for my neighbour too.

If you drive the solenoid directly via a FET you may be able to shave off quite a bit of power consumption by doing peak-and-hold.  Once the valve is open it should take only a fraction of its nominal operating current to hold it there, so you would apply full power for some fraction of a second, and then switch to PWM at whatever duty cycle you find keeps the valve from chattering.  Be mindful of snubbing, as a solenoid of the size typically used in sprinkler valves will give a heck of a kick when turned off. 

Good idea, I'll experiment with this!

Actually, a really fun twist on this project would be to attempt to make it solar powered.  8 hours of insolation is plenty of opportunity to store up enough charge to run a solenoid for 10 minutes, just a matter of sizing the solar panel right.

You might be able to buy some solar garden lights at the dollar store / harbor freight and rip the solar panels out of them.

I considered using solar but then it involves a rechargeable battery, and one of the valves will be in a half shaded area. I guess I can mount the solar panels on the roof and have wires coming down. I haven't completely discounted this idea yet. Maybe I'll try both ways (mains and solar) just for fun.

[jimjam]

Upon closer inspection yes it's 24VAC!

http://www.bunnings.com.au/holman-25mm-solenoid-valve-with-flow-control_p3112814

[Psi]

Usually garden watering timers use either a pressure assisted latching system (triggered by tiny solenoid pulse) or a geared motor attached to a ball valve.
A 9V just cant supply the juice required to hold a typical solenoid on.

[ajb]

Upon closer inspection yes it's 24VAC!

http://www.bunnings.com.au/holman-25mm-solenoid-valve-with-flow-control_p3112814

Yep, that's a pretty typical pilot-actuated diaphragm valve.  The valve is really opened and closed by water pressure, and the solenoid only directly actuates a small internal valve that controls the pressure behind the diaphragm. 

I've used these valves on several occasions and found them to work perfectly at well below 24V.  I think on one occasion I even ran one at 12VDC, but this thread piqued my curiosity so I went and hooked one up to a bench supply.  Winding the voltage down from 24VDC gives a progressively less enthusiastic THUNK from the solenoid, but it seems to work just fine all the way down to ~7VDC!  Below that it may still work for irrigation purposes, but the valve will probably chatter quite a bit when opening  (I was testing it dry).  I suppose it makes sense that the manufacturers would account for quite a bit of voltage drop from poor cables and connections, but I wouldn't have that that it would work at less than 30% of spec!

(Of course your sprinkler valve may vary)

[jimjam]

Typical Australian pricing. USA $12, AUS $20.

[22swg]

If you are using 'mains' water supply a solenoid / valve will be OK. I actually rewound an old washing machine inlet valve 230ac to 12v dc. but was not suitable for low pressure from a rain water but . I also built a motorised plumbing service valve , worked well but loads of current on and off , hence the bilge pump and relay.   

[eneuro]

In my garden anty mole project I will use simply old car battery 12V VARTA L2 400.
Those batteries are useless in vehicle while it needs a lot of rush current to start engine, but it still powers up 55W 12V light bulb for several hours.

It would be perfect connect it to PV panel, but it still has a lot of Ah, so one can connect to battery charger when undervoltage protection diode or buzzer switches on.

Why not to use mosfets with protection against sparks with fast Shotky diode to controll this solenoid?