Quote Originally Posted by lerameur View Post
the only fix voltage I have now is my Lm7805 at 5v.

and also, what is the lowest voltage for 4 Nmhi batteries, (when needed to be changed)

well I removed the Lm7805, , i dont have any fox voltage reference. also, leaving the LM7805 there, i could not get enough voltage for the unit to operate
Read the datasheets for the 7805 and you'll find out why you can't get enough voltage from them, they need a couple of volts of overhead to operate correctly.

Lowest volts for NiMH batts - really depends on the current draw, the higher the draw, the higher the end voltage. I suspect for your application, you might want to charge them up when they hit 1v/cell (4v total). That will give you some time to get to it. I wouldn't let them get much below .8v/cell, but then they're on their way out anyways. As far as charging, take them up to about 1.4v/cell depending on the charge rate, faster charge, higher voltage (up to a point of course), slower charge, lower voltage. Overall, 1.4v is a good round number.

More on the easy voltage reference later. Here's a bit of a teaser for you: take a diode, any old diode, a resistor and a 9v battery. Hook it up as follows, +9v to resistor, resistor to one side of diode, other side (banded side) of diode to -9v. Read the voltage across the diode, just the diode and only the diode. Now change the value of the resistor, read the voltage across the diode again, change the resistor again, read the diode again, swap the 9v battery for the 4cell NiMH pack, read the diode again. Let me know your results.