RM640R mercury battery adapter

thingiverse

This is a simple adapter to replace the obsolete RM640R mercury cell used in old cameras, light meters etc. with a modern button cell. I made it for a vintage Pentax spot type exposure meter I bought. It will fit LR44 alcaline, SR44 silver oxide or 675 air-zinc hearing aid battery. I currently use a 675 air zinc which has approximately the same voltage and a flat discharge curve as the original mercury cell so it's preferable for voltage sensitive devices but has a short shelf life after being activated. They are quite cheap though so not a huge problem. SR44 also has a flat discharge curve but the higher voltage would cause the meter to read incorrectly. LR44 has a sloping voltage curve so it's suitable only for devices that do not require a constant voltage to be accurate. I used a strip of aluminium tape as a conductor through the hole which is offset as there's a raised contact point in the center of the battery cover. A piece of wire or a strip of thin metal glued on will also work. I'm planning on making one with a schottky diode to drop the 1.55V of a SR44 down to the required 1.3-1-35 volts.