Patent ID: 8659550

Claim:
A computer input apparatus, comprising: a light source to project light on a working surface; a sensor chip to capture reflected images from said working surface; and a control circuit having a fault detection circuit to control power provided to said light source, wherein said fault detection circuit comprises: a first current source to provide a current flowing through said light source; a first resistor having a first terminal coupled to said first current source and a second terminal coupled to said light source; an amplifier having a first input connected to one terminal of said light source, a second input and an output; a transistor having a gate connected to said output of said amplifier and one terminal connected to said second input of said amplifier, wherein said transistor is not connected between said first input of said amplifier and said light source, and is not connected between said second input of said amplifier and said light source; a second resistor having two terminals, a first terminal being connected to said second input of said amplifier, wherein said second input of said amplifier is connected between said first terminal of said second resistor and any other part of said amplifier; and a second current source coupled with said second terminal of said second resistor to provide a current such that a voltage level of said second terminal of said second resistor is similar to a voltage level of said second terminal of said first resistor, wherein said current flowing through said light source is controlled in accordance with a comparison between said voltage level of said second terminal of said second resistor and said voltage level of said second terminal of said first resistor.