When
I finally got a working panel voltmeter (measuring range 200 mV) I got an
unpleasant surprise; it needs a separate supply, other than the circuit, but it
wasn't specified in the catalog I bought from. There are also voltmeters able
to share the same power supply with the rest of the circuit (the so-called
common-ground type), but unluckily not this one. So I needed first to make
somehow a split power supply with the same battery for both the sensor circuit
and the voltmeter, with the available resources. So I redesigned the circuit a
bit. The aim is to keep both voltages (for the meter and the sensor) as »far
away« as possible from each other, preventing to get faulty readings. Below,
see the circuit schematics.
After
that, I needed to do a quick test to such a circuit on the breadboard. I also
needed to do a short-circuit on one contact pair on the voltmeter, in orderto
get into the 200 mV range (following the instructions). Oddly, there was already
one short-circuit present (factory-made). These »shorts« also set the decimal
point. So two decimal points are now present. Nothing dramatic, only funny
somehow (note in the photo).
As
the weather wasn't really appropriate (for several days) to fully test the
meter outside and in sunny conditions, I relied on my instinct so I soldered
the final circuit on the prototype board.
I
then made the apertures on the enclosure for the display, the sensor and the
activating switch. The battery holder is just an ordinary AAA battery holder
shortened by about 15 mm and glued together (the batteries A23 and AAA are of
the same width).
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the battery holder |
Finally,
I put everything inside the enclosure: switch, panel meter, sensor, circuit and
battery-using solder and glue. As it seems, it works nicely! Now, a necessary
addition should be to mount (below the panel meter) also an exposure
calculator wheel to be able to set the exposure settings. But I first need
to buy some plastic printable film to do that.
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The inside of the exposure meter |
|
The finished "product" |
Should
the precision of the meter be a bit off, I will still need to change the Zener
diode in charge to power the panel meter, i.e. using one with a bit lower
voltage-just enough to ensure the meter to work, by changing the existing diode
in the circuit with another one.