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I'm trying to make an Arduino controlled irrigation timer. My idea is to use an Arduino pro mini and a cheap solenoid valve, wired to a TIP120.

However, from my calculations, it's impossible to run it on batteries for long. From what I've seen they draw a considerable amount of power. (the one I'm using is similar to this one https://www.adafruit.com/product/997). If i power it with 9V, it draws 240mA, which is 2.16W. And if i power it for 30 minutes, it draws 1 watt-hour. That is way too much to use with batteries.

How do battery powered controllers work then? (like this one http://www.ebay.com/itm/Home-Water-Timer-Garden-Irrigation-Timer-Controller-Set-Water-Programs-/231932984347?hash=item3600487c1b:g:HAcAAOSw9ZdXKbTA)

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    \$\begingroup\$ This thing normally draws 240 mA, but if you power it for 30 minutes it draws 1 A? This makes no sense. Something is messed up somewhere. \$\endgroup\$ – Olin Lathrop Jun 10 '16 at 17:31
  • \$\begingroup\$ The linked controller works from batteries but maybe the solenoid is powered from a converter from the AC? \$\endgroup\$ – Andy aka Jun 10 '16 at 17:32
  • \$\begingroup\$ @olin-lathrop Well, I'm a noob starting out on electronics. Am I calculating this wrong? \$\endgroup\$ – Miguel71 Jun 10 '16 at 17:39
  • \$\begingroup\$ @olin-lathrop Oops, I ment 1 Wh, If an AA battery hold about 1000mWh, I'll be draining one each time \$\endgroup\$ – Miguel71 Jun 10 '16 at 17:40
  • \$\begingroup\$ @andy-aka I don't think so. I've seen multiple battery powered ones, and this one even says it takes 2 AA batteries \$\endgroup\$ – Miguel71 Jun 10 '16 at 17:42
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It's most likely a motorised valve.

enter image description here

Figure 1. Motorised valve attached to water hose line.

An industrial version of a motorised valve would have feedback potentiometer so that the actual position of the valve could be confirmed and, perhaps, proportional control could be achieved. Alternatively limit switches could be fitted to stop the motor once full travel had been reached.

In this case and in the case of the garden hose timers you linked I suspect that the motor would be run for a time and if the valve doesn't move then "too bad". With five or ten cycles per day the run time would only be half a minute or so and battery life should be good.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Simplified representation of control circuit for open / close control including optional limit switches. With SW1 and SW4 closed as shown the motor would run CW until the CW limit switch opened.

Figure 2 shows how a simple DC motor would be switched by a 'H-bridge' to effect forward and reverse motion. SW1 to SW4 would, in a practical circuit, be replaced by transistors (with protection diodes).

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  • \$\begingroup\$ This seems like a good solution. At least on ebay they are too expensive for this project. I could build one myself, though. But what motor should I buy? Does it have to be low RPM? How do I know it has enough torque to turn the valve? And how will I connect it to the valve? Thanks for the answer! \$\endgroup\$ – Miguel71 Jun 11 '16 at 11:30
  • \$\begingroup\$ There are two basic valve types: 1/4 turn ball and multi-turn screw type (as in a standard tap / faucet). The motor will need a small gearbox to increase the torque. If you can find a valve you may be able to estimate the torque required using a torque wrench or else a lever with weights to calculate the newton-meters required. This will be a good mechanical engineering exercise. With this information you can spec your motor-gearbox. You may find it hard to compete on price with the ready-made solution. \$\endgroup\$ – Transistor Jun 11 '16 at 11:35
  • \$\begingroup\$ I see. So, a way I could calculate the torque required would be to put a weight on the end of the valve lever, calculate the force gravity put on it(F=m*g) and multiply that by the lever length, right? \$\endgroup\$ – Miguel71 Jun 11 '16 at 11:42
  • \$\begingroup\$ That sounds right. I'd pressurise the valve in case that increases friction. Then I'd give a safety factor of 3 or 4 because you want to move it at some speed, not just barely be able to move it, and it may get stiff with age. Have a think about where else similar devices may be salvaged? e.g., Most cars have electrically operated valves on their heater water circuit. Are these motorised? Window-winder motors / wiper motors? All powerful, 12 V and with gearboxes. Even if oversized they'll only pull the current they need. \$\endgroup\$ – Transistor Jun 11 '16 at 11:49
  • \$\begingroup\$ This project is a lot more complex(and expensive) than I first thought. If i could use a solenoid valve it would be awesome, but It wouldn't be an elegant solution to have it plugged to the wall 24/7. I don't think I'll find anywere to salvage a valve. Thank you for helping me! \$\endgroup\$ – Miguel71 Jun 11 '16 at 12:14

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