PURPOSE: To remarkably facilitate the manufacture by functioning one of thin film transistors (TRs) as a photodetector for a prescribed time and holding the other in the conducting state during the time so as to suppress the increase in output conduction lead wires thereby decreasing number of components on a substrate.
CONSTITUTION: An optical switch P consists of two thin film TRs FETT 1 , T 2 connected in series. In giving a gate voltage to the TR FETT 2 only, the TR is turned on as a switching element but the TR FETT 1 keeps the off-state. In irradiating a light to the channel of the TR T 1 in the off-state, the entire sensor P is turned on. That is, the TR FETT 1 acts like a photodetector and the sensor P is turned on or off depending only on the presence of the light sensing. Thus, in opening an analog switch A, the presence of the photodetection of the TR FETT 1 is converted into the presence of a current flowing to ground from a DC power supply 4 via the sensor P, the switch A and a resistor 7 and represents the presence of the output of an amplifier 8. In giving a gate voltage to the TR FETT 1 only conversely, the TR FETT 2 acts like a photodetector and the similar detection is attained.