Maximum voltage allowed in trigger pin of 555 bistable
Per the 555 datasheet, absolute maximum voltage on the trigger pin is equal to VCC.

The easiest way to protect the pin is to use a series resistor and a schottky diode to VCC:







simulate this circuit &ndash; Schematic created using CircuitLab

The resistor should be sized sufficient to limit the current shunted through the diode; anything in the range of 10k to 1M is likely to be fine in your application.

In a pinch you can omit the diode and rely on the protection diodes in the 555, but it's generally considered a bad idea to do so.

Since your output is AC, too, you will need an additional diode to block negative voltages, or a resistor divider to shift the voltage appropriately.