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I've been working on a Wifi gardening project that allows me to water plants from anywhere I want. Of course, this means I need to use a water valve and a MOSFET to control it. The MOSFET feeds power from a 9V battery to the solenoid, and a Huzzah ESP8266 (on a separate 9V battery) controls the MOSFET gate.

I found that the ESP8266 would not open or close the valve, so I started testing the valve and I realized that I could not open the valve at all. The valve is a plastic 12V solenoid valve (https://www.adafruit.com/product/997), but the information on that page indicates that it can be run using 9V. As far as the 3 PSI minimum pressure requirement goes, the valve is connected to a resevoir that is 3-4 feet above the valve, so I figure that I have a decent amount of pressure from gravity.

I have almost no idea what could be going wrong - I connected the 9V battery directly to the solenoid through a breadboard and nothing happened. The only thing that seems off to me is the diode I use for reverse current protection - see the picture below. Is that diode set up correctly?

Thank you very much for your help.

https://drive.google.com/file/d/0BwEoZ1ajLf1uWUhrS2JIU2NnZDQ/view?usp=sharing https://drive.google.com/file/d/0BwEoZ1ajLf1uVlJHUm5YS0VVNlE/view?usp=sharing

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  • \$\begingroup\$ A schematic of your circuit is needed. Have you simply tried to turn the valve on and off directly from your 9 volt battery? A typical 9 volt battery will not be able to supply enough current to operate the solenoid. \$\endgroup\$
    – Glenn W9IQ
    Jul 6, 2017 at 20:22
  • \$\begingroup\$ 3 psi = 0.2 bar. At 10 m water per bar you need >= 2 m head. \$\endgroup\$
    – Transistor
    Jul 6, 2017 at 20:25
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    \$\begingroup\$ THe cost of water to fill a rain barrel is cheaper than the battery that might take a couple hours to empty even if it were 2 m above the valve. \$\endgroup\$
    – Tony Stewart EE75
    Jul 6, 2017 at 20:30
  • \$\begingroup\$ @Transistor I don't quite understand what you mean - do you mean I need 2 m between the resevoir and the valve? \$\endgroup\$
    – Anthony C
    Jul 6, 2017 at 20:33
  • \$\begingroup\$ @GlennW9IQ I have tried to turn the valve on and off by connecting it directly to the 9V. \$\endgroup\$
    – Anthony C
    Jul 6, 2017 at 20:35

2 Answers 2

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Unless the 9V battery is a Lithium cell, Alkaline will not last very long driving a load of 9V @ 240 mA (from datasheet) or about 37.5 Ohms.

The 9V battery internal resistance with 6 tiny 1.5V cells in series is in the 5 to 20 Ohm range which makes a 9V Alkaline a poor choice for this application with only ~500mAh capacity or 0.5Ah= 0.5A*9V = 4.5Wh.

From the solenoid specs of 4.8W at 12V that with each application of about 4 watts for 10 seconds resulting in 40 watt-seconds giving you maybe 400 pulses of life. But battery capacity decreases with rising current due to losses.

If the solenoid was on all the time, it might yield 1 if it is a good industrial Panasonic Alkaline fresh battery and you had sufficient water pressure for flow.

  • If not then it wont work at all.

It is best to use a small DC water pump than a solenoid. given you don't have enough pressure nor enough voltage or current with a suitable sized or use a proper DC supply with suitable waterproof and rodent proof cable insulation.

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  • \$\begingroup\$ So, a 9V wouldn't cause any results at all? I'm not particularly concerned about getting long life out of the battery, and the valve will only ever be open for 10 seconds at a time, at most. \$\endgroup\$
    – Anthony C
    Jul 6, 2017 at 20:28
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    \$\begingroup\$ The best practice is to design the system properly based on energy stored in cell and load demand rather than willy nilly guesswork. So far we dont have enuf data to do this. A voltmeter will confirm the voltage when energized and 3psi seems to imply you also dont have enough pressure to guarantee flow. \$\endgroup\$
    – Tony Stewart EE75
    Jul 6, 2017 at 20:33
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The valve requires a certain amount of pressure to overcome the stiction of the gasket. The spec calls for 3 PSI which means your gravity feed water supply must be at least 7 feet above the valve. Anything less than this means the valve will probably not open - especially when powered by a 9 volt battery.

The solenoid has a diode across it. This is to protect any attached circuits from the reverse voltage that occurs when the solenoid is turned off. If you have ever accidently connected the valve power backwards, there is a good possibility that the diode is now shorted. As a result, the solenoid will not work. You can test this by temporarily removing the diode and powering the solenoid directly with a suitable battery.

The 9 volt battery is probably marginal for this type of application. You could test your solenoid with a 12 volt car battery for comparison. Remember, however, to have the proper water pressure first.

To test the efficacy of your 9 volt battery, measure the voltage across the terminals of the solenoid when the battery is connected. If it drops much below 8.5 volts, it probably will not work.

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