Convert 0 to -150V DC to 0 to 3.3V
Something like this should work if the source of your pulse has a reasonably
low impedance. The MCU input would preferably have a Schmitt trigger input type, or add a ST buffer before the input. If the load on your 3.3V supply is at least 1.5mA it will be able to handle +150V/-500V transients (limited in the negative direction by the resistor surge rating and voltage rating). Transition (1.65V) is at about -33V, so the pulse at the MCU should be around 150usec. 







simulate this circuit &ndash; Schematic created using CircuitLab