How to remotely trigger buttons on a PCB?

Discussion in 'The Projects Forum' started by TelluriumCrystal, Nov 14, 2014.

  1. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    Hi, I am working on designing a High Power Rocket designed to fly to 3000 feet. It will have a small HD camera mounted in aerodynamic shell on the side of the electronics bay, and the camera's PCB and battery will be mounted inside. However, the camera was not designed to be used this way, and is controlled by two small buttons on the board. These will be inaccessible unless I completely open up the bay, which is impractical before a launch. Thus I need to come up with a way to activate them from the exterior of the rocket.

    Here is an image of the PCB with the buttons located on the upper-left part of the board (click for full size):
    [​IMG]

    And here's the other side, just in case:
    [​IMG]

    I am open to wiring buttons to the board, but my soldering experience is limited and the board is tiny (it's about as wide as a quarter). Space in the electronics bay is extremely limited, and setting up some sort of mechanical mechanism to press them is probably out of the question (unless I try to press them by sticking rods through holes in the side of the rocket, or something along those lines).

    Any suggestions are welcome! I have very little experience working with electronics and have never tried to do anything even remotely like this before, so assume I know nothing.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Do the switches need to be pressed and released,or is pressing and hold OK?

    I would replace them with slide switches and use a string on the outside to activate or a shorting plug/socket and pull the plugs out. Do they need to be pressed in sequence or at the same time? You could also run jumper wires to them and have the switches on the outside, which could be yanked off before launch.

    The existing switches could be activated mechanically, but I think that solution is likely to add more weight. I would think in terms of a spring strong enough to activate the switch held in compression with a thread to the outside that could be pulled lose.

    John
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the goal is to remote the switches so they can be operated, say using remote radio control:

    1 Set your DMM to DC Volts mode.

    2 Connect the - black lead of the DMM to the camera's battery - end.

    3 With camera on, using the DMM's + red lead, measure eight voltages as described below:

    4 There are two switches, call them A and B for reference.

    5 Each switch has two electrical connections, call them X and Y (even though it looks like four pins on the pcb, two pins are the same electrical node on each side of the switch.) It is important that you keep track of the placement of the red probe so you can come back to the same point on the pcb later.

    6 Now make four measurements: V(AX), V(AY), V(BX), V(BY). Write them down as you make them.

    7 Now with switch A pushed, make two more measurements: V(ApX) and V(ApY). Should be nearly the same voltage, but write them down.

    8 Now with switch B pushed, make two more measurements: V(BpX) and V(BpY). Should be nearly the same voltage, but write them down.

    9 Report back with the eight voltages tabulated using the nomenclature above.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    That's not such a bad idea - just leave two tiny holes thru which you could stick a straightened paper clip, like the little hole in a disk drive that allowed you to force a disk ejection. You could use a straw or such to guide the paper clip to the button, if there is a gap between the switch and the hole. Would you be able to tell the button had been successfully pressed?
     
  5. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    The sequence to shoot video is to hold the rightmost button for about 2 seconds to boot the camera, then press the leftmost button once to begin recording. To stop, you press the leftmost one once which stops the recording and then hold the right one until the camera turns off. So the left one does need to be pressed and released.

    The camera in question is an 808 car key camera (probably a #16).

    I'll give it a shot as soon as I find time. I assume by DMM you mean a digital multimeter?

    I should point out there is not enough room to fit a radio transmitter/receiver in the electronics bay, so by "remotely" I mean from outside the rocket without needing to directly access the camera PCB.

    One of the problems with this camera is that it can be tricky to tell if it's recording even if it is being directly manipulated. There is a small light that indicates its current status, but because the camera is meant to be used for spying on people, the light turns off while recording. For this reason I would prefer to have an electronic means of activating the camera so I at least know for sure I'm pressing the buttons. The clearance between the side of the rocket and the board (~0.3 inches) would mean I would have to design a guide shaft for that sort of external manipulation, which would be tricky to mount. It isn't impossible, but I would like to see if it is doable electronically before resorting to physical means.

    Perhaps I should also try wiring the indicator light to another one mounted externally so I can verify that it is starting properly.
     
  6. wayneh

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    I wonder if you could build your own "on" detector, maybe by sensing a higher current when it is recording. I think not having positive feedback that the camera is working will lead to frustration and failed shots.
     
  7. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    Believe me, it already has - I've had more failed shots than successful ones while testing it. If it's possible to make a recording indicator then I would definitely give it a shot!
     
  8. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    Here are some screenshots of the avionics bay designs. The rocket's diameter is 2.56in, so hopefully this makes it easier to understand the space I'm dealing with.

