Looking for a quality substitute for a 640 style
mercury cell for
your Pentax Spotmeter? Here it is!
This device is designed to allow you to use a common, inexpensive,
three-Volt lithium CR1632 cell in your spotmeter without the problems
of zinc-air or silver oxide cells. This device fits down inside
the Asahi Pentax Spotmeter 21/1 and the Pentax Spotmeter III's PX640
battery compartment, accepts the
three Volts of a
CR1632 lithium cell, and regulates the voltage to a rock-steady 1.35
Volts. Gone are the short-idle life spans of Zinc-air cells, or
the voltage-drift of silver oxide cell solutions. This device
provides a stable 1.35 Volts to your spotmeter for the life of the
lithium cell, and the voltage won't drift downward as the
lithium cell discharges.
Accurate readings of the light levels is crucial to getting a good
exposure. There are silver oxide cell solutions to the problem
that use Schottky diodes, but the voltage drop of a diode
increases with the current through the diode, so high light level
readings will be too low compared to low level readings.
Additionally, as the silver-oxide cell decreases in voltage over its
life, the output voltage level will drop correspondingly. There
are zinc-air cell solutions, but once installed, zinc air cells will
expire in a month or two even when not used. The
regulator circuit in this new device doesn't suffer from voltage droop
caused by higher operating currents as do diode approaches. And
the lithium CR1632 cells are inexpensive compared to the silver oxide
cells, pack more operating time, and don't self discharge like zinc-air
cells.
You only need to buy this device one time for your spotmeter, and
simply replace the three-Volt lithium cell as necessary. Since
the device fits down inside the bottom of the battery well, you won't
even need to remove it between battery changes. This device has
an extremely small standby current requirement, and a new CR1632 cell
used
with this regulator will self discharge in two or three years of idle
storage. The CR1632 lithium cells may be bought on the web, and I
also offer the cells here.
These adapters are for use only on those photometers that have a
metal-sided, single battery well. The device contacts the sides
of
the
well to
get the positive voltage from the cell. The adapter will not
operate inside an insulated sleeve.
These regulator devices are hand built, so please refrain from ordering
more than
two per order. They cost $30 each in U.S. dollars, plus a
fixed-rate shipping cost of
$1.50 per order to anywhere in the world. I
include
one lithium CR1632 coin cell per unit as a starter, as well as
installation
instructions. You may also purchase extra CR1632 coin cells at
$1.25 each. To order, print and fill out the order form below
and send it to
me with your check, money order, or cash of the appropriate amount
payable to Duane Becker. My bank accepts and converts all
international monetary units for me for free.
Updated - April 19,
2008 - I
do still sell these adapters, and their popularity is growing.
Instruction Sheet (pdf)
Order Form (pdf)
Solving An Intermittent Spotmeter Problem (pdf)
Email me with any questions (please
include a subject in your note)
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