Trigger the same set of safelights from independent timers?

Discussion in 'Darkroom Equipment' started by clayne, May 15, 2010.

  1. clayne

    clayne Member

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    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.
     
  2. ic-racer

    ic-racer Member

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    I'd just leave the lights on, unless you like creating 120V logic circuits :smile:
     
  3. bdial

    bdial Subscriber

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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. tiberiustibz

    tiberiustibz Member

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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.
     
  5. Rick A

    Rick A Subscriber

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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.
     
  6. Sirius Glass

    Sirius Glass Subscriber

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    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
     
  7. Worker 11811

    Worker 11811 Member

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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.
     
  8. clayne

    clayne Member

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    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?
     
  9. Sirius Glass

    Sirius Glass Subscriber

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    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! :wink:

    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
     
  10. Worker 11811

    Worker 11811 Member

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    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! :wink:

    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. :wink: :wink:
     
  11. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    One relay with a form A (Common/Normally Open) contacts.

    The output of Timer 1 drives the relay coil.

    The output of Timer 2 goes to the safe light on one side and the the relay C contact on the other. The other wire of the safelight is wired to the 'NO' contact of the relay.

    For the safe light to be one both timers must have energized their safe light output AC sockets.


    It is possible to do it with no relays - but you don't want to try it as it has a good chance of making a short circuit - and violates every wiring code on the planet.

    * * *

    The reason for switching the safelight off is to:

    1) Make focusing easier as there is more contrast on the paper

    2) Make it easier to position dodgers and burners

    3) Allow the use of meters and analyzers as the safelight illumination interferes with the readings
     
  12. Sirius Glass

    Sirius Glass Subscriber

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    But using an electrician would not be as much fun as watching a engineer solve your problem just to prove you are wrong. Both sides win that way.

    Steve
     
  13. Worker 11811

    Worker 11811 Member

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    Yes, the best way to get something done is to tell an engineer that it can't be done! :wink:
     
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  15. Steve Smith

    Steve Smith Subscriber

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    That was going to be my suggestion. You don't need anything extra, just modify the timers so both internal relays are in series.

    But as the op is asking the question I assume he is not interested or able to do these sort of modifications.

    There are no rules in the UK to prevent you doing this yourself in a domestic situation.

    The other option would be a parallel connection with separate on/off switches for each timer so only one was powered at a time.


    Steve.
     
    Last edited by a moderator: May 16, 2010
  16. Sirius Glass

    Sirius Glass Subscriber

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    Short circuits do not violate any laws or codes. The lifetime of a short circuit is usually short due to fuses and circuit breakers. Short circuits can burn out, therefore the open circuit is more reliable and will not burn out.

    Steve
     
  17. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    Wellllll, most timers only use one relay inside -- the NC contacts go the to safelight outlet and the NO contacts go to the enlarger.

    To do it with no relays you need to hook one timer up to the AC line in a backwards configuration (reversed Hot/Neutral or Brown/Blue wires) and then wire the safelight between the two Hot sides of the safelight outlets. Really not recommended.
     
  18. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    Your insurance may balk at paying if dodgy DIY wiring is responsible for setting the house on fire. In the US the 'National Electrical Code' is part of the housing regulations in most states and cities.
     
  19. RalphLambrecht

    RalphLambrecht Subscriber

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    There you go!
     
  20. Steve Smith

    Steve Smith Subscriber

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    I'm sure they would (if they knew) but there is nothing to stop you connecting whatever you like to a socket. From incoming supply to outlet there are regulations but no one is going to check or test anything you connect to an outlet yourself.


    Steve.
     
  21. Steve Smith

    Steve Smith Subscriber

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    Yes that does complicate it a bit. So far, your single relay method seems to be the best solution..... other than just using one timer and only plugging in one enlarger at a time.


    Steve.
     
  22. Sirius Glass

    Sirius Glass Subscriber

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    Well, there is another solution: daisy chain the timers [use the first timer switched outlet into which plug the next timer] with the safelights using the last outlet in the series. That way any timer in use will turn off the safelights! Now the requirement has been met without opening any boxes or touching a soldering iron!

    Or to quote a poster sold at www.despair.com =>
    "None of Us is Dumb as All of Us."
    ##### http://www.despair.com/meetings.html #####​

    Steve
     
    Last edited by a moderator: May 16, 2010
  23. Steve Smith

    Steve Smith Subscriber

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    That's far too simple and elegant!


    Steve.
     
  24. Sirius Glass

    Sirius Glass Subscriber

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    And it is free, so there is no relief for GAS. :surprised:

    Steve
     
  25. clayne

    clayne Member

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    Need to use the timers concurrently.
     
  26. glbeas

    glbeas Member

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    You need to use a relay for each timer and run the coils separately via each timer and the contacts (normally open) in series on the safelight power line. The safelight is plugged into it's own wall outlet and the safelight outlets on the timers power the relay coils. All power circuits are separate from each other and no chance of a short circuit.