Convert 1.5 Vdc circuit work with 5 Vdc
The question rambles too much for a single clear answer, but look up something called a voltage regulator or linear regulator.  These take a high and possibly varying voltage in, and produce a lower and steady voltage out.

There are voltage regulators available with fixed output voltages.  I haven't looked whether any are available with fixed 1.5 V output, but that would be easiest if you can find it.

In this case, you're starting with a reasonably steady 5 V from USB.  It can vary, but so can a "1.5 V" battery.  A simple voltage divider driving a emitter follower would probably work well enough as a crude voltage regulator.

Here is what I'm talking about:



This is not a good approach in general because the output voltage is a direct function of the input voltage.  In this case it's probably good enough since the USB voltage is close to 5 V.  In effect, this circuit uses the USB voltage as the reference that the output voltage is derived from.

More general voltage regulators contain their own independent references.