122-28176 Parallax Inc, 122-28176 Datasheet - Page 240

• Manufacturer Part Number: 122-28176
• Description: GUIDE STUDENT PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 122-28176

• Accessory Type: Manual
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Propeller Education (PE) Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Additional Information
The op-amp can be employed here to control an on-off system without the expense of a
microcontroller, ADC, and all the additional hardware. While we won't ask you tear
down your circuit for this, you may wish to try it at a later time. Consider the schematic
in Figure 7-6.
Recall the output of the op-amp attempts to drive the inverting input (−) towards the non-
inverting input (+). In this configuration, if the temperature is lower than the setpoint
(1.015 V = 101.5 °F), the op-amp output will go high in an attempt to drive the inverting
input down to the non-inverting. But the output is NOT connected to the inverting input,
so the op-amp will drive fully to the positive supply-voltage rail in a futile attempt at
affecting the inverting terminal. This causes the transistor to go into saturation, turning
on the heater.
Once temperature rises above the setpoint, the op-amp will drive to the other rail voltage
(ground/Vss) in an attempt to lower the inverting input towards the non-inverting input.
This will have the effect of turning off the heater. Thus, the heater will cycle around the
setpoint.
This op-amp configuration is called a comparator because its output is based on a
comparison of the two inputs. A digital signal output is based on comparing two analog
voltages.
Figure 7-6
Op-Amp On-Off
Control Circuit
DO NOT BUILD