28128 Parallax Inc, 28128 Datasheet - Page 15

• 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

Figure 1-4 shows what happens inside the pot as it is adjusted. The jagged line represents
a resistive element, typically made of carbon. One end of the resistive element is wired to
Vdd on the Board of Education, and the other end is wired to Vss. The middle of the
three terminals is connected to the “wiper”, and it’s where the variable output voltage is
measured. The wiper stays in contact with the carbon element as it moves.
As the wiper gets closer to Vdd, the voltage measured at the wiper terminal will approach
the value of Vdd, which is 5 volts. Likewise, when the wiper is closer to Vss, the voltage
at the wiper terminal will be closer to Vss, which is 0 volts. As the wiper terminal travels
between Vdd and Vss, the output measured at the wiper terminal varies between these
two values in a manner analogous to a door as it opens and closes.
An op-amp (operational amplifier) is a building block commonly used in analog circuits.
Figure 1-5 shows the circuit symbol and block diagram for the LM358 op-amp used in
this experiment. The op-amp circuit used in this experiment is called a voltage follower
because the same voltage comes out as goes in. In other words, the voltage at the output
"follows" the voltage at the input. The reason it's used in the circuit in this experiment is
to electrically separate a potentiometer circuit from an LED circuit. We'll learn more
about the usefulness of a voltage follower in Chapter #4.
The LM358 Op-amp
Vdd
Vss
+
0-100 k
Pot
Ω
Figure 1-4
Potentiometer Wiper
Showing how the wiper
in a potentiometer
travels along the surface
of the resistive element
as it’s adjusted.