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For the class I am taking, we are to design a circuit with a thermistor in a wheatstone bridge. When the thermistor senses a temperature above 100°F or below 60°F it is supposed to turn on a red LED otherwise a green LED is supposed to be on. The circuit I have turns on the red LED at the 100° set point then turns on the green LED at the 60° set point. I am not sure how to fix that. Any help is greatly appreciated.

Function table (to be edited by OP).

Temperature   Green   Red
    < 60°F    Off     On
60 - 100°F    On      Off
   > 100°F    Off     On

enter image description here

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    \$\begingroup\$ Your question is not clear. You said "turn on a red LED otherwise a green LED", which suggests there is a single setpoint. If you need two setpoints, why not use two op amps? \$\endgroup\$
    – Elliot Alderson
    Commented Jan 30, 2020 at 16:51
  • \$\begingroup\$ You have two setpoints in that the reference voltage at PR10 changes with the state of the output, i.e. R9 is causing some hysteresis. Ramping the value of the thermistor up until it changes state, then back down will cause the changeover to occur at different point. \$\endgroup\$
    – Phil G
    Commented Jan 30, 2020 at 17:03
  • \$\begingroup\$ Sorry for the miscommunication. There is a temperature range we are supposed to use; for my circuit it is 60 to 100 degrees Fahrenheit. So above 100 degrees the red LED should turn on and below 60 the red LED should turn on. But between the two a green LED should be on. I would use two op amps like you suggest but I am unsure how to connect them. \$\endgroup\$
    – Joey Sams
    Commented Jan 30, 2020 at 17:05
  • \$\begingroup\$ Don't leave important information in the comments...fix your question. And add all of the information you can. \$\endgroup\$
    – Elliot Alderson
    Commented Jan 30, 2020 at 17:16
  • \$\begingroup\$ Thanks Elliot Alderson. I will edit my question. \$\endgroup\$
    – Joey Sams
    Commented Jan 30, 2020 at 17:21

2 Answers 2

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Without knowing the exact constraint, here is one idea with using two single ended, rail to rail output op amps and some transistors.

enter image description here

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I believe what you are after is a window comparator. Adjust R1,R2 and R3 to get the thresholds that you want.

The graph on the left describes the output with respect to the Vrefs (red horizontal lines) and the input voltage (blue line).

enter image description here http://www.circuitous.ca/Comparators.html

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