Analog Interfacing to Embedded Microprocessor Systems, 2nd Edition

frequency that will work—typically around 10 kHz. Because filtering the signal

takes some time, it takes several cycles for a mechanical change in whatever is

being measured to show up at the output. So, while the sensor may be able to

operate at 10 kHz, the system may be able to handle a rate of only 1000 Hz or so. In

the example shown, if the ambient light causes so much DC offset that the

phototransistor saturates, no amount of filtering will recover the signal.

Mechanical Instability

Mechanical jitter can cause strange results with reflective sensors. I saw a system

once that used a reflective sensor to look at a shiny strip on a flat black motor shaft

to count rotations. The output of the sensor circuit generated an interrupt to a

microprocessor. ...