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4051 multiplexer current

Discussion in 'Electronic Basics' started by Data888, Aug 24, 2003.

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  1. Data888

    Data888 Guest

    Hi.
    Can anyone hell me what the maximum current through the signal
    input-outputs on a 4051 analog multiplexer would be? Because I want to
    drive a small reed relay, with a coil resistance of 500 ohms. So if I
    put 5v on the input, and the relay coil on the output the current
    through the IC would be 10mA. Is this ok, or will it burn out the IC?

    Nick
     
  2. Hi Nick,

    I don't think this is gonna work. The ON resistance of the 4051 is about
    the same as the resistance of your relay (at 5V supply voltage). So this
    leaves only 2.5V for your relay.

    You'd better use a transistor to drive the relay. And don't forget the
    protection diode across the relay coil.
     
  3. Yep, a BC547 is just fine.
    Connect the lot like this:
    +5V
    |
    +------+
    | |
    relay --- Protection
    coil / \ diode
    | --- 1N4148
    | |
    +------+
    |C
    +---+ B|/
    4051 --|10k|--| BC547
    +---+ |\
    |E
    |
    ---
    GND

    BTW, the 4051 is acually meant to switch analog signals (like audio or
    video). Since you only want to drive a relay, you may want to use a 3-to-8
    decoder, such as a 74HC138. It might be somewhat cheaper. For more info, go
    to http://www.semiconductors.philips.com/pip/74HC138D.html.

    I just saw your previous post on the AV switch. You want to switch one
    video source between 2 devices. You may still try using the 4051 to switch
    the video signal. It really doesn't such a bad job. Even TV/VCR
    manufacturers use similar switches. Just give it a try and see if it's good
    enough for you.
    The video amp comes between the 4051's output and the input of each target
    device. If you have 2 target devices, you'll have to build the video amp
    twice. Actually, the circuit around T1 (in
    http://jap.hu/electronic/avswitch.html), isn't an amplifier, it's a video
    buffer. Just like the 4051 isn't poweful enough to drive a relay, it also
    isn't powerful enough to drive the video input of a TV or VCR. The buffer
    doesn't amplify the voltage of the signal, it just makes it able to drive
    low-resistance loads. A 5V power supply voltage is therefore not too high.
    If you're absolutely sure, there will never be more than 2 target devices,
    you may consider using a 4053.It contains 3 switches with 2 inputs and one
    output each. See http://www.philipslogic.com/products/switches/4053/

    Hope this helps,
     
  4. Data888

    Data888 Guest

    Thank you very much, you're being very helpful. I know I could just have
    used an ordinary multiplexer to drive the relays, I just happened to
    have an extra 4051 instead of going and getting another IC.
    Basically I'm designing it using a 4029 to feed the control inputs of
    the 3 4051s, to switch between up to 8 AV inputs. I might later use some
    additional logic to allow the 4029 to reset itself when it gets to a
    certain output, to limit the switching between a certain number of inputs.
    The second output I'm just planning to manually switch between the
    inputs with a rotary switch, so it's possible a video signal will be
    switched 2 ways at the same time, but unlikely. So I need to put one of
    these buffers on both of the outputs?

    Nick
     
  5. Data888

    Data888 Guest

    I haven't terminated the lines, although I've been reading and it seems
    like a good idea. Does the 75 ohm resistor go from input to
    ground/shield, or just in series with the input?
     
  6. Byron A Jeff

    Byron A Jeff Guest

    --
    -> Gert van den Heuvel wrote:
    ->> ->>
    ->>
    ->>>Basically I'm designing it using a 4029 to feed the control inputs of
    ->>>the 3 4051s, to switch between up to 8 AV inputs. I might later use
    ->>>some additional logic to allow the 4029 to reset itself when it gets
    ->>>to a certain output, to limit the switching between a certain number
    ->>>of inputs. The second output I'm just planning to manually switch
    ->>>between the inputs with a rotary switch, so it's possible a video
    ->>>signal will be switched 2 ways at the same time, but unlikely. So I
    ->>>need to put one of these buffers on both of the outputs?
    ->>>
    ->>>Nick
    ->>>
    ->>
    ->>
    ->> OK, so you have 8 inputs and 2 outputs. 1 output is switched using a
    ->> 4051 and the other ouput is switched between the same 8 inputs with a
    ->> rotary switch.
    ->>
    ->> I assume all 8 inputs are grounded via 75 ohms resistors (to minimize
    ->> reflection)? In that case you need buffers on both outputs. Yep, even
    ->> on the output with the rotary switch. It you don't, the internal
    ->> impedance of the device on the output will become parallel-connected
    ->> to the 75 ohms resistor at the input.
    ->>
    ->>
    ->
    -> I haven't terminated the lines, although I've been reading and it
    -> seems like a good idea. Does the 75 ohm resistor go from input to
    -> ground/shield, or just in series with the input?
    ->
    ->
    -
    -The resistors go from input to ground.

