Cottage Tip: Building a small exposure meter-Part III

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).

 

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.

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.