Put a light sensor in my Omegalite-D

Discussion in 'Darkroom Equipment' started by Bill Burk, Mar 31, 2012.

  1. Bill Burk

    Bill Burk Subscriber

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    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.

    [​IMG]

    [​IMG]
     
  2. ic-racer

    ic-racer Member

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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:
    [​IMG]
    [​IMG]
     
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  3. john_s

    john_s Subscriber

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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. john_s

    john_s Subscriber

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  5. john_s

    john_s Subscriber

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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. ChuckP

    ChuckP Subscriber

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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. Bill Burk

    Bill Burk Subscriber

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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...

    [​IMG]

    As installed...

    [​IMG]

    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. ic-racer

    ic-racer Member

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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. paul ron

    paul ron Member

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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?
    .
     
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  10. Nicholas Lindan

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  11. ic-racer

    ic-racer Member

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    WIth respect to the Aristo 1414 head pictured the light output is proportional to the temp of the bulb up to a certain temp beyond which the light output becomes inversely proportional to the bulb temp. The housing temp is proportional to the bulb temp. The ballast is not in the housing, it is on the floor and stays cold.
     
  12. paul ron

    paul ron Member

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    Your cold light has no balast transformer in the head?... I think that is strange since the manufactures want both bulb n transformer to stay warm for quicker starts. Generally the heater is on a separate line, is that plugged in? You aren't mixing up the balast transformer with the power supply or conditioner or comp timer?

    I found someting interesting about the airista heater in a cold head...
    http://www.light-sources.com/sites/default/files/omega_67c_cold_light_head__mounting_info.pdf

    It's interesting about the light output as the heat goes up. I'll have to check my Zone head vs the Arista to see if this is typical of the bulb or just the Aristas. Mine are are both the round housings for the Besseler 45. I also have the old tubes in both, the W45.
     
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  13. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    The purpose of the ballast is to supply a somewhat constant current to the lamp. The temperature of the ballast has no effect on light output. If anything a cold ballast will work better than a hot one, though the difference is minuscule.
    http://en.wikipedia.org/wiki/Ballast_%28electrical%29

    A cold lamp is at low internal pressure and operates at a low voltage - as the lamp warms up the pressure of the gas in the lamp increases and the mercury and the other metals vaporize, adding to the gas pressure. The voltage of a plasma - the glowing gas in the lamp - is proportional to pressure. As power = voltage x current, the higher the lamp voltage the greater the lamp power. If the lamp gets too warm, and the lamp voltage rises, the current through a simple ballast will decrease somewhat thereby reducing the power to the lamp.
     
  14. ic-racer

    ic-racer Member

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    Yes you should check that and see. The thermostat controlling my 1414 heater (the heater being just a few power resistors shorting the mains) came set around 35c. From there the lamp gets slightly dimmer as it heats beyond that temp. Other cold cathode systems may share a similar behavior. They recommend limiting lamp-on time to under 5 minutes. The 1414 transformer is quite massive and sits in a tray by the floor on the legs of the enlarger. The special cables to the head carry 400v.
    [​IMG]
    [​IMG]
     
  15. paul ron

    paul ron Member

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    WOW! Both my 4x5 cold lights have small balast transformers built into the head, no where near as massive as yours. My 8' florecent ceiling lights have noting that big either.

    Thanks for the FAQ on cold lights.
     
  16. Bill Burk

    Bill Burk Subscriber

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    Do the lamps get hot enough to pop a neg in a glassless carrier? The Omegalite-D, because it is a large round bulb, stays cool. Sounds like they might. If I were to replace it with a grid-type fluorescent, I might introduce a neg popping problem that so far hasn't plagued me...
     
  17. cardiomac

    cardiomac Member

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    Very clever work. Another option would be to go with LED's which are more stable than fluorescents, run cooler, last longer and require less energy to run. Toward this end I have nearly finished an LED diffusion head for the Omega D series enlargers which I plan to offer for sale soon at a reasonable price. It works fairly well now but I would like to bump up the light output a bit using the new Cree XM-L series of LED's. Also I need to tweak the LED placement a little to get absolutely uniform brightness but otherwise it is done. Attached are photos of what it will look like. Power consumption is about 25 Watts from an external power brick and light output will be about 1000 lumens.
     

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  18. Bill Burk

    Bill Burk Subscriber

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    Nice work!
     
  19. Bill Burk

    Bill Burk Subscriber

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    Still working out the rhythm of it, but basically I'll leave the lamp on for a couple minutes as if focusing/composing... Then check for stable ohms and make test strip. After the enlarger is off for 5 minutes or so (processing the test strip), it takes only about a minute in focus mode to reach the stable ohms again... Then I switch to time and put the paper in and print.

    One print tonight I forgot to warm it up and noticed right away the ohms were "high", so after the alotted time I just hit "focus" and added a third-stop's worth of light at the end. Came out OK.