A foldable LED lamp that is powered by USB costs as little as $3-4. The overall design seems relatively pleasing; the foldable and adjustable features are practical. One single touch button controls three levels of brightness and three light color temperatures. We take a look at its electronics.
5V comes in from the micro USB connector and goes through a diode. There is an unidentifiable IC driving two what appear to transistors (likely NPN) and receiving a capacitive touch button input. There are unpopulated footprints, possibly for the battery powered variant. There are total 32 LEDs on two PCB strips, with two different types of white LEDs, warm and cool installed next to one another. The brightness control is probably through PWM. With the base resistors of 3300 Ohms, each transistor perhaps runs at about 120mA driving 16 LEDs, only 7.5mA per LED, which seems a little low. The LED forward voltage is perhaps 3.4V; the dissipation of each transistor is 120mW, which is tolerable for a SOT23 package. We'll take a few measurements to check our speculations.
We measured the outputs from the IC (using AN8008 multimeter's frequency and duty measurement function), and found the PWM frequency to be 20KHz and the duty cycles of 10%, 40%, and 99% for the three brightness settings respectively.
