with the proliferation of low cost, low drop out voltage regulators, I'm surprised that nobody has come out with a simple electronics based replacement. One would only need a 3.0v lithium cell, a 1.35V LDO regulator and a few discrete components. I've plans to retrofit the low voltage side of my Pentax Spotmeter with such a circuit but powered from the 9V battery that's already there...just haven't "got around to it" yet.
Such has been proposed and done. For instance with the microchip TC1015 for devices running on two mercury cells.
However this chip necessitates a on/off-switch, otherwise it would drain the battery itself.
That is one reason to use instead of a seperate control circuit a Schottky diode in-line.
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