In Reply to: Re: Why do you want to do that? posted by sicminds on 11/30/02 at 11:52 AM:
to give you the complete circuit would almost (will) give away the perceivable purpose of such a wonderful school project (i'm sure).
i'd be happy to give you some hints and tips towards the design if you like.
now, if i understand you correctly you need to compare two voltages against a particular level that would define the switching of a higher voltage (namely, 4.5V).
From a 12V source you can regulate it down to 4.5V. Compare the voltages (B and C) to a divided voltage of 1.5V. This can be done using op-amps (look this up in a good op-amp book, or read your text book). If right, switch output to receive the 4.5V. This could be done in a number of ways - i'll leave that to you to work out. (transistor, relay, etc.....)
current may be a concern. be wise with the circuit's requirements. read the data sheets to be sure it can handle it.
have fun. i hope it helps.