Automatically adjusting drive circuit for light emitting diode

A light emitting diode driving circuit for supplying a driving pulse from a drive power source to a light emitting diode to thereby cause the light emitting diode to emit light intermittently, the driving circuit comprising:

• a clamping circuit connected between said drive power source and said light emitting diode, said clamping circuit including a diode having an anode connected to the drive power source and a cathode;
• a photodiode disposed in the vicinity of said light emitting diode for detecting the optical output power of said light emitting diode and for generating a detected output signal;
• an AC amplifier having a DC operating point stabilized by means of a DC feedback circuit for AC, the AC amplifier amplifying the detected output signal from said photodiode;
• a shaping circuit for shaping the output amplified signal from said AC amplifier;
• a comparison circuit for comparing the output shaped signal from said shaping circuit and a reference voltage, and for generating an output signal determined by a difference between the detected output signal and the reference voltage;
• and
• a transistor having an emitter connected to the cathode of the diode, a base connected to receive the output signal from the comparison circuit, and a collector connected to ground;
• wherein a clamping voltage of said clamping circuit is controlled by the output signal of said comparison circuit by adjusting a resistance of the transistor in response to the output signal of said comparison circuit such that the optical output power of said light emitting diode is kept constant.