Patent ID: 8212775

Claim:
A computer input apparatus, the apparatus comprising: a light source to project coherent light on a working surface; a sensor chip to capture reflected images from said working surface; and a control circuit having a calibration circuit to control power provided to said light source irrespective of intensity of said reflected images, such that said power provided to said light source is in a range between a threshold value of said light source and a predetermined safety value of said light source, wherein at said threshold value Said light source starts to project coherent light, and above said predetermined safety value said light source project coherent light which is too strong that has a risk to damage human eyes, wherein said calibration circuit controls power according to a reference voltage without optically measuring the projected coherent light, 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; an external resistor having one terminal connected to a second input 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; and a third transistor having a gate connected to one terminal thereof, wherein a signal at said gate of the 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 said power and the other terminal of said second transistor is connected to said gate of said third transistor; wherein the other terminal of said third transistor is connected to said ground; wherein said first transistor has one other terminal electrically connected the second input of the amplifier, wherein a signal at said one other terminal of the first transistor and a signal at said second input of the amplifier are substantially the same; and wherein said first input and said second input of said amplifier form a virtual ground; whereby current through said first transistor is controlled to regulate current through said light source.