Single supply Schmitt trigger with hysteresis voltage. In reality this circuit for me works only with op amps that have low slew rate ( few hundred mV/us to 2V/us ) aka with slow op amps. The value of the base resistor in reality needs to be checked for stability. Reference voltage is applied to the non inverting input. R1 is the input resistor 1k, R2 is the other one, 2k. Hysteresis voltage can be calculated with this equation:
U_hyst = U_ref * R1 / R2
