Much like this question, I am trying to interface a Raspberry Pi 3 Model B to an alarm system, but this time is is a GE NetworX NX-8V2. I have snooped on the control box with a voltmeter and it looks like when a sensor is tripped (i.e. a door is open, a motion detector detects motion) the corresponding pin goes from 0V to about 0.3V. What I would like to do is monitor the pins with my RPi, so I figure that the input voltages to the RPi will need to range from 0V to 3.3V in order to work with the GPIO pins.
However, I have a problem.
My idea is to use an op amp (I have a couple of LM2902N's lying around) to multiply the voltage. Using the op amp as a non-inverting amplifier, and choosing my resistors as shown in the schematic below, I should see a gain of 11, which should give Output = V0 * 11. When I simulate the circuit, it works exactly as I intend. However, when I implement the circuit on a breadboard the output stays at a constant 3.9V, regardless of if I implement the circuit as designed or disconnect the V0 input. (Another data point: when I tested with V1 set to 3.3V but everything else the same, the Output stayed at a constant 2.1V)
Any ideas on why my circuit is behaving like this? (Or if you have another way of solving the problem, that would help too. Another idea I've considered is to implement a voltage comparator with Vref at 0.15V)