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Okay, so basic idea is that when signal voltage is changing from 0V to 5V and vice versa (from 5V to 0V) its triggering some kind of circuit with external power source to generate 0,5s - 1,0s voltage pulse with a value of 5V and ability to handle around 100mA. Signal voltage change freq. is around 1 voltage change per 8 hours, so it does not have to be very fast ;) I couldn't find any device online, if something like this exists pls let me know, also I have no idea how to design such a circuit. I would appreciate any help.

Im not a native English speaker, so if someone feels hurt by grammar Im sorry:)

By these days, a very common and relatively cheap way to do that is by using an MCU (micro controller unit) like an Arduino. You will have to set two of its GPIO pins (General Purpose Input Output) to the signal and, by software, fire an interrupt on a raising edge (passing from 0 toward 5 V) and the other pin on the descending edge ( 5 V to 0 V). Next, in the interrupts handler subroutine, you may raise the signal on other pin(s), as long in time as you please, eventually to a BJT or MOSFET if you need more than 5 Volt. Since your montage will be based on the front (raising or descending) of the signal, if the signal STAYS low or stays high for a long time, that won't matter, since only the change would raise the interruption. And using software, that make the montage simple to modify ( duration of the generated pulse, include other "sensor", be depedant of the time of the day, or of the weekday, etc. ).

NOTE: for 100mA, you probably need to inferface the OUTPUT of the MCU to a transistor (BJT of MOSFET) since most MCU are of limited current for output.

NOTE: There is a new MCU star in town, the Raspberry Pi Pico, which allows you to use Python code, as software. That may be an interesting plus. And why not, allowing you to use WiFi too!

Thanks a lot! I think I'll try to do this with arduino. Wish me luck :D

An Arduino is generally agreable to work with, even for experimentation or for acquiring experience. :-)