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  1. #1
    Bill Burk's Avatar
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    Put a light sensor in my Omegalite-D

    After getting a few uncontrolled variations in some recent prints, I decided to add a light sensor to my Omegalite-D.

    Took a few simple supplies: A couple banana plug sockets, a miniature light socket and a CdS cell.

    I smashed a small light bulb and soldered the CdS cell to the filiment leads.

    Then I soldered a small tube of brass to the base to protect the cell, cut a slot in the brass tube where I can insert ND filters, painted the inside black and the outside white.

    Installing only required some drilling in the upper shell. Pretty basic work.

    Satisfied with the work but wished I had pointed the sensor to a better "integrated" portion of the dome. As it is I have it pointing towards the ends of the bulb, where there would tend to be more variation than average.

    I had done a breadboarded metronome, but I get either no tiks or two tiks per second nothing much else. So if I am going to make that work I'll need some kind of "amplifier" circuit to magnify the resistance changes.

    For now, I am just going to use an ohmmeter to monitor the resistance of the CdS immediately prior to test printing and final prints.

    Can't justify an RH Designs unit because, after all, it is only an Omega DII. But if I put too many hours into the project I might be looking for a real compensating timer.




  2. #2
    ic-racer's Avatar
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    I always figured it would be easier to rig a Packard-style shutter and keep it on all the time.

    Can you post the metronome circuit and the range of resistance values you are getting with the CDS cell. Seems like you are 90% there. Do you have a way to compensate for non-linearity in response of the CDS? For it to work it should give one-half the ticks per minute when the light is twice as bright. If your range of brightness is small it may be ok as long as the CDS cell is working in a straight line portion (if it has one?) If the resistance is not proportional to the light output, then you do need an amplifier to converted it to a linear output. Assuming the metronome responds in a linear manner for ohms to tick speed.

    Another way to do it would be to forget about the CDS cell and just point an infra-red thermometer at the housing:

    Last edited by ic-racer; 03-31-2012 at 05:38 PM. Click to view previous post history.

  3. #3

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    Bill, the Zone VI Compensating Timer might be available on the evilBay at not a lot of money. It uses a different sensor (identified in a post of mine maybe a year ago) which is easy to get. The RH Designs Stop Clock Vario is better in that it has f-stop type time scale (and many convenient features available) but if you just want to set times in "nominal seconds" the Zone VI unit is good. Don't get the Zone VI Stabilizer though: it does something different.

    Just because your Omega is, in your view, not worthy of going too far, an enlarger is good enough if it works. You might never need another one.

  4. #4

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    The sensor used by both RH Designs and Zone VI is identified at:

    Post #17 on this thread:

    http://www.apug.org/forums/forum43/8...an-2011-a.html

    They use different connectors. See later posts in the same thread for details.

  5. #5

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    I just looked on evilBay and there aren't any Zone VI Compensating Enlarger Timers, but there are Compensating Development Timers which are not what you want: they compensate for developer temperature.

  6. #6

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    If you want to go the cheaper way look for a Zone VI Compensating Metronome. You would still probably have to use a Zone VI type sensor. Does your cold light have a heater? If not maybe you could add one. If I remember right the Aristo heaters were some power resistors installed in the case.

  7. #7
    Bill Burk's Avatar
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    Hey ChuckP,

    I picked up this project after being outbid on the Zone VI Compensating Metronome couple weeks ago. I might pick up the sensor linked in this thread, an advantage of making it modular... I would just have to solder it to a light bulb base.

    Here's the project... A Jobo minilux and a Beckman Circuitmate DM73 in a cardboard box...



    As installed...



    The light from the circular fluorescent bulb is fairly steady once "warmed up" but the housing never really gets "hot" (the bulb stays cooler than I imagine the Aristo grids).

    The resistance is about 6.0 Kohms cold and about 4.5 Kohms warm. Light at the baseboard (27" height 135mm f/5.6) is near EV 1.0 when warm and near 0.5 when cold. When warm it seems to stay in a narrow range from EV 0.9 to 1.2 so it really isn't that bad. This might be all I need to keep tabs on print exposures.

  8. #8
    ic-racer's Avatar
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    Another thing to try is using the probe and box from a baseboard color meter. Most all of them had a "white" channel that is essentially a light meter. The little one I have is a null meter. You can zero it and then the meter indicator reads off in + or - fractions of an f-stop as the light dims or brightens. It should work with the light sensor up in the head like you have setup. The difference is that you won't have to do the math to get from resistance to light intensity.

  9. #9

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    BTW the Zone VI cold lights have the sensor built in em. I've used it via a VOM to see when my light is up to full brightness. Becuase the built in heater keeps it warm, plus while focusing I keep it on about a minute or more, I can get her up to max in a split second (I have the old type bulb). Trick is to keep it on for a minute prior to exposure.

    Measuring the temp of the housing is not what the heater is for.. it's the balast that has to be hot to fire it up faster.

    Now here is an idea got the DIYers. Install an electronically controled iris so when the bulb is up to max it opens n starts your exposure timer automatically via the sensor's reading?
    .
    Last edited by paul ron; 04-06-2012 at 08:26 AM. Click to view previous post history.
    Anyone can make a Digital print, but only a photographer can make a photograph.

  10. #10
    Nicholas Lindan's Avatar
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    No man can be a prophet in his own country.

    Click image for larger version. 

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    http://www.darkroomautomation.com/em.htm
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

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