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  1. #1
    clayne's Avatar
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    Trigger the same set of safelights from independent timers?

    Trying to determine the best way to trigger a set of two safelights (or even one safelight for that matter) from two separate timers, both of which use a standard switched off circuit (safelight) during exposure. Connecting a single safelight to each timer is a piece of cake and obviously works independent of each other - but what I'm trying to accomplish is basically have 2 outputs drive switching for a single output, i.e. if any timer has prevented current to it's safelight circuit - then I want all safelights off - regardless of which timer did it. Both safelights would also use the same power strip/extension cord/etc.

    Seeing as the typical 120vac output for a safelight circuit is usually switched by a relay within a timer itself - I don't see how I can realistically make use of the output as a control signal. Using a single N-O relay with 120v capacity on the input side connected to both safelight legs seems a bit crazy and asking for trouble - especially if both timers were by chance not using the same supply circuit. But maybe I'm not thinking it all through.

    Remember that my desire is if any safelight leg is off, I want them all off (more than one enlarger+timer being used concurrently).

    Another alternative would be to say screw it and leave the safelights on all the time.
    Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.

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  2. #2
    ic-racer's Avatar
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    I'd just leave the lights on, unless you like creating 120V logic circuits

  3. #3

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    I've never quite understood the point of the timer safelight switching, not to mention that it's not practical for most darkrooms, for the reasons Clayne is running into. I much prefer having the safelights run independantly of the timer.

  4. #4
    tiberiustibz's Avatar
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    Well lets say you have a relay. Current to the relay could be set to interrupt the flow to the safelight. That way, if either of the two timers is sending current, the relay will be triggered and the safelight will be off. Is this practical? I have no idea that's your problem.
    --Nicholas Andre

  5. #5
    Rick A's Avatar
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    I've never used that recept on any of my timers. Turn my safelight on when I enter my DR and turn it off when I'm finished. I suppose the old logic was no extranious light while exposing paper, fear of fogging paper.
    Rick A
    Argentum aevum

  6. #6
    Sirius Glass's Avatar
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    Quote Originally Posted by ralnphot View Post
    I've never used that recept on any of my timers. Turn my safelight on when I enter my DR and turn it off when I'm finished. I suppose the old logic was no extranious light while exposing paper, fear of fogging paper.
    What you and others have said to the OP is leave the light on all the time. That is a solution for black & white processing. It is not a solution if one is printing on color paper.

    As suggested by tiberiustibz build a relay system that interrupts the power when current is running in any of two or more circuits. To do this take the switched output [example when the enlarger is on use the 120 v AC out, even better is it has a low voltage DC output] of each enlarger and run it through its own relay that is normally open when there is power in its relay. The output side of each relay allows say 5 volts DC to run through it. Now take the DC power and connect each relay output in series followed by a resistor connected to ground at the end. Now when any enlarger is on the relay will not allow the DC voltage at the resistor will be high. Connect the high side of the resistor to another relay that switches 120v AC off when its coil is excited [use the normally closed side] and on otherwise. This output allows the safelights to remain on normally and turns the safelights off when either enlarger is on.

    Steve
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

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  7. #7
    Worker 11811's Avatar
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    Use two Single-Pole, Single-Throw, Normally-CLOSED relays. (SPST-NC relay.)
    Make sure they are rated for 120v and can handle at lest 1 or 2 amps.

    Assume two relays "A" and "B".
    The NC contacts of the two relays are wired together.
    The incoming hot line is connected to the movable contact of relay "A."
    The hot lead of the light is connected to the movable contact of relay "B."
    The neutral lead of the light is connected to the neutral in the usual way.
    Connect the coils of the relays to their respective timer outputs.
    Be sure everything is grounded and insulated.

    Essentially, you have placed two relays in series with your safelight. If EITHER ONE of the relays is energized its contact is OPENED, causing the light to go out. When the timer expires and the relay de-energizes, its contact closes again, thus relighting the safelight.

    Imagine this as your model: You have two light switches in line with the same light bulb. BOTH of them have to be ON for the lamp to light. If EITHER ONE of the switches is turned off, the lamp can not light. You are using the same paradigm with your relay setup. The only difference is that the magnetic coil of your relay is throwing the switch instead of your hand.

    This is not difficult to do. It will take some time and effort but it is a relatively easy task.
    If you don't know enough about electricity, just get an electrician to rig it up for you.

    P.S. - You might not be able to find NC relays very easily. You can do the same thing with two double pole relays. You only use the normally closed half.
    Randy S.

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  8. #8
    clayne's Avatar
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    Re: Trigger the same set of safelights from independent timers?

    Ahh yes two relays in series achieves the OR goal. So simple and sounds like just the solution. The reason I said normally open relays is because I figured since by default the safelight output is ON that the coil would be energized during the time the exposure is not being made and hence close the circuit. Wouldn't an NC relay go closed upon losing current to the coil and allow current to the safelight (opposite of what we want) when the timer sl circuit is off or do I have something backwards?
    Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.

    http://www.flickr.com/kediwah

  9. #9
    Sirius Glass's Avatar
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    Quote Originally Posted by Worker 11811 View Post
    If you don't know enough about electricity, just get an electrician to rig it up for you.

    P.S. - You might not be able to find NC relays very easily. You can do the same thing with two double pole relays. You only use the normally closed half.
    An electrician will be expensive and may not have the knowledge to do this. Ask an electrical engineer to help you and tell him/her that you were told this is impossible to do. The electrical engineer will build a solution for you just to prove that it can be done!

    How do I know, I was one before I became a controls engineer, then a systems engineer and now a hardware-software systems engineer.

    Steve
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

  10. #10
    Worker 11811's Avatar
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    Quote Originally Posted by clayne View Post
    Wouldn't an NC relay go closed upon losing current to the coil and allow current to the safelight (opposite of what we want) when the timer sl circuit is off or do I have something backwards?
    I think you're right. For some stupid reason I had the picture of the relays being plugged into the outlet for the enlarger lamp instead of the safelight outlet. Dyslexia strikes again!

    Sirius, an engineer would be better. But any electrician worth his salt would have learned how to wire a three-way switch on his second week of tech school. This circuit is only half that. If you drew it out on a big piece of paper with a nice, big crayon I bet you could teach an electrician to do it. It just might take a while.
    Randy S.

    In girum imus nocte et consumimur igni.

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    http://www.flickr.com/photos/randystankey/

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