I repaired a guitat tuner today - blown 78L05 as
no supply reversal protection.
I was intrigued by another 3 pin TO92 device marked
8054
HN
3C31
Presumably working order - just wondered what it did.
Probably 8054HN not 8054 datecode , by Seiko,
activation voltage 3.8V to 4.2V, battery monitor.
One line is connected to INVERSE READ of the 80C51 micro
, I didn't trace any other lines. But what would it
do presumably when 5V drops to 4V or so. Just kill
the LED drives or shut down completely via some other
undiscovered trace. Seems odd having this sophistication
but no diode protection against battery or ext ps reversal,
especially in a product destined for musos.

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse