I'm trying to build a circuit to monitor the temperature of an IC, it's a TPA3118D2 in a guitar amp. I know I can do this with an Arduino but thought I might try something of my own and see if I can learn something. The sensor is a
TMP36 which outputs a voltage between 100mV and 2V, 750 mV at 25°C and each degree Cº increments the output by 10mV.
I attached the schematic of the circuit I came up with. It consists of a voltage amplifying opamp feeding a voltage follower transistor as a current source for the three LEDs, each with different cutoff voltages to have them lit up at different temperatures.
The LTSpice simulation looks ok to me, the green LED reaches 0.5mA and gets lit at about 1V in the input (50ºC), the yellow LED reaches 0.5mA at 1.2V or 70ºC, and the red one at 1.4V / 90ºC
So far I have breadboarded the opamp section, and it's not working at all. I put 0 to 2V in the input through a voltage divider made with a 1M resistor to a 9V battery and a 250K pot to ground. With the pot at 250K I get 1.8V as should be, but as soon as I connect the opamp input to it, the voltage drops to zero and of course nothing at the opamp output either. Aren't opamps supposed to have high impedance inputs? I'm at a loss
Any suggestions please? Thanks.
EDIT: added the LTSpice asc file in case it helps