Patent ID: 7783205

Claim:
A circuit, comprising: an infrared photodiode having a first terminal and a second terminal; an adaptive reference signal circuit having an input lead and an output lead, wherein the input lead is coupled to the second terminal of the photodiode; a comparator having a first input lead, a second input lead, and an output lead, wherein the first input lead is coupled to the second terminal of the photodiode, and wherein the second input lead is coupled to the output lead of the adaptive reference signal circuit; and a timer, the timer having an input lead that is coupled to the output lead of the comparator, wherein a photocurrent flows through the infrared photodiode during an operational signal on-time, the photocurrent having a carrier frequency component and a low frequency component, the photocurrent having a positive peak and a negative peak during a cycle of the carrier frequency component, wherein the negative peak is a non-zero current, wherein the positive peak is a non-zero current, and wherein the negative peak is at least twenty percent of the positive peak.