I put most of my projects on hold due to the lack of time, but Im getting back to them very soon.Are you people still building this ? Created an account just to ask. This was easily the most interesting craft from Metro for me.
EIt is looking great so far. I think I spot some Lego bits in there.
The first thing to try is turning the screw on the front of your volt meter. It is there to allow calibration of the meter, but it probably wont turn far enough to compensate for the 5 volts.
The next thing to try is to take apart the existing meter you have now, and change the internal resistor. After all, a volt meter is just an amp meter with a fixed resistor added to make it work at a certain max voltage. Take a look at this analog panel meter clock for images of the resistor inside.
If it doesn't happen to have a resistor inside, then you will need to get a Amp meter instead. I searched eBay for "Analog Panel Meter Amps" and found some cheap models that look identical to your volt meter. You can always swap out the gauge face like the clock guy did above.
If you get an amp meter say rated for 10mA, it would take less power than a single LED light to actuate that meter. You then wire up a potentiometer in-line with the meter. This allows you to vary the resistance of the meter. Allowing you to calibrate the meter's maximum setting to match the maximum output your already getting from the generator. You may even end up with enough power left over to drive some LED lights.
Also, a Brushed DC motor should work better as a DC generator. Apparently, a NEMA 17 (Stepper) motor can be used as a generator, but needs a set of bridge rectifiers to use the full output of the motor.
Regarding the 5V output: Your hand crank has a fixed amount of hand power (Watts) that you adding to the system. Any gearing can't actually add any power to the system. It can only multiply/divide the rotations per minute, changing the low RPM of the input to a high RPM at the output. (The law of the conservation of energy.)
As an example: This brushed motor runs at 3500RPM, and consumes 3.2W @ 24VDC. (0.13 amps). To get 24V and 0.13A from this motor, you need to spin the input shaft at 3500RPM. However, if your pulling more than 0.13A from a connected circuit, then the voltage will drop.