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74hct4020 question

Discussion in 'General Electronics Chat' started by jack0987, Mar 20, 2017.

  1. jack0987

    jack0987 Member

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    I have a 74hct4020 that I am clocking. I connected a led to Q0 as a test. If I have my hands close to the circuit it does not seem to light the led.

    Do I need to ground unused outputs?

    Anyway, in general how does one work with it?

    What if I want to connect a led to each output?

    thanks
     
  2. AnalogKid

    AnalogKid Well-Known Member

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    Speaking only for myself, schematic, Schematic, SCHEMATIC.

    ak
     
  3. JLNY

    JLNY Active Member

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    I don't think you need to ground the outputs, but you will probably want to ground the "MR" reset pin and add a 0.1uF decoupling capacitor across the power and ground pins if you haven't already. If the reset is left floating, it could be reacting to ambient electric fields and could behave as you describe.

    I'm not sure what you mean by "work with it" exactly... is there something specific that you want it to do? It should count up in binary for each clock pulse. If you are already sending a clock signal, then it sounds like you pretty much have the gist of it already.

    The maximum rated supply current at Vcc is 50mA, so you would probably want to buffer the outputs if you plan to run an LED from each output.
     
  4. dave

    Dave New Member

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  5. jack0987

    jack0987 Member

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    Thanks. The capacitor did not help but grounding the reset pin seems to have done the trick.

    Is there a particular driver IC that I might use and I'll look in parts box to see if I have one or two on hand.
     
  6. JLNY

    JLNY Active Member

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    Hmm, that's what I figured. Your description sounded like a pretty textbook case of a floating CMOS input. The decoupling cap is not always strictly necessary, but is generally considered good practice.

    There are LOTS of buffer/driver ICs that would be able to drive an LED. I don't really have any specific recommendation off the top of my head...

    If you can't find anything on hand and really want to test, you might be able to use some discrete transistors to drive the LEDs (along with some current limiting resistors, of course), but that might be a bit cumbersome to wire up for 14 separate inputs.

    I mean, if it's just to test and you don't need the LEDs to be super-bright, you could always limit the current to the LEDs so that the combined current wouldn't exceed the rating of the IC. Some 10k or 4.7k resistors in series with each LED would probably work just fine.
     
  7. AnalogKid

    AnalogKid Well-Known Member

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    The ULN2003/2004 is a great general purpose driver for experimenting. 7 drivers in a package, base resistors built in, a single ground pin, and rated for 50 V and 0.5 A. 8 in a ULN2803/04.

    ak
     
  8. Mikebits

    Mikebits Well-Known Member

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    I don't think you would ever want to ground an output, input maybe, but never an output. Of course I know you were thinking input :)
     
    Last edited: Mar 20, 2017
  9. JLNY

    JLNY Active Member

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    Oh, I guess I meant more in the sense of "not floating" rather than "shorted to ground." That would be a bad idea. The RF engineer in me wants to put termination loads on everything lol.
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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  11. Mikebits

    Mikebits Well-Known Member

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    Not always. It depends on how the input affects the output driver (if you care about power consumption). In some cases grounding the input as opposed to pulling it high may consume more power. So, it would be more accurate to say "Unused inputs should not be left floating". Pulling high or low depends on the part being used. Of course you cant go wrong pulling low :)
     
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  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Yes your correction about the inputs is good.
     
  13. jack0987

    jack0987 Member

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    LOL. I am looking to do something like this, only in TTL as I am only using a 5VDC power supply, have a lot of those type of parts on hand, and the TTL is, IMHO, kind of bomb proof for a little guy like me.

    SCHEMATIC: http://ronj.eu5.org/rt.html

    In the above circuit, I would also like to replace the 4001 stuff with a 555 one shot.
     
  14. jack0987

    jack0987 Member

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    I have lots of 7490s on hand and (still looking) and I think a few 7 segment driver ICs as well to replace the 744020. This would work quite nicely for me.

    What do you think?

    EDIT: I do have some 7447s but now think I'll just stay with the binary. Still looking for a binary driver of sorts.
     
    Last edited: Mar 21, 2017
  15. jack0987

    jack0987 Member

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    Thanks. This is not a part I will have on hand. Is there something similar you can think of I might use without base resistors built in?
     
  16. aardyvarky

    aardyvarky Member

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    OptoSw.jpg
    Perhaps the OP meant one of these (commonly known as a photo-interrupter or opto-switch)? If mounted upside-down at the top of the slot then some filtering of smaller coins can be achieved (tut tut @ naughty people). Also, although the clock input should be normally low, the coin will create a high-low-high pulse, which is enough to clock the 4020, as long as it is fast enough... the sensor is normally a long way down the coin tube so that the coin is travelling quite fast by then - that also stops lolly-stick triggering (tut tut again!).
     
  17. AnalogKid

    AnalogKid Well-Known Member

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    I don't think there is a TTL part 744020. AFAIK there is no TTL part with an on-board oscillator and counter-divider. You can build the oscillator out of two TTL inverters or inverting gates and drive a string of 7490s with it.

    For output drivers, a small-signal NPN transistor with a 4.7K base resistor will work. 2N4401, 3904, 2222, etc.

    ak
     
    Last edited: Mar 21, 2017
  18. JonSea

    JonSea Well-Known Member

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    Just FYI. The ULN2003 is an awesome, cheap driver chip (and very common). But it's ok 7/8s as awesome as the ULN2803. The same part with an addition channel. For some reason, itdoesn't seem as well known.
     
  19. jack0987

    jack0987 Member

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    I do not understand. What I am doing has nothing to do with coins. Did I misstate something?
     
  20. jack0987

    jack0987 Member

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    Thanks. I will look into this.
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    TTL is so old that I do not look at it.
     

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