Patent ID: 8519959

Claim:
A computer input apparatus, comprising: a light source to project a light on a working surface; a sensor chip to capture reflected images from said working surface; a control circuit having a fault detection circuit, a first current source to provide a current flowing through said light source, and a calibration circuit to control said first current source by a first control signal to provide said current in a specific range between a threshold current and a safety current of said light source; wherein said fault detection circuit controls said first current source by a second control signal to avoid excessive current flowing through said light source, and wherein said calibration circuit comprises: an amplifier having a first input of a reference voltage; a first transistor having a gate connected to an output of said amplifier; a second transistor having a gate connected to said gate of said first transistor, wherein a signal at said gate of the second transistor and a signal at said gate of said first transistor are substantially the same; a third transistor having a gate connected to one terminal thereof, wherein a signal at said gate of said third transistor and a signal at said one terminal of said third transistor are substantially the same; wherein one terminal of said second transistor is connected to a power source and another terminal of said second transistor is connected to said one terminal of said third transistor; wherein the other terminal of said third transistor is connected to ground; and an external resistor having one terminal connected to a second input of said amplifier and also connected to one other terminal of said first transistor, wherein a current flowing through said external resistor is substantially the same as a current flowing through said first transistor; wherein said one other terminal of said first transistor is electrically connected to said second input of said amplifier, wherein a signal at said one other terminal of said first transistor and a signal at said second input of said amplifier are substantially the same, whereby current through said first transistor is controlled to regulate current through said light source, and the light emitted from said light source is not fed back to said calibration circuit by opto-coupling.