28128 Parallax Inc, 28128 Datasheet - Page 153

• 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

The difference in time it takes the three different circuit outputs to decay can be used to
determine a resistance value if the capacitance value stays the same. Likewise, if the
resistance is fixed but the capacitance varies, the capacitance can be determined by
measuring the time between the step input and the threshold voltage.
A PBASIC command called
the amount of time it took the RC output to decay from 5 volts to 1.4 volts, which is the
value of
The value of t
from 5 volts to 1.4 volts.
It turns out that the amount of time it takes for the voltage to decay is directly
proportional to R multiplied by C. R ´ C is called the RC time constant, and it is often
represented by the Greek letter tau.
At the instant the voltage at P15 reaches 1.4 volts, the input value at pin P15 goes from 1
to 0, and the BASIC Stamp stops counting 2 us intervals.
Voltage
Voltage
Output
(Volts)
(Volts)
Input
t
in the exponential decay equation. The value of
1.4 Volts
-0.02
-0.02
5
4
3
2
1
0
5
4
3
2
1
0
is the amount of time it takes for the voltage at I/O pin P15 to fall
-0.01
-0.01
RCTIME
0
0
V
1
0.01
0.01
can be used to make the BASIC Stamp determine
V
2
V
0.02
0.02
3
V
V
V
V = 1.4 V
1
2
3
t
→
→
→
0.03
0.03
R = 5 k
R = 10 k
R = 15 k
2
3
1
Time, (Seconds)
0.04
0.04
t
Ω
Ω
Ω
that gets measured is t
V
t
0.05
0.05
Figure 8-2
RC circuit
Responds
To a negative
step input for
three different
resistance
values. Note
how each output
takes a different
amount of time
to decay from 5
volts to 1.4
volts.
1.4 Volts
.