    This is the magic thread for me. This is the next hardware project on my list.
    It must be the fact that the new television season is right around the corner.

    I'm planning on using 74HC4051s to switch video and stereo line level audio.
    The thread has been helpful, but I have 3 issues:

    1) What is the setup for line level audio? Does it require the impeadance
    matching resistor too?
    If so then what value?
    Is an audio buffer required?

    2) I know it's silly, but I presume that all of the video/audio shields are to
    be grouded
    to the 74HC4051's ground right? The reason I ask is while researching this
    task I came across Zoomcat's parallel port switcher here:

    http://www.geocities.com/zoomkat/ppswitcher3.htm

    On the page he has the following statement:

    -----------
    All the grounds (RCA plug outside) are connected together, keeping audio
    and video seperated. Note that there are no connections made to the ground
    side of the parallel port connector (that would ground out the video and
    audio signals). The chip grounds are connected to the common video out
    (not video ground) going to the capture card.
    -----------

    And in fact on the diagram it does show the 74HCT259 ground connected to
    the common video out.

    3) The primary reason for using the HC4051 instead of individual transistors
    was for packaging reasons. The video (and possibly audio) buffers will cost
    another transistor per output (which certainly isn't as bad as a transitor
    per input!). However it would simplify my task if I could use an opamp
    package with a circuit such as:

    http://ourworld.compuserve.com/homepages/g_knott/elect254.htm

    #2 the only confusing thing I've found about the whole process. Any insight
    would be appreciated.

    BAJ
     
  7. Data888

    Data888 Guest

    I haven't buffered anything yet. Unfortunatly the video buffer in the
    design I previously linked to is not possible for me, and none of the
    electronics places near me carry that transistor. I possibly have a
    design for another, and I'll be posting that soon, so someone can tell
    me if it's suitable.
    I'm not planning on buffering the audio, as it seems to be fine as it is.
     
  8. Byron A Jeff

    Byron A Jeff Guest

    -> I'm planning on using 74HC4051s to switch video and stereo line level audio.
    -> The thread has been helpful, but I have 3 issues:
    ->
    -> 1) What is the setup for line level audio?
    -> 2) ...I presume that all of the video/audio shields are to be grouded
    -> to the 74HC4051's ground right?
    -> 3) ...opamp video buffer?i
    -
    -I haven't buffered anything yet. Unfortunatly the video buffer in the
    -design I previously linked to is not possible for me, and none of the
    -electronics places near me carry that transistor. I possibly have a
    -design for another, and I'll be posting that soon, so someone can tell
    -me if it's suitable.

    The only thing you have to worry about with the transistor is the switching
    speed. Even a lowly nickel 2N2222 can easily buffer video with it's 250 Mhz
    switching frequency.

    -I'm not planning on buffering the audio, as it seems to be fine as it is.

    Good to know.

    How did you handle the grounding issue I asked about above?

    BAJ
     
  9. Byron A Jeff

    Byron A Jeff Guest

    ====
    I'm trying to get wider coverage to my questions. The original thread can be
    found in sci.electronics.basics - BAJ
    ====

