Book Analog Interfacing to Embedded Microprocessors
versions and comes in an 8-pin SMT package.
The National LM75 is similar to the LM74, but uses the I2C interface. The
LM75 has a narrower operating temperature range: -55°C to +125°C. The
LM75 produces a 9-bit output and includes a comparator that can indicate
when the temperature exceeds a limit. The limit temperature can be pro-
grammed via the I2C bus.
Optical Sensors
Slotted Switches
Figure 3.6 shows a slotted optical switch. An LED is mounted in a plastic
housing, facing a phototransistor. A gap separates the two, so if something
Figure 3.6
Slotted optical switch.
Sensors 59
moves into the gap, it blocks the light path between the LED and the photo-
transistor. Slotted switches are often used to detect motor speed by placing a
slotted wheel on the motor shaft; as the shaft rotates, it alternately blocks and
unblocks the light path. Another use for slotted switches is as indicators when
annaabi.ee 
