If you feel this idea works, please share it with other people who need to grow things.
For the watering can shown in the video, the straw/spout dimensions are about: 3.75mm to 4mm outside diameter (OD) 1.75 to 2mm inside diameter (ID) but try to stay within 1mm to 2mm. 1.5 mm is probably the ideal size of the pipe opening. The hose/straw in a spray bottle can vary from bottle to bottle. An ink pen often has about a 1mm tube in it. A coffee straw/stirer might work. If you find that too much water is comming out, maybe you can heat the tip and melt some of it to a smaller opening or use hot-glue to make the opening smaller. Use any length you want/need.
Be careful not to over-water, it's easy to get carried away with it. You don't want your soil to become mud. Even if the upper layer of the soil appears dry, the soil right below that might be damp enough, and that is the key I think, just keep it damp and not over-water creating mud. Make sure your cups or containers have enough drainage holes in the bottom that wont clog up or cause dirt to easily come out if you move the container around. I think when you put the soil into the pot, just before you put the seed in, it is best to get that soil lightly damp first so that watering required is minimal and you know that the entire volume of soil is in-fact damp. A good soil mix with good drainage is recommended.
Know how much water comes out of the watering can. Take it and see how many seconds of time it takes to fill up a table-spoon. Since my system (the size of the containers, volume of dirt, exposed surface area, temperature, humidity or dryness where the containers are) seems to require about a tablespoon and a half a tablespoon per day (I'm still doing tests on exactly how much for these small plants). If it takes 4 seconds to fill one tablespoon, it will take 2 seconds to fill a half a tablespoon. I will then need 4+2=6 seconds of watering for my system.
Once the soil is slightly damp when you put the seed or germinating seedling in, no extra water is needed at that point (maybe perhaps a few drops to ensure the seed is tended to), and the only watering needed later is to replace any (a relatively small amount) that has evaporated away at the surface and perhaps a small amount lost through the bottom holes.
As mentioned in the video, when you open the valve (ie remove your finger from being over the hole) in the container, you let the air pressure in so that the water can flow out due to that pressure. If you look, you can usually see air bubbles getting forced into this hole both from the air pressure and the partial vacuum created when the volume of water leaves the container. When the valve is closed the water will slow down to a stop, a partial vacuum is inside the can and the air pressure is preventing the water from coming out the tube/nozzle/spout. Well that's my take on it, let me know if I'm wrong.
Everyone should grow something: plants, flowers, food, trees, fruit. Don't forget the concepts of "grafting" such as grafting a pear branch onto an apple tree. Grow, give away, and maybe sell, for others, give them the seedlings they need.
I have made something very similar to this out of a plastic mayonnaise jar and I'm not sure if I even considered it for plants at that point. I think it might be in my video about a homemade timer and electrical switch.
This video also shows a good method for a fine watering device/sprinkler for seedlings and small plants: http://www.youtube.com/watc...
There are other ways to water plants, but from the top is the most natural, like rain, however, rain can damage very small/weak plants. Some systems "water from the bottom", that is, the water is "wicked up" when the soil or other conduit of water, ie a wick, is sitting in water. In hydroponic(s) farming, the roots of the plant are literally bathed in the water itself...seemingly without problems to the life of the plant.
Be careful no one can get poked by the device shown in the video. Perhaps even put some kind of ball or plug over the end of it. I don't use anything yet, since I fill the container up with tap (city) water and I want to de-chlorinate it somewhat by venting any fumes to the air.
Also consider water conservation, such as a miniature greenhouse/plastic covering for your small seedlings. This will help eliminate the loss of water, and also keep your plant warm.
Song is a YT selection: Harmonix by Evelyn Glennie