    The avionics bay with the camera shroud:
    [​IMG]

    Walls hidden:
    [​IMG]
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    In that case, ignore my previous post. I would arrange the pushrods through the side of the rocket to depress the buttons, and I would use a short piece of optical fiber to remotely view the LED on the camera.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    If those switches are simple SPST, you could experiment with putting a similar switch in parallel. This switch could be mounted with easy access for the operator.

    The switch's job is merely to connect the two poles, so if you do it right, you'll have two wires for each switch that bypass the switches and allow you to control the camera.

    Those switches each have 4 solder pads, but I think 2 may just be for mechanical stability. But I could be wrong - they could be DPST. You can deal with that too, it's just a little trickier to keep it all straight. Use your multimeter to see which pads are shorted when the button is pressed.
     
  11. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    I would like to try and get an electronic solution before resorting to using rods. I like the idea of using optical fiber to extend the LED's light - I'll probably end up doing that.

    I think this would be the best plan. If I can get a pair of buttons mounted to the side of the rocket, along with the light's optical fiber indicator, I should be able to operate the camera externally.

    I've looked up and understand the difference between SPST and DPST, but I'm not sure how to test for it. Please forgive my ignorance, but what am I looking for when I test the buttons? How can I differentiate between SPST and DPST?

    Also, assuming I go through with soldering wires to the board, how should I go about doing that? Despite having good fine motor skills, I have trouble imagining myself soldering wires to something so small. Are there any "micro" soldering irons I could get for this?

    Thanks for all the help thus far!
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    I agree with wayneh. These switches are called membrane switches. Even though a SPST switch has only 2 functional connections, these have 4 connections to give it stability on the PCB.

    [​IMG]

    Two legs on each side are connected together, When you push the button all four legs are connected together.

    If you have a multimeter you can test this.
     
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  13. takao21203

    Distinguished Member

    Apr 28, 2012
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    use relays, I guess transistors would do as well.
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Could you use a bilateral switch there?
    An 4066 would be one, that can be controlled by an incoming voltage.

    Bertus
     
  15. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    Ok, but as I have asked twice before, how do I test this? I have a good multimeter (an ExTech EX205T), but I have no idea how to go about determining if the switches are SPSTs or DPSTs. If someone could tell me what to test for (voltage, current, resistance), I can get some results back and we can move forward with determining how to wire up some external switches.

    Again, sorry for my ignorance, but this is my first time trying to wire anything to a PCB (or test one for that matter). I might as well have no idea what I'm doing.
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As said there are two pairs of pins connected to the switch as in the picture below:

    [​IMG]

    Bertus
     
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  17. wayneh

    Expert

    Sep 9, 2010
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    Put your multimeter on the lowest ohms scale. Confirm that placing the leads together causes the meter reading to approach zero.

    Probe any two contacts on your switch. If they are already connected, choose another pair. When you find two that are not connected, press the button to confirm that now they are.

    Continue until you are comfortable that you have mapped out the relationship between all 4 contacts.
     
  18. TelluriumCrystal

    Thread Starter New Member

    Nov 14, 2014
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    Ok, I tested both of them and they are pretty much exactly what Lestraveled and bertus suggested:

    Now that I know where to solder wires to in order to wire another switch to the board, I need to figure out how. This thing is tiny. One of my friends has a micro soldering kit, so I'll see if it will be possible to solder wires to them using that. In the mean time, how would you guys recommend connecting wires to something as small as this?

    Here's an image with a quarter for scale:
    [​IMG]
     
  19. wayneh

    Expert

    Sep 9, 2010
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    The right tools will make all the difference.
    1) Stranded wire. You want tiny wire that will tin easily. I don't have a recommendation. Wish I did.
    2) The right solder. I'd use 60/40 or 63/37 tin/lead rosin core.
    3) A soldering iron with as small a tip as you can find. Mine is the sharpest point here, which I think would be up to this job.
    4) Good lighting.
    5) Magnifying glass. I use those cheapo helping hands. Obviously there are fancier options.

    Strip and tin the wire ends. Hold the wire onto the target spot. With a bit of solder on your iron, touch the wire and hold until it melts and the transfers enough heat to melt the solder on the pad. Hopefully the solder from the tip and the wire will become one with the solder on the pad.

    Once the wires are connected, think about strain relief. I sometimes twist the wires together and put a blob of hot glue to hold things steady.
     
  20. Lestraveled

    Well-Known Member

    May 19, 2014
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    Again, I agree with wayneh.
    When I do soldering like this I will wear a pair of reading glasses under my Opto-visor for added magnification. (I probably have older eyes than you.) I suggest that you practice before you do the real thing. As wayneh said, tin the wires. Use a damp sponge to clean the soldering iron tip. Get your wire in position, place the tip, let the heat flow, melting the solder, remove the tip, count to four, and you are done. As I said, practice, it's not hard.
     
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