Start with a reliable power source, a wide range of constant current sources. LED requires cross-flow drive, and the current various LED power supplies are also cross-flow. It is generally about 300mA. The power size depends on how many lamp beads the output voltage can drive. But there is a limit to the range of voltages that can be automatically adjusted by the power supplies available today. For example, 7W power supply can often drive 5W LED lights normally. Because the interior of the 5W and 7W LED lights are in series, the current is the same, but the output voltage is different. But to make the 7W power supply to drive the 3W lamp can not be done, there is the possibility of exceeding the current. The 7W power supply also cannot drive the 9W lamp, because the output voltage is not enough. In order to achieve your requirements, you need to have a lamp that can drive a maximum of 15 leds in series, and can drive one LED at a minimum. The 15 leds are connected in series, and each LED is connected with an electronic switch, such as a MOSFET. Short circuit the LED directly when it needs to be turned off.