    --
    -> Gert van den Heuvel wrote:
    ->> ->>
    ->>
    ->>>Basically I'm designing it using a 4029 to feed the control inputs of
    ->>>the 3 4051s, to switch between up to 8 AV inputs. I might later use
    ->>>some additional logic to allow the 4029 to reset itself when it gets
    ->>>to a certain output, to limit the switching between a certain number
    ->>>of inputs. The second output I'm just planning to manually switch
    ->>>between the inputs with a rotary switch, so it's possible a video
    ->>>signal will be switched 2 ways at the same time, but unlikely. So I
    ->>>need to put one of these buffers on both of the outputs?
    ->>>
    ->>>Nick
    ->>>
    ->>
    ->>
    ->> OK, so you have 8 inputs and 2 outputs. 1 output is switched using a
    ->> 4051 and the other ouput is switched between the same 8 inputs with a
    ->> rotary switch.
    ->>
    ->> I assume all 8 inputs are grounded via 75 ohms resistors (to minimize
    ->> reflection)? In that case you need buffers on both outputs. Yep, even
    ->> on the output with the rotary switch. It you don't, the internal
    ->> impedance of the device on the output will become parallel-connected
    ->> to the 75 ohms resistor at the input.
    ->>
    ->>
    ->
    -> I haven't terminated the lines, although I've been reading and it
    -> seems like a good idea. Does the 75 ohm resistor go from input to
    -> ground/shield, or just in series with the input?
    ->
    ->
    -
    -The resistors go from input to ground.

    This is the magic thread for me. This is the next hardware project on my list.
    It must be the fact that the new television season is right around the corner.

    I'm planning on using 74HC4051s to switch video and stereo line level audio.
    The thread has been helpful, but I have 3 issues:

    1) What is the setup for line level audio? Does it require the impeadance
    matching resistor too?
    If so then what value?
    Is an audio buffer required?

    2) I know it's silly, but I presume that all of the video/audio shields are to
    be grouded
    to the 74HC4051's ground right? The reason I ask is while researching this
    task I came across Zoomcat's parallel port switcher here:

    http://www.geocities.com/zoomkat/ppswitcher3.htm

    On the page he has the following statement:

    -----------
    All the grounds (RCA plug outside) are connected together, keeping audio
    and video seperated. Note that there are no connections made to the ground
    side of the parallel port connector (that would ground out the video and
    audio signals). The chip grounds are connected to the common video out
    (not video ground) going to the capture card.
    -----------

    And in fact on the diagram it does show the 74HCT259 ground connected to
    the common video out.

    3) The primary reason for using the HC4051 instead of individual transistors
    was for packaging reasons. The video (and possibly audio) buffers will cost
    another transistor per output (which certainly isn't as bad as a transitor
    per input!). However it would simplify my task if I could use an opamp
    package with a circuit such as:

    http://ourworld.compuserve.com/homepages/g_knott/elect254.htm

    #2 the only confusing thing I've found about the whole process. Any insight
    would be appreciated.

    BAJ
     
  10. Data888

    Data888 Guest

    I just grounded the shields and the ICs to the same ground as the power
    supply.
     
  11. (Byron A Jeff) wrote in
    1) There's no audio buffer required. The input resistance of of audio
    devices is usually 10k or more. The 4051 just adds 500 ohms or so. This
    also means that there's no need to terminate the audio lines.

    2) Yes, all audio and video grounds must be connected to pin 7 of the 4051.
    The digital ground is connected to pin 8.
    Zoomcat uses an NPN transistor to switch audio and video. The ground of the
    74HCT259 is NOT connected to ground, but to the emitter of the switching
    transistor, which is connected to the center (signal) pin of the RCA
    connector.

    3) You only need to buffer the video signal. Opamps for video frequencies
    are quite expensive, but if that's not an issue, feel free to use them.


    HTH,
     
  12. (Byron A Jeff) wrote in
    OK, if you use a symmetrical power supply (I should have noticed that
    before), things are a bit different. I suppose the digital part just uses
    the positive 5V supply. In that case:
    * All grounds are are connected together and to pin 8 of the 4051
    * -5V is connected to pin 7 (Vee).
    * The video buffer must use the full power supply.
    Hopefully, this answers your four questions.

    I don't like much what Zoomcat is doing. The transistors aren't used as a
    buffer, but as a switch. If a pin on the parallel port becomes '1', a base
    current (DC!) will flow from the port, through the resistor, the base, the
    emitter, and the connected device. This will saturate the transistor,
    allowing the video or audio current to flow from the input to the output.

    I think the OP uses a asymmetrical supply. In that case, you can connect
    pin 7 to pin 8 (and all other grounds), but make sure that the DC component
    at each switch must be about half the power supply voltage. Take a peek at
    my Macrovision killer that uses a 4053 switch:
    http://home.planet.nl/~heuve345/electronics/projects/MacrovisionKiller.html
    Here, R3 and R4 add the DC component. C1 ensures this DC component doesn't
    reach the input.
     
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