a google of '555 timer' produced lots of stuff, one of which was this https://www.eleinmec.com/article.asp?3 about half way down the page, it had the circuit you need. You say it "shold ganrate strong output" I assume this means you want to drive something quite powerful, in which case connect the output of the 555 (pin 3) to the 'base' lef of a suitable transistor.
do you mean you want a square wave that goes high at 23v and low at 17v? If so, can i ask why? or do you mean you want a square wave that goes high at somewhere between 17v and 23v?
The maximum supply voltage for 555 is 18V so you can't use them into your case of 23V. The logical CMOS is also limitted at 18-20V supply.
If your application don't request short edges of square wave, you can use a operational comparator as LM393(dual).
But if your app request short edges you are limitted to transistors implementation or usage of some special IC working over 23V.
ANother version is to make a voltage step down regulator to some volts for supply a 555 which will drive an output transistor that will work with collector or drain at 23V.
If you post the complete requirements for output signal we can help you with a schematic.