10.5.1.2 Rail to rail range

As described in the previous section, a TTL/CMOS device only guarantees to register a zero when the input voltage is below about 1.35V, and only guarantees to register a one when the input voltage is above 3.15V. Unfortunately, some transmissive sensors cannot produce an output voltage range that exceeds this range of 1.35V and 3.15V.

The problem is usually due to two physical properties of an optical encoder. First, some optical encoders have ``weak'' phototransistors. This means the phototransistors cannot let much current through the device, even when it is 100% on. As a result, it is often necessary to use a large value pull-up resistor. This, in return, change the response time of the phototransistor. As a compromise, sometimes it is necessary to use a less-than-optimal resistor value, making it impossible for a phototransistor to yield a low voltage when it is fully turned on.

The other limiting factor, surprisingly, is the encoder disc. Industrial encoder discs are machined from thin aluminmum sheets. A thin foil of aluminum is 100% opague to infrared. However, it is far more cost effective to create encoder discs from transparency sheets (for projector use). A stripe pattern can be easily generated by various programs, and the pattern can be printed to a transparency sheet using a laser printer.

Although the color black from a laser printer is dark enough for the human eye (in the visible spectrum), it is not guaranteed to be opague enough in the infrared spectrum. This is because infrared has a longer wavelength than visible light. As a result, a toner density that is sufficient to block the transmission of visible light is not necessarily sufficient to block the transmission of infrared.

As a result, a cheap homemade encoder disc may be somewhat transmissive even at the opague (black) area. This infrared leak through means that the phototransistor is always on a little bit, and it is never completely off. The corresponding output of the phototransistor never reaches the high end of the rail.