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Thread: Interlock circuit for daisychainable projector

  1. #1
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    Lightbulb Interlock circuit for daisychainable projector

    I'm in the process of finishing three home built projectors. But I'we not found a description of a daisychainable interlock.

    If anyone know of anything woking or comments to my current idea, please shout out.
    ---
    All my projectors will have built-in ethernet switches for dac, arduino with network (monitoring, cooling control, safety and remote startup), and passthrough to next projector.

    I was thinking that i would have one powered master emergency switch with a 20mA constant current source. this would be connected spare pairs of the ethernet wiring (100mbit) and passed through to the port, and then looped back.
    Each projector would then have a optocoupler driving a dpdt relay, where one pole passes power to the laser drive and the other is connected to the arduino for interlock detection. That way the interlock would be carried in the same cable as ethernet, and i would only need a loop terminator at the end of the chain.

    This would also allow me to add any number of emergency switches to the chain and even have the arduinos shut down all projectors in case of malfunction (would probably be configurable for show-must-go-on purposes).

  2. #2
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    24v, 20 mA source, with a series diode and opto-isolated from the rest of the system, in both power and feedback. With a backup current limiter for just in case. Each projector has either a 360 ohm resistor or an led for the sense. Also optically totally isolated. Like the 4-20 mA signaling system, it should be nearly bulletproof as long as no common mode voltage is induced. Of course the Chinese will instantly remove all isolation, to be cheap. I've dreamed of this as a standard for years. Would perhaps help to put a little modulation on the DC for recognition. With a bypass jumper for test. Follow the old CDRH reporting standard and make the jumper stick above the lid level when installed, as a warning to remove the bypass after alignment and repairs.

    A fused, current limited source of 20 mA with 12 to 24 V of compliance, and totally isolated current loop. Optocouplers are cheap and reliable, and easy to prevent damage with clamping, etc.

    Besides that, current sources usually can be in series with no fault if there is a wiring mistake.

    CL2N3-G by Microchip


    Comes to mind.

    NCR402T by Nexperia.


    I'll draw an example up tonight.

    Perfect.


    Steve
    Last edited by mixedgas; 04-10-2023 at 15:21.
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  3. #3
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    NSI45020T1G was the regulator i planned to use.

    I made a quick schematic. The current line fuses will most probably be 150mA littelfuse poly fuses (they have 50mA nominal hold).
    The wire selection is deliberate to throw error if plugged into poe switches or short passive injectors.
    The numbers are incorrect for the projector on the drawing, they shoud be swapped.

    Sub e-stops will be identical to master except for the voltage and current supply.

    Click image for larger version. 

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  4. #4
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    The current indicator / valid interlock led should be a 150 mA or 350 mA led in series with the current. Not another opto.
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  5. #5
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    nice block diagram but it'd missing a few things. You font wantboptos directly driving the relays.

    For US use its missing a required delay.

    I know ethernet cables are the cat's meow of cheap, but I've had too many fail shorted
    over thesupply.

    You need the isolated power supply.

    Steve
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  6. #6
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    The interlock will run on the same ethernet cables as the dacs, not as rugged as separate xlr, but It's not like I make any money from this.

    This is not us, but the delay is not a part of the interlock chain, but controlled by the arduinos that also detect broken interlock and will require start button on projector, start on the web page or api call to start same as with power on (in addition to valid voltages, no scanner fail and temperatures.

    150mA led will not light up as this is a 20mA chain. I did initially consider just adding a led in the indicator, but my e-stops have lighted base with 4 white leds and one red in the button. Plan was to use the base lights as ok (makes the estop easy to see) and the red light as interlock broken, the base lights turn off.

    The master e-stop has a isolated power supply and the interlock chain will be isolated from the other supplies.

    The micro relays in the estops can easily be driven directly from the opto but the projectors will probably have an added transistor.

    Good idea about the service jumper. I could mount a extra key switch that makes the key be in the way of the cover if enabled. I got a bunch of them that needs to be turned off for the key to be removed.

  7. #7
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    I've had very good luck with high power leds st 20-30 mA, mostly Cree. 60 mA automotive Ted or yellow in a Paranah package might work just fine as well. Might want to put one 250 Ohm resistor in the return lead. This would give you 3-4 volts to confirm the active chain and prevent any bizarre capacitive oscillations in the source. I'd add a few 0.1 uF caps to prevent RF pickup and dump static spikes with unshielded. cable.
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