  An IoT optical sensor based on the series-connected RGB LEDs is designed,
which exhibits the light-controllable optical-to-electrical response like a
phototransistor. The IoT sensor has the maximal AC and DC responsivities to the
violet light mixed by blue and red light. Its responsivity to the blue light is
programmable by the impinging red or green light. A theoretical model based on
the light-dependent impedance is developed to interpret its novel
optoelectronic response. Such IoT sensor can simultaneously serve as the
transmitter and the receiver in the IoT optical communication network, thus
significantly reduces the system complexity.
