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06-27-2019, 09:06 AM   #1
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Honeywell Pentax 1/21 Spotmeter DIY Replace Mercury's battery with a regulator


A few days ago I bought a Spotmeter for $ 40 dollars.

I thought it was the V version but when I had it in my hands it turned out to be the first version that uses two mercury batteries: a dry 9V battery and a 1.35V button type.

I have done some research on mercury batteries:
Mercuric Oxide: Mercury batteries were commonly used in many classic cameras of the 1960s and 1970s for their CdS (cadmium sulfide) metering systems. Mercuric oxide batteries provide a constant and stable 1.35 volts over most of their life. This means that the camera does not have to have voltage regulation circuitry, considerably lowering the cost of production. Unfortunately, mercury batteries were outlawed in the U.S. in the 1980s and are not readily available anymore.

So, this meter uses mercury batteries because of the voltage during discharge remains practically constant at 1.35 volts and 8.4 volts. A very stable voltage indeed.
The 9V (8.4volts) battery feeds the LO meter and the 1.35volts battery feeds the HI meter mode and the lamp:

How do we get a steady voltage for the HI mode meter without modifying the internal components (much)?

I have been thinking and there are no simple solutions but I have concluded that two modifications can be made:

The first is to take advantage of the 9V battery and hang a 1.35V linear regulator to power the HI Mode INSIDE the meter. The drawback is that we will have to be attentive with the voltage level of the 9V battery so that it stays in the black bold line suggested by the manual.

The second option is to apply the first option and make an "24V to 9V" adapter: consisting in two A23 batteries in series, which I believe fit perfectly in the size of the 9V battery space, but additionally we would install a linear voltage regulator of 9V.

The spot meter has NOT power on switch and 'cos the manual says that the 1.3V battery last about a year so the power consumpsion is very little. Maybe the housing power for the 1.3V can be recycle to insert a micro swich to avoid drain the power from the regulators.

I'll be using a LM317L for the 1.35V and the LM78L09 for the 9V for this DIY hack and I will be posting images and progress updates soon.

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06-27-2019, 11:44 AM   #2
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As long as you are in there modifying things, you might consider using the lamp switch to enable the power circuitry, and replace the lamp with a dim (night only) LED that is on whenever the circuitry is on.

Also, the "dry" battery is a carbon Leclanché cell that alkaline batteries have almost completely replaced. These dry cells had/have a voltage-charge curve that is not flatter than that of alkaline batteries, and without analysis my guess is that the unit is designed to compensate by the balance resistors across the meter. Whether these are internal calibration or set by the user is unclear. You may be able to get away with an alkaline replacement and limit the power supply function to mercury battery emulation.

Further, I recently rebatteried a TDS meter that used mercury batteries. There exist hearing aid batteries that are zinc air based and nominally 1.4V. However, the data sheets show that they typically operate at 1.27V under light load. The limitation is that they age out even unused, once exposed to the air, so they provide only short term service. I'm not sure how long short-term is; the TDS meter was rebatteried about 3 mos ago and still works. These batteries (in the size my TDS meter used) are denoted 675 size. I expect other sizes of hearing aid batteries are similar. Even if you go to a power supply solution, you could test your meter with these batteries.

Last edited by kaseki; 06-27-2019 at 12:09 PM.
06-27-2019, 01:54 PM   #3
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QuoteOriginally posted by kaseki Quote
Even if you go to a power supply solution, you could test your meter with these batteries.
That Zinc-Air batteries's steady voltage... What a nice surprise!

Energizer Az312dpa8 with 8pz for only $5usd

This Weekend I'll try to make a "pack" of 6 x 1.4v to get the 8.4V

Thank You
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Last edited by carabez; 06-27-2019 at 01:55 PM. Reason: price correction
06-27-2019, 02:23 PM   #4
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You're welcome. If this becomes your permanent solution, I recommend keeping a full set of spares handy for whenever the precipitous decline occurs. 200 hours service life is only a bit over a week for a hearing aid; so I imagine these batteries are changed often in that application. However, I haven't come across a lifetime specified for an opened but barely used Zn-Air battery.

Edit: Found this comment in a July 2005 forum thread at"
FYI, the life expectancy of the Wein cells, or generic Zinc-Air cells, is a function of the humidity of the climate you live in. In a dry climate like southern California or Arizona, they will only last 4-6 weeks, once activated by unsealing the anode. In a humid climate, they might last 4-6 months.
I should add that technical comments I have found suggest that low humidity dries out the gel, but high humidity makes it too wet. There is probably an optimal value of humidity, not yet found by me.

Last edited by kaseki; 06-27-2019 at 03:26 PM.

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