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Logic Gate problems (AND 4081)

Discussion in 'General Electronics Chat' started by Maarten, Aug 7, 2017.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I guess you didn't see the Texas Instruments datasheet for the CD4081. With a 5V supply the typical shorted output current from the ordinary CD4081 is only 3mA and 4mA. Into an LED without a current-limiting resistor the current is even less.
    Maybe you are thinking about a high speed Cmos gate (74HC series) that can supply much more output current.
     
  2. ci139

    ci139 Active Member

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    instead of this "Quanted Voltage Follower" describe your further goal

    . . . i assume (never tested) that these LED-s would show out better using them @ the std. duty 1:10 where you manage to flash them with 5 and more mA . . .
     
  3. Maarten

    Maarten New Member

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    Well actually in these topics my only goal was to understand a few things... Because putting up a circuit found online won't really give any problems.. So I tried to follow the circuit step by step and came across simple problems.. and later on I did actually find in the other circuits the poll-up/down resistors etc.. But this time I did understand why those resistors were used.. And did learn about the floating pins, which JimB actually told me before when i asked about a Timer555.. But didn't really fully understand it that time.. But this time i understood it better..



    Ah, I see i made actually a mistake in those numbers... Kinda weird i skipped the number 4... And in the post I mentioned the supply..
    Yeah about the LED and the input... It is not the best idea.. But it worked out on the board.. But those LEDs were actually more for me to understand and see how it actually works.. But if I did actually need and output which both gives a Light(like signal that it is on) and at the sime time i want to give it a signal to another output pin.. How should this be done then? Is there a better way for than i did on the circuit?

    Thanks a lot for the information.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An LED with a current of only 0.51mA will be very dim. That is why I said that an ordinary Cmos gate with a 5V supply cannot drive an LED and have a logic high voltage at the same time. The gate can easily provide a low input current to a transistor that can provide plenty of current to the LED then the logic high voltage will be fine.

    Why is the value of your current-limiting resistor R3 only 200m ohms which is 0.2 ohms and is the same as a piece of wire? If the LED is a 2V red one then the 0.2 ohms will have a voltage drop of only 0.51mA x 0.2 ohms= 0.0001V!
    Actually, the datasheet shows that logic levels for ordinary Cmos gates using a 5V supply are a minimum of 3.5V for a logic high and a maximum of 1.5V for a logic low.
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    For test monitor purposes, wouldn't a high-brightness type LED be quite readable at that current?
     
  7. ci139

    ci139 Active Member

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    depending on led the daylight background threshold is from 270 to some 800+ µA - so the clear lens red (← has a lowest FW drop) may show out quite good at the 500µA - it may considered too much fuzz to set up stand alone indicators until you make one (= get the experience + get already that much better on making such . . . http://www.ti.com/product/CD4050B)
     

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