I am designing a circuit that will turn on a Green LED at normal room temp. When room temp exceeds approx 82° Fahrenheit a Red LED will turn on. I am using a PNP transistor as a switch to turn on one LED and an NPN transistor as a switch to turn on the other LED.
I am also wiring the LED's as a parallel load to the transistors.
PARTS I AM USING:
Thermistor, op-amp wired as an open loop comparator, NPN & PNP transistors, red & green LED and a bi-polar power supply that is supplying +5V and -5V.
Thermistor is a 10KΩ @ 25° Celsius with a Negative Temperature Coefficient of 460Ω per degrees Celsius
SO FAR:
NEED HELP:
The thermistor works and is at around 990Ω at room temperature (72° Fahrenheit) and around 680Ω when I pinch the thermistor so roughly (98° Fahrenheit).
The op-amp IC is working properly with PIN 4 to +5V and PIN 11 to -5V with proper voltage levels.
All resistors are showing their ohmic values and the LEDs are working correctly and are biased correctly.
I am stuck on why this circuit is not working.
I appreciate any feedback. I hope my description helped.
You can also text me when you are going to post so I can see the follow ups. 309-716-4176 (AT&T Wireless)
-EmptyFireFly
I am also wiring the LED's as a parallel load to the transistors.
PARTS I AM USING:
Thermistor, op-amp wired as an open loop comparator, NPN & PNP transistors, red & green LED and a bi-polar power supply that is supplying +5V and -5V.
Thermistor is a 10KΩ @ 25° Celsius with a Negative Temperature Coefficient of 460Ω per degrees Celsius
SO FAR:
- Wired a voltage divider on the negative input of the op-amp using a 100KΩ resistor pull up resistor and a 100KΩ resistor to the pull down (to ground).
- Wired a voltage divider on the positive input of the op-amp using the 10Kohm thermistor pull up and a 100KΩ resistor to the pull down (to ground).
- Output of the op-amp is split into to two 25KΩ resistors, each running into the base of each transistors acting as a switch.
- The NPN has a pull up resistor of 180Ω to +5V. The emitter is wired to the collector of the PNP and the emitter has a pull down resistor of 180Ω to -5V.
- The LEDs are wired in parallel to each transistor to light whether the transistor is in cut-off or saturation.
NEED HELP:
The thermistor works and is at around 990Ω at room temperature (72° Fahrenheit) and around 680Ω when I pinch the thermistor so roughly (98° Fahrenheit).
The op-amp IC is working properly with PIN 4 to +5V and PIN 11 to -5V with proper voltage levels.
All resistors are showing their ohmic values and the LEDs are working correctly and are biased correctly.
I am stuck on why this circuit is not working.
I appreciate any feedback. I hope my description helped.
You can also text me when you are going to post so I can see the follow ups. 309-716-4176 (AT&T Wireless)
-EmptyFireFly
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