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Thread: Avoiding relay use

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    Default Avoiding relay use

    I while ago I saw a possible way to close switch contacts on a camera without using a relay. I am trying to short out switches to ground which are OSD settings on a camera. I found the menu modes would open and operate if I put 1k or 10k resistors across the pushbutton switches. I seem to remember it being done with a transitor and some resistors on the output from a Picaxe. Can anyone remember this, thanks

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    have you determined what value of resistance actuated the camera? Should be simple with a bipolar or MOSFET if you don't need electrical isolation from your driver electronics... [determine the polarity of the camera's port voltage as well so you know to use PNP or NPN sort of transistor]. If you DO need isolation there are FET driven by LED type isolators... sorry I don't know what OSD is - but almost any simple circuit should work- On further thought if you can make the PICAXE V- common to the camera then perhaps you can just put the 'AXE pin direct or through 200 ohm resistor to the camera control... if speed is not too critical you could put LDRs turned on by LEDs connected to the 'AXE giving isolation and control...
    Last edited by premelec; 23-12-2016 at 23:52. Reason: mental activity

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    Quote Originally Posted by ZOR View Post
    I while ago I saw a possible way to close switch contacts on a camera without using a relay. I am trying to short out switches to ground which are OSD settings on a camera. I found the menu modes would open and operate if I put 1k or 10k resistors across the pushbutton switches. I seem to remember it being done with a transitor and some resistors on the output from a Picaxe. Can anyone remember this, thanks
    What camera?

    What plug?

    e

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    Thanks premelec, got it working. My series resistor was too big, 200 ohms works. OSD is on screen display, it's a menu system that allows configuring the camera for AGC, Language, Day/Night settings etc.

    Thanks again and all the best for Christmas and New Year, and to all the others out there who have helped a lot, hippy etc

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    Thanks eclectic, it's a security camera (board camera) that has inline OSD setting switches at the camera end, and I am remoting them with parallel switches at ground level.
    All the best wishes.

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    Hello ZOR

    There are a number of ways to do this. For starters, are you absolutely certain that the switch contacts actually "short" to ground? In fair number cases, a keyboard (particularly when there is many keys) is arranged in a matrix and doesn't actually go to ground. This is done to reduce the number of lines (pins) used on a micro. Also, if these are carbon based contacts, these usually don't actually "short", but instead offer maybe 100 Ohms or more of resistance. This may (or may not) be of importance when it comes to the actual circuit design. As a general rule, I wouldn't actually short out your switches. I would tend to suggest using a 100 Ohm resistor in the circuit. You can measure the actual resistance (often provided by a carbon pad and carbon tracks). With no power on the camera, find contact locations either side of the switch and using your DMM measure the resistance when the key is pressed. This will give you an indication of the switch "resistance".

    You are on the right track, I definitely wouldn't use relays!!! In my opinion they are "too mechanical" and would tend to create more contact bounce when compared to your average key press. They are also a bulky item to use when compared to discrete or integrated components. The coil is also an inefficient use of power. Basically it's overkill for the application!!!

    In my opinion you have two discrete options and one (or maybe more?) integrated options. You could (as you suggest) use regular transistors (or to reduce your component count, digital transistors). If the keys in question do actually "short" to ground, then a simple circuit could be used to interface the PICAXE with the camera. You might even be able to use the PICAXE direct (with a 100 ohm resistor), if the camera and PICAXE circuit share the same ground. If the keys do not "short" to ground then I wouldn't use the transistor approach as it might prove difficult to correctly bias on (and maybe off) the transistors to provide correct operation.

    The second discrete option is to use FETs (signal FETs would be more than sufficient). They put basically no load on the PICAXE and if you chose one with a low switch on (gate) voltage then this might be a good option to drive directly from the PICAXE. If they are used as a "switch" then I would still suggest putting a 100 Ohm in series with the Drain/Source connection. Again, problems might arise with correct FET operation if the camera switches don't operate from the switch to ground. I would have to get more information on the exact keyboard configuration and give more thought to if the FETs would switch correctly within a matrix.

    The integrated option is probably the least complicated. There might be more than one way to do this. This would work particularly well if the PICAXE and camera DON'T use the same (common) ground. If you use this idea and the PICAXE doesn't share the same ground as the camera, then the integrated component/s MUST have the same ground (and supply) as the camera to ensure correct operation. I've used a 4066 in the past to do this and it works very well. It's a quad analogue switch. It will give you isolation from the PICAXE circuit and close the switch contacts. You can also include the 100 Ohm resistor (or whatever the carbon switch reading is from your measurements) in series to ensure this doesn't change the circuit for the camera. The series resistor may (or may not) matter but it would have be factored into the original engineering.

    http://www.ti.com/lit/ds/symlink/cd4066b.pdf

    There are probably more ways to do this. If you need more help just ask but please do the following and post back.

    1. Measure the carbon switch resistance.
    2. Measure if the switches (both) actually short (allowing for carbon resistance) to ground.
    3. If it isn't too complicated, try to "reverse engineer" (draw the circuit) of the keyboard section.

    This additional information would be very helpful and help me/others to assist further.

    Regards,
    Brad
    Darb

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    Took me that long to write my novel that the issue was resolved before I hit "post reply".

    Bugger!!! This forum is fast!!!
    Darb

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    Many thanks Brad.

    I pulled to pieces an inline joystick kind of device which does the camera OSD switching, and was able to verify the switches do get grounded when pressed. They were also in a circuit which I cannot verify inside the camera that kept the switch high, between 2 and 3.5 volts. I could not get the output pin of the Picaxe to pulldown the switch using a 1k resistor between the Picaxe pin and switch. However changing the resistor to 200ohms it works. The camera seems happy i.e no picture dimming or distortion so I think I am home and dry.

    Thanks for taking the time to writeup so much, and I will keep that as reference.

    Many thanks again, best wishes for XMAS and New Year

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    Thanks again Brad, yes I know what it's like. However you provided a lot of useful information. I must get some FET's sometime and see what they do, it's one component that's gone amiss with me. Regards

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    Quote Originally Posted by ZOR View Post
    Thanks again Brad, yes I know what it's like. However you provided a lot of useful information. I must get some FET's sometime and see what they do, it's one component that's gone amiss with me. Regards
    Hello ZOR

    You are very welcome. FETs are a bit of a mystery to most people, probably partly due to their lack of popularity (when compared to BJTs) and maybe partly due to the slightly more obscure way in which the specs are expressed.

    Here is some reading for you that might be of use and interest.

    http://www.nutsvolts.com/magazine/ar...ircuits_part_1

    Also check out the rest of the "Learning Electronics" section on the right handw sidebar towards the bottom of the page.

    Regards
    Brad
    Darb

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