Remote switching suggestions

Discussion in 'The Projects Forum' started by dhessel, May 17, 2016.

  1. dhessel

    Thread Starter New Member

    May 17, 2016
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    My first post here so hello to everyone.

    I am wanting to wire up a switching circuit to allow an external video recorder to trigger the recording mode on a video camera using the recorder and cameras remote ports.

    1. The camera has a 3 pin port that has a ground, +24v DC out and a +5v remote pin. To trigger the record mode on the camera you short the +5 remote pin to the ground pin, the camera will be in recording mode while these are shorted. Therrecord mode quites down the fan in the camera and illuminates the tally lights.

    2. The recorder has 4 pins and I have posted a link to a wiring diagram for a remote switch below. It has a +5V record trigger, gnd recorder trigger, +3.3V(could be 5V will confirm soon) and a ground pin. Recording is triggered by shorting the +5V with the gnd trigger, momentary or toggle is fine and when recording the unit outputs current for lighting up a tally light via the +3.3V and GND.
    https://convergent-design.com/odyssey7-knowledge-base.html?kbartid=30

    So what I want to do is wire up a circuit that will allow the +3.3V and GND of the recorder to short the +5v and GND on the camera. I know there are other options but this way would allow me to use the touch screen on the recorder to trigger recording. The simplest approach I thought would just be a relay but the with the low voltage in play I wasn't sure if it would be enough.

    Lastly since I am running cables from the camera to the recorder I am also wanting to use the +24V DC out and GND from the camera to power the recorder. It know it will work fine just for power, that is what the port is also used for but I am unsure about using it for power while shorting the +5v agains the GND at the same time.

    Any suggestions would be greatly appreciated.

    Thanks,

    -David
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    You can definitely find a relay that will work at that low voltage. But I'd use a transistor. Cheaper, faster, no moving parts.

    The 3.3V or 5V signal would go to the base of the transistor through a current-limiting resistor. A value of 1K-10KΩ would probably do it. The emitter of the transistor would be connected directly to ground. The collector would connect to the 5V line that gets grounded when you want to activate the camera. When a voltage appears on the base of the transistor, it will conduct and effectively connect the 5V line to ground.

    Normally you would add a resistor in series with the collector to limit the current through the transistor. Since your 5V line is intended to be directly grounded without a resistor, we have to assume is has its own internal current limiting.

    You don't need the relay (or transistor) to latch, do you? This means turning on and staying on even after the signal is gone, until a reset. There are latching relays. The transistor approach obviously gets a little more complex if you need it to latch.
     
  3. dhessel

    Thread Starter New Member

    May 17, 2016
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    0
    Thank you for the reply. I doesn't need to latch in that I only want it on while the 3.3v is being output. Put another way normally the 3.3v is used to illuminate an led while recording is in progress, I want it to illuminate the tally lights on the camera instead.

    I already have a NPN2222 transistor and some 1K resistors would those work?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Yup! Certainly worth a test. A 1K resistor will limit the base-to-emitter current to I = ∆V/R = (3.3 - 0.65)/1000 = 2.65mA. That will guarantee a collector-emitter current up to 27mA without concern that the transistor is "in the way". I doubt the sensing of the 5V line being pulled low needs any more than that, so it should be fine. But one experiment is worth a lot of speculation.

    Check and double-check the pinout of the transistor as you wire it up. It's so very easy to get it wrong.
     
  5. dhessel

    Thread Starter New Member

    May 17, 2016
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    Thank you, I gave it a try and it didn't seem to work with only 1K, going to pick up a 10K and try again.
     
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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  7. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Well that won't get better, it'll likely get worse. Test to see what the voltage at the collector drops to when the transistor is turned on. It may be some fraction of a volt but not quite low enough to trigger the switch. A different transistor might be called for. A darlington would be a good choice.
     
  8. dhessel

    Thread Starter New Member

    May 17, 2016
    5
    0
    I will give these suggestions a shot, thank you.
     
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