Simple solar powered pocket submarine with depth control.

Step 4 Some words about the diving water

You need the diving water to be in direct sunlight. You could design a version wit a larger number of solar cells working under artificial lighting, but that will become more expensive. I would also advice against improvising high power lighting around water, unless you know very well what you are doing.

The water should be more or less clear. Originally I expected the solar energy caught by the solar cells to diminish with the amount of water above it, thus diminishing the down force (dynamic diving) and stabilising the dive at a certain depth. In practice this effect is far to small in clear pool water and the dive goes down all the way to the bottom. Even a test in quite turbid water made the sub dive to the bottom (be it only 40cm deep, but already hard to see). My guess is that if the sight is limited in the water, the sub is quite likely to dive below the level you can see it. So my advice is to keep to reasonably clear water where you can still see the bottom and use the depth control methods explained later. Of course it is also wise to choose a diving water where you can reach the bottom in case your trimming is not correct.