battery education sought

I am not at all convinced that the environmental harm done by mercury batteries (particularly if correctly recycled is worse than the damage done by all the equipment that was replaced becasue you can no longer get the right battery. But anyway... To go OT for a momnet, a lot of 1950's/1960's cameras used a mercury cell to power the exposure meter. The constant voltage meant the circuit could be very simple (just the CdS photoresistor, the mercury cell, a microammeter movement and a couple more resistors, generally). Now that such cells are unobtainium, there are various kludges, some of which work better than others, but it's still a pain. Incidentally, I was told that the Eastern Bloc cells were anything but constant voltage, which means that cameras from those countries, like Exaktas and Prakticas tended to use a wheatstone bridge circuit for the meter, with correct exposure being at the balance point of the bridge. Of course that design doesn't care about the battery voltage, so such cameras are still easy to keep going. Unlike that ridiculous circuit in the Pentax Spotmatic. It's a weatston bridge alright, but the correct exposure point is not when the bridge is balanced, it's when a certain, small, current flows through the meter. It therefore is dependant on battery voltage. The official justification is that a totally flat battery can't give the correct exposure indication, to me it appears they got the design wrong.... I've not pulling mine apart yet, and the 'service' manual on hpmuseum.net doesn't include schematics, but I will bet you're right. I am pretty sure the HP150 (original version [1]) used a similar battery arrangement, and it maintains a 5101 CMOS RAM chip. I have schematics for that. [1] On the 150-II, there are a couple of lithium (?) cells soldered to the PSU/sweep PSB. With a bit more circuitry, they supply a backup supply to a pin on the Elmer custom chip, which contains the configuration RAM. Or at least that's what happens on most machines. On some early ones, including mine, the Elmer chip wasn't fitted, becasue apparently it wasn't ready in time. Instead, there's a header plug soldered to the main PCB in placeof the Elmer chip, and a little daughterboard containing a handful of chips is plugged into it. One of thsoe -- and the one that the battery maintains -- is a 5101 CMOS RAM. FWIW, I do have HP150 schematics (both versions). -tony