28128 Parallax Inc, 28128 Datasheet - Page 160

• Manufacturer Part Number: 28128
• Description: KIT BASIC ANALOG/DIGI PARTS ONLY
• Manufacturer: Parallax Inc

Specifications of 28128

• Accessory Type: ADC, DAC
• Product: Microcontroller Accessories
• For Use With/related Products: BASIC Stamp® 2 and Board of Education
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Do the Math
The RC time measurements along with the exponential decay equation can be a powerful
tool for determining an unknown resistance or capacitance.
Since we'll use PBASIC to make the BASIC Stamp tell us the measured amount of time,
t, we have the following information shown in Table 8-1
Algebra can be applied to the exponential decay equation to solve for R or C. First, take
the natural log of both sides of the equation. It eliminates the e term.
Rearranging terms yields:
ln
V
R
P
(
Values
×
15
V
t
C = Can solve for
R = Can solve for
VP15 = 1.4 volts
Vdd = 5.0 volts
C
P
=
15
Table 8-1: Known Values at the Instant VP15 Crosses the
t = Known
=
e
V
)
=
ln
dd
ln
(
2.718
V
×




dd
e
V
R
)
dd
−
×
−
t
C
×
ln
e
(
V
R
−
×
P
t
C
15
Comments
If the value of resistance (R) is known, then the




)
The BASIC Stamp counts the time for us in 2
If the value of capacitance (C) is known, then
The value of e is a constant found in many
Threshold Voltage
The initial condition of the voltage at P15.
The final condition of the voltage at P15.
⇒
capacitance (C) can be determined.
resistance (R) can be determined.
algebra books, physics texts, etc.
ln
microsecond increments.
(
V
P
15
)
=
ln
(
V
dd
)
−
R
×
t
C