How to duplicate a momentary switch with a relay?

Discussion in 'The Projects Forum' started by SteveWilson333, Dec 28, 2014.

  1. SteveWilson333

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    Dec 13, 2014
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    I have a camera (HackHD) that needs a momentary switch pressed to turn on (>60ms, but <1000ms). The switch simply grounds Pin 3. The camera will be connected to a timer so it turns on once a day. I need to duplicate the momentary switch via the timer to activate the recording.

    So I have 12v SLA into the timer. Out of the timer I have + and - into the coil of a DPDT relay. And that is where I'm stuck. Seems like it should be simple, but I'm not getting it.

    I found a automotive relay that seems to be what I need using a cap and resister, but I need to convert this to my relay:

    [​IMG]

    Thank you.

    [​IMG]



    [​IMG]
     
  2. MikeML

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    What is the dc resistance of the relay coil? 12Vdc coil?

    How long between simulated button pushes?
     
  3. SteveWilson333

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    Dec 13, 2014
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    Last edited: Dec 28, 2014
  4. MikeML

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    Here is the way I would do this:

    It provides a 1/2sec wide relay closure every time 12v is applied. It recycles within about 1sec when power is interrupted. NFET is not critical. Almost any logic-gate NFET with a Rds of less than an Ω will work...

    RD.gif
     
  5. MikeML

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    Are you sure that you need the relay? It occurs to me that the NFET in the circuit above probably can produce the equivalent of a contact closure to simulate the pushbutton...
     
  6. SteveWilson333

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    Dec 13, 2014
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    Simpler is better. If I can do it without a relay, then great.
     
  7. MikeML

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    Can you make some quick measurements?

    Switch your DMM to VDC (5V or 10V full-scale).
    Put the black probe of your DC voltmeter on pin1 of your HackHD.
    Put the red probe on pin 3.
    With the device powered, measure the DC voltage. Write this down.

    Now switch your meter to mADC (10mA or 100mA full-scale). You might have to move one of the probes to a different jack.
    Put the black probe of your DC voltmeter on pin1 of your HackHD.
    Put the red probe on pin 3.
    With the device powered, measure the DC current that flows from pin3 to pin1. Write this down.

    Report back.
     
  8. SteveWilson333

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    Will do.
     
  9. SteveWilson333

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    Across 1 and 3 I'm getting 2.85V on or off.
    Across 1 and 3 I'm not getting any current.
     
  10. SteveWilson333

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    Dec 13, 2014
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    When I try and measure current through the mA side I get nothing, switching over to the 10A side, when I probe 1 and 3 this seems to perform a restart on the HackHD.
     
  11. MikeML

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    Fluke meter with the internal fuse blown?

    Even the 10mA scale should restart the HackHD. I am having you measure how much current it takes to do that?
     
  12. SteveWilson333

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    This is a Vellman, and it looks like the fuse is blown. I'll try another meter.
     
  13. SteveWilson333

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    Dec 13, 2014
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    2.168 mA
     
  14. MikeML

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    So if you are starting the HackHD normally, you would be using a momentary switch wired between pin 1 and pin3?

    From your measurements, it looks like the mom. switch only has to stand off <3V when it is open, and only pass <3mA when the switch is closed. Per your earlier description, the mom. switch is supposed to be closed only for about 1/2sec when some external input happens?

    Now, what is the external input that actually is supposed to initiate the start-up of the HackHD?

    Can the external input share a ground with the HackHD (or is this why you think you needed a relay)?

    What voltage does that external input provide? 12Vdc?
     
  15. MikeML

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    Duplicate post due to web site outage.
     
  16. SteveWilson333

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    Dec 13, 2014
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    The switch is actually wired between PIN3 and PIN5. PIN5 is ground though so it's essentially PIN 3 and PIN1. The camera uses a 1S LiPo for power, so 3.7V. I came up with the relay from their description:

    PIN3: Button Input - Connect a switch here to control all camera operations (turn on, turn off, record, stop, take photo, etc). Whenever terminal 3 is grounded, the button is activated. This can be connected to a push button, relay, motion controller, microcontroller (Arduino), or any other sensor that can ground it.
     
  17. MikeML

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    Ok, but you haven't specified what you will use to initiate the momentary grounding of pin 3?
     
  18. SteveWilson333

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    Dec 13, 2014
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    Sorry, I have a 12v timer hooked up to a little 12v SLA battery. When it turns on, it passes the 12v though.

    ZYT16G-2a/3a microcomputer timer switch
    http://www.zhuo-yi.com/upload/file/1310005063.pdf
    [​IMG]
     
  19. MikeML

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    Ok. Try this:

    First, I am assuming that the NO contacts in the timer are totally isolated from the 220VAC line input! Pay attention. If this is not the case, do not use this circuit. To make this circuit work, the timer contacts must be available as shown below.

    There are only four additional parts added, R1, R2, D1 and C1. Rhd1&Rhd2 are the Thevenin equiv of what is inside the HackHD. No need for a 12V battery.

    TR.gif
     
    Last edited: Dec 28, 2014
  20. SteveWilson333

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    Dec 13, 2014
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    I'll give it a shot. Thanks.
     
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