The nutrient film technique (NFT) is what most people envision when they think of hydroponic farming. This system provides the plants with nutrients on a constant basis and does not need a timer.
The nutrient solution is continually cycled up to the growing tray then drains back to the reservoir underneath.
The NFT does not use a growing medium at all; the plants are growing in pots or other container that allow the roots to grown down into the nutrinet solution.
pic: San Diego Hydroponics & Organicsより