Mercury PX625 batteries are obsolete.

My friend Martin asked me if I could mod his light meter, so it could be powered by normal AAA batteries.

The required schematic is very simple, a mercury battery gives an exact 1.35V, so the 2 batteries from the light meter together, give 2.7V. I just use a suitable small regulator, in this setup :

To make it small enough, I built it all on a tiny footprint adapter, which fits into the battery compartiment.

Add the 2.7V regulator to it :

Add the 2 SMD capacitors (can you spot them ?)

And added a clip to it, so it can slide over the battery contact (when I bend the battery contact upwards)

Added some isolation, the whole thing plugs into the battery compartiment like this :

Now just add a 3xAA battery holder and... ready ! It works well.

The light indicator works !