Circuit for generating one pulse?

[Technological]

Thanks everyone for your replies!

The BMS is activated by applying battery voltage (Li-ion 20S 85V) to it's "power in" pin and then applying 3-5 V to separate two "boot pins" for a short moment of time to activate boot.

I'm going to activate the BMS with a mechanical switch and with a wireless link. The activation signal is full battery voltage on a wire for as long as the BMS is needed to be powered on. The activation signal is going to control a MOSFET switch consisting of two MOSFETs which applies battery voltage to BMS power in and 3-5V to "boot pins".
The manufacturer suggested to remove voltage from "boot pins" afterwards.
The manufacturer didn't think that anyone would want to power the BMS on and off. They designed it to boot once and leave it on.
When I need to power the BMS down, the voltage on the control wire is removed thus removing voltage on the "power in" pin.

I was able to find LR8N3 linear regulator in my local electronics store. It can accept 17 - 450 V of input voltage to produce 3.5 V of output. I'm going to connect it to BMS "boot pins" for as long as BMS is needed to be powered on and neglect power consumption of 2mA on this pins.

I would use the solution provided by EPAIII using 74HC if my arrangement shows any drawbacks.
I think using 74HC is the most simple and compact circuit for the task.

[RogerThat]

Simplest way: 74HC14

on one of the inputs you put a capacitor in series and then a pull down resistor (AC coupled). Adjust the capacitor and resistor values to adjust the 'on' time of your pulse.