130-28176 Parallax Inc, 130-28176 Datasheet - Page 137

• Manufacturer Part Number: 130-28176
• Description: KIT PARTS PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 130-28176

• Accessory Type: Education Kit
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Board of Education Full Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

ACTIVITY #7: INCREASING SENSOR RESPONSE
While testing the various encoder wheels in Activity #6, you probably encountered a
point where the RPM and count is abnormally low or zero. The response time of the
sensor is insufficient to read the rapidly changing light levels in order to produce a
sufficient change in signal level for the BASIC Stamp to count.
How can the response time be increased? Recall from Chapter 3 that the response of a
transistor is largely dependent on the size of R
still drop asufficient voltage? If R
drop to cross the threshold and produce a LOW signal when in conduction.
The circuit needs a low value of R
Vss and Vdd. One solution is to use a Darlington Pair arrangement, like the one shown
in Figure 4-25. A first stage transistor, Q
stage transistor, Q
into saturation. This allows a fast response time by lightly loading the first stage, but also
a high voltage swing, or gain using the second stage.
Often R
to Vdd. It is used in this circuit as a safety measure for the components.
C1
2
. Q
is not used in Darlington pairs, and the collector of Q
1
only needs to provide sufficient current to drive the second stage
C
C
, but also needs an output that swings fully between
is too small, there may be an insufficient voltage
1
, is used to provide base current for a second
C.
Can a smaller value of R
Figure 4-25
Example Darlington
Pair Arrangement
DO NOT BUILD
1
is directly connected
C
be used and