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counting input from a massive array of simple sensors

Discussion in 'Electronic Basics' started by Avital, Nov 4, 2003.

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  1. Avital

    Avital Guest

    Hi,

    I need help figuring out a relatively basic circuit, I'll break my
    question into two parts:

    1) Simple 8-sensor case

    I have a circuit that takes a clock signal from a 555 timer and feeds
    it into a 7490 counter which generates a 3 bit BCD number (I ignore
    the 4th bit) that's fed into a 74151 (8:1 Multiplexer). My sensors
    (all very simple and independent), generate TTL level output of logic
    '1' and '0', these inputs are fed into the multiplexer inputs. As the
    count comes into the multiplexer, it "connects" each sensor input to
    the output in turn.

    **I would like to be able to count these inputs and display that
    count**

    I have a set of 7-segment LED displays that are driven by chained
    7490s (and the 7-segment decoders obviously - 7448s), all that is
    required to set the number is a serial set of pulses into the clock
    input of the first 7490 in the chain. This is NOT a rigid requirement
    of my circuit - just the way things are set up right now.

    My thinking behind the setup was to simply feed the clock input of the
    "master" 7490 that drives the LED displays from the output of the
    multiplexer. This caused erratic counts, so I added a "debounce"
    circuit in between the multiplexer output and the "master" 7490
    clock-input. This seemed to give less erratic counts (this is, after
    all on a breadboard with relatively loose wiring...noise is to be
    expected, or so I figured.) I also hooked up the chained 7490s feeding
    the LED displays to reset them to the '0' digit whenever the count
    reached 8 on the multiplexer by using an AND gate and the 'C' input on
    the 7490 that feeds it.

    This approach is not working, I'm not sure why and I'm looking for
    a) suggestions on current design problem
    b) better ways of doing what I want it without making major changes to
    the entire circuit

    2) Extension of (1) to 128 or more sensors

    Say the scheme figured out for part (1) works, will it extend to 128
    or more sensors? What is the best way of doing this? I realise that a
    circuit of this type is better designed with a PIC of some sort but at
    this stage, I would like to implement it with "off-the-shelf" TTL and
    passive components.

    Thank you for any help in advance!

    **PLEASE USE THIS EMAIL ADDRESS INSTEAD:**


    (remove the letter that sounds like "wye" and the words "nospam" to
    get actual email)

    Avital
     
  2. -------------
    You want a faster clock so you can use higher order counter bits as
    address and use lower bits for preventing timing conflicts and for
    gating. Then you can speed it up for more sensors without fear.

    -Steve
     
  3. Avital

    Avital Guest

    Could you be more specific? I realise I can use a larger count to work
    with more sensors. Was wondering if there are better ways of doing
    this? And, ideas on why the current design does NOT work.

    Avital
     
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