ROTARY ENCODER – a rotary switch that just keeps turning – uses gray code
An LED can be used as a photodiode used for light detection as well as emission.
http://en.wikipedia.org/wiki/LED_circuit
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD (Cathode Current Departs).
Cathode polarity is not always negative. Although positively charged cations always move towards the cathode (hence their name) and/or negatively charged anions move away from it, cathode polarity depends on the device type, and can even vary according to the operating mode. In a device which consumes power, the cathode is negative, and in a device which provides power, the cathode is positive:
* In a diode, it is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device. Note: electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows “most easily”), even for types such as Zener diodes or solar cells where the current of interest is the reverse current.