Real time Digital to Analog converter
First and foremost, try to use the signal digitally.  The usual strategy is to convert analog measurements to digital early in the process, then do everything digitally from there.  Digital values don't degrade over time, drift, or pick up stray noise.  They can also be way more accurate than analog signals just by using sufficient numbers of bits.

Sometimes, after all the digital processing, you still need to control a analog real-world system.  However, even that is often not done by converting the final digital value to a voltage, then using that to drive the system.  Nowadays, we try to drive the system with pulses, which are easy to produce directly from the digital values with PWM generators built into just about every microcontroller.  Even things that appear analog, like motors and solenoids, can be controlled by pulses.  In fact, driving motors and solenoids with pulses is more efficient.


If you really really need to convert a digital value to a analog voltage, use a D/A chip.  Others have already done the engineering and put it all in a nice and convenient package for you.

Consider accuracy.  With 1% resistors, you won't even get 7 bits.  You can 12 bit and more D/A chips for very little money.  There is no way you will be able to compete with those by building your own from discrete parts.