Buying from e-bay a common valve usually is not gravity feed.
Needs a lot of pressure in order to allow the water to flow when the coil is energized.
The tools that we are going to need in order to make this modification are :
A glue gun
A water bottle
A screw driver
A dremel with 1.7 mm drill bit
A 12V 1A AC to DC adapter / power supply
With a screwdriver open the pump by removing the 4 screws.
Now start heating the glue gun. Before getting too hot the gun (it is important) place a straw in the glue gun nose and remove it fast.
If the temperature is "correct" then the straw will deform like the photo below. Do not overheat the glue gun.
Test the modified straw with the core of the coil like below :
Now take the solenoid with the spring and the core.
Press down the spring put the conical straw over the core and glue the straw.
Do not overheat the gun because you will destroy the straw.
Do not push the straw all the way down. Just enter in into the hole and glue it.
Do not glue all around the straw. Leave a track unglued in order to allow the air to flow.
Once done by sucking on the straw you should not be able to get air, if not remove the glue and try again with another straw.
Connect the diode between the two ends of the coil. The use of the diode has been explained in this article.
Connect the coil into a 12V power supply and try blow or suck though the straw.
If you receive air you are good to go !
Now it is time to prepare our bottle.
Make 9 holes using a 1,7mm drill bit on the cap of the water bottle.
Important notice. Avoid making one big hole on the cap. We are going to take advance of surface tension.
The smaller the holes are, the bigger the surface tension force is.
Surface tension gamma=F/L
F of the force exerted parallel to the surface of a liquid
and L is the length of the fluid surface.
Now drill a hole at the bottom of the plastic bottle and insert the straw like the photo below :
Use tape in order to block the solenoid into a certain position.
Use the glue gun into a not too hot temperature and glue the straw with the bottle in order to avoid leaks.
You can use silicone as a better solution.
Now remove the cap fill your bottle with water and place the cap again.
You are done !
Let's see a more detailed diagram in order to understand how this hack work.
The dia diagram can be downloaded from here : GravityFeedValve
The forces will be calculated into the second page of this article.
Let's see what happens by placing the bottle upside down.
If you are not giving voltage to the coil then the bottle will start leaking for a while but after a couple of seconds will stop.
This happens because no air can flow into the bottle.
The only way to enter the air into the bottle are the 9 tiny holes but the surface tension does not allow the air to enter.
The surface tension is caused from the water molecules which attract one another and does not allow the air to flow.
If this effect does not happen control the glued joint between bottle and straw.
So by not entering air into the bottle the water can not exit from the bottle !
Now by giving voltage into the coil an electromagnetic field is generated.
The coil acts like a temporary magnet so attracts the metallic core inside the coil (Fcoil).
Since the core is down the air can enter into the bottle.
The air pushes the water out of the bottle from the 9 tiny holes.
A demonstration video can be found below :
Into the next page we will do some calculations of what is happening into our circuit.