Mailing machine having optical sensors with improved immunity to ambient

A mail processing system comprising:

• a controller to control operation of the mail processing system;
• a transport device to transport mail pieces along a transport path of the mail processing system, the transport device being controlled by the controller;
• at least one optical sensor located along the transport device for determining a location of a mail piece along the transport path, the optical sensor including a light emitting diode that is powered based on a pulse width modulated signal, and a photo-detector, the photo-detector causing a voltage level at a photo-detector output node to vary based on an amount of light being received by the photo-detector;
• a filtering circuit having a diode coupled to the photo-detector output node and a low pass filter coupled between the diode and an output of the filtering circuit;
• and
• a comparator having a first comparator input, a second comparator input, and a comparator output, the first comparator input being coupled to the photo-detector output node, the second comparator input being coupled to the output of the filtering circuit, the comparator output indicating when an object is detected by changing states based on voltage levels input to the first comparator input and second comparator input, the voltage levels at the first comparator input and second comparator input being dependent on the amount of light being received by the photo-detector,
• wherein the filtering circuit adjusts the voltage level at the second comparator input to control a threshold at which the comparator changes states to reduce false detections caused by the photo-detector receiving ambient light.