Rangefinders with bridge circuits.

Discussion in 'Rangefinder Forum' started by Kiron Kid, Mar 6, 2018.

  1. Kiron Kid

    Kiron Kid Member

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    I have read on numerous occasions, that quite a few of the older rangefinders have circuits that automatically compensate for voltage difference with batteries. Is this true? If so, are they listed somewhere?

    Russ
     
  2. jim10219

    jim10219 Member

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    If you wanted to, you could have someone good with circuits fix the issue in just about any rangefinder you want. Most of the time the problem is newer replacement batteries have too much voltage. It’s easy to just add a resistor or zener diode to get the proper voltage while still using modern batteries.
     
    Last edited: Mar 6, 2018
  3. polka

    polka Member

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    There is one kind of automatic rangefinders that I know do not mind battery voltage : those with so called "electronic shutters" like the yashica-s "electro something" or the minox35-s.

    Because they build the exposure time by comparing the progressive charging of a condenser through the cell resistor with voltage references given by fixed resistors (depending on the Fstops).

    As both the cell resistor and the fixed reference resistors are attached to the same battery, the compensation for battery voltage is indeed automatic.

    On the opposite, all the camera-s that use a galvanometer to move a needle or to automatically close the diaphragm are sensitive to battery voltage.

    The case of the minox35-s is somehow special : they use an electronic shutter to build the exposure time,but besides, have a second cell attached to a galvanometer which moves a needle in the viewfinder to warn for overexposure (shorter than 1/500) or slow speeds (less than 1/30). This may not be taken exactly at face value, but anyway, it does not matter for the actual shutter speed.

    Polka
     
  4. AgX

    AgX Member

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    A number of Agfa cameras from that period (types 135 and 110) have got 2 sensor cells.
     
  5. E. von Hoegh

    E. von Hoegh Member

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    Not at all true. Pentax Spotmatics, for one example. One of the Gossen meters has a bridge circuit as well, possibly the N-100?
    As a very general rule (with exceptions), any meter either in camera or handheld in which the needle indicates correct exposure with all power removed, has a bridge circuit.


    As for the obsolete cell problem, there are tiny low dropout voltage regulators that will give a regulated voltage from most any source, say a 3 volt lithium cell for 2.7v out the regulator.
     
  6. ic-racer

    ic-racer Member

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    Most older rangefinders of which I am familiar, don't have meters. In terms of stable voltage for a built-in meter, the Yashica FX-3 (1980s) had a voltage regulator in its simple meter circuit though that camera never used mercury batteries anyway. So I have added nothing to your quest.
     
  7. AgX

    AgX Member

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    What do you mean by "with all power removed" ?
     
  8. jim10219

    jim10219 Member

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    He means with the battery removed. Basically, a bridge circuit will most likely cause the needle to rest in the center when no power is supplied. Where as in most other circuits, the needle will rest at the bottom (or top) indicating an underexposed (or overexposed) shot when no power is supplied. A bridge circuit basically has two paths and compares the two currents against each other, one being controlled by known resistors, the other being adjusted by the light sensor in the camera. So when the light hitting the sensor causes the second path to equal the first path, which is calibrated to be the value that is expected by a correctly exposed image, no current is flowing through the meter. Thus the needle galvanometer has no current running through it, so it comes to rest in the middle, just like it would if no voltage was being applied to the circuit. That's why bridge circuits don't depend on battery voltage, because if you increase the voltage on one path, it also increases in the other, but the ratio between the two paths doesn't change. It relies on the balance between the two paths instead of absolute values calibrated against an expected voltage.

    Other types of metering circuits just take the current flowing through the by the light sensor in the camera and use that to directly power the needle's galvanometer. The galvanometer is calibrated to show a properly exposed image when the pre determined amount of current flows through it. So if you change the voltage of the battery, you'll change the amount of current flowing through the sensor, and will thus change the current flowing galvanometer at all light levels versus what is expected. That's why these types of circuits depend on using specific types of batteries.

    Of course, there are easy ways to change the voltage of a battery, so those type of circuits can be adjusted for newer batteries. I suggested a resistor or zener diode, but I like E. von Hoegh's suggestion of using a tiny voltage regulator better. I've only ever used them for dropping a full volt or more in larger circuits, and didn't know you could get one for dropping such small voltages. But they're really the way to go for this type of job. Oddly enough, I can't find any zener diodes that would work for this type of application at such small voltages (I could have sworn I saw some once), so I was way off in my initial suggestion (though a resistor would still work, provided the new battery had a fairly steady discharge curve over it's lifespan).

    I probably didn't explain all of that very well, and it's a bit more complicated than how I laid it all out. But that should give you the general idea of what's going on.
     
  9. E. von Hoegh

    E. von Hoegh Member

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    Thanks you saved me some typing on this tiny screen.
    I'll add "without an ohmmeter connected to the meter circuit".
    I'll find some info. on the regulators when I get to a real keyboard later.

    edit - there's a huge variety of LDO regulators, for instance here is a page listing 1.35 and 2.7 volt regulators, albeit with a far higher output current than needed for camera meters. http://www.onsemi.com/PowerSolutions/parametrics.do?id=366

    I still use 675 hearing aid cells with all but one of the airholes blocked off and a rubber or metal bushing to align it/them in the cell compartment.
     
    Last edited: Mar 8, 2018
  10. E. von Hoegh

    E. von Hoegh Member

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    No battery, and no ohmmeter connected to the circuit.
    Some galvanometers are so sensitive they can be damaged by an ohmmeter, so don't go fishing around with the probes unless you actually know what you are doing.
     
  11. Eugen Mezei

    Eugen Mezei Member

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    Unfortunately the problem is not the voltage difference but the voltage variation. In the old, non-bridged, meters the voltage of the (stable mercury oxide) battery itself is the reference for the measuring. Alcaline batteries have no stable voltage but a continuously declining one with the age of the battery.
    If it were only the voltage difference,it would be simple: you simply set a different ISO.
     
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