First of all is it ok? to use Bjts (2n222) in an inverter rather than Mosfets
secondly i have designed a circuit i i'm having trouble with the ratings of resistors , plus the circuit is operating accordingly when both the voltages(the voltage to be inverted 5v and the source voltage of the 555 is 5v ) but as i change the voltage to be inverted to 12v keeping the 555's supply at 5 the inversion is not as according to the principle ,is it need to keep both the voltages same?

 

If the '555 is operating with a 5V supply, but the '7404 is running on a higher supply voltage then the output from the '555 (max 5V) will not cross the input threshold of the 7404.

 

If the '555 is operating with a 5V supply, but the '7404 is running on a higher supply voltage then the output from the '555 (max 5V) will not cross the input threshold of the 7404.

And why is that?

 

Hello,

The 7404 is a TTL chip. It has a supply voltage range of 4.75 to 5.25 Volts.
Running it at a highr voltage will damage the chip.

Bertus

 

If there were no other issues, you would get about 2-3V out of that bridge. You need PNP transistors for the high-side switches. That will get you up to about 4V. MOSFETs would bring it close to 5V, which is why they are preferred.

Bob