I want to design a schmitt trigger which triggers 3.3V above -8V (independent of whether inverting or non-inverting schmitt trigger) when the input voltage is greater than 14V and less than -6V. my total supply voltages are +15V and -8V and 3.3V above -8V (i-e -4.7). My comparator is a open collector comparator which can be connected to 3.3V to match to the output logic. The output signal of Schmitt trigger should go to microcontroller.
As my output voltage is allowed to be in range of -8V and -4.7V (uC GND is -8V and Vcc = -4.7V), I dont know how to calculate R1 and R2 to reach this. If I calculate using +Vsat = -4.7V and -Vsat = -8V, I get negative values of Resistors.
e.g to trigger at upper threshhold of +14V.
14V = -4.7V (10K) / 10K + R1 => R1 = -13.35 KOhm
Could you please tell me where I am wrong. How to do this ? Should I use voltage divider at inverting input to scale +15V - (-8V) = 23V to 3.3V frame ?
I expect hysteresis like shown in a graph.