TTL voltage regulator for low voltages

Discussion in 'General Electronics Chat' started by Bilnigma, May 28, 2008.

  1. Bilnigma

    Thread Starter Member

    Mar 10, 2008
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    i have a circuit with three ICs (so far), with output from one serving as input to the other and so on. the first is a PIC which passes a logical HIGH or LOW to an OR gate, and finally to a D-flip-flop. the problem is that the HIGH output from the PIC is about 3.8V. this is acceptable to TTL but the output from the OR gate is less than 2V.

    <image here>

    does any1 have an idea of how to make all the outputs from each IC to be only either 5V or 0V (HIGH or LOW)?
    i once used a voltage regulator to accomplish a similar goal but i was having voltages that were higher than 5V volts. the voltage regulator made them five volts. right now i am thinking of using a BJT like PN2222 to accomplish that but i want to know if there is another way, an easier way.
    unless the problem is from how i connect to my IC inputs and outputs (i use pull-down resitors), this should be a problem faced everyday by others.

    any suggestion is appreciated
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is not very clear. What are the IC's that you are using? What is the Vcc applied to them? Is it the same as the Vcc to the PIC? Why pull-down resistors? If you have a limited output switg with TTL it is possible to use pull-up resistors to improve the situation (a pull-down is tied to circuit ground, a pull-up is tied to Vcc).

    As far as voltage regulation, the LM7805 is a dependable device.
     
  3. Bilnigma

    Thread Starter Member

    Mar 10, 2008
    11
    0
    the ICs i am using are PIC16F877A, 74LS32, and 74LS273. i am not using a pull up resistor, will that help? but i am using pull down resistors. all the ICs have the same VCC, the problem is that the outputs from each IC, which should be equal to VCC or ground, are less than VCC. the output decreases with more ICs.
    pls refer to the image i attached to the first thread. :)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    The image does not show the resistors. Why are you using pull-down resistors? They may be preventing the logic from going to an acceptable level.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Look at the spec's for old 74LSxx TTL logic ICs on their datasheet. Their minimum output voltage is only +2.7V when they do not have a pull-down resistor and is lower if they do.

    I think a 74LSxx output voltage is typically about +3.4V when it is high and without a load.

    74LSxx TTL outputs can sink 8mA but can source only 0.4mA max.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Audioguru, your statement, "Their minimum output voltage is only +2.7V when they do not have a pull-down resistor and is lower if they do." is confusing?

    Low-level output should be below 0.4v with a 4mA current. High-level output may be as low as 2.7v. You may need to use pull-up resistors to get closer to Vcc.

    LS TTL is very limited in sourcing current from high-level output.

    If you went to 74HC series components, or 4000 CMOS, you'd likely have better results.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Sorry, I missed the HIGH word:
    "Their minimum output high voltage is only +2.7V when they do not have a pull-down resistor and is lower if they do."
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Why not go to a different family of 74xx with the same pinouts?
     
  9. Bilnigma

    Thread Starter Member

    Mar 10, 2008
    11
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    beenthere: i am using the pulldown resistors because the outputs were not stable when i was not using them earlier. now i'll try doing without them again.

    Audioguru n Sgtwookie: yes, those voltage levels are approximately wut i get. my problem is after gettin 3.8V from an output as logical HIGH, i get 2.0V as output from the next whose input was the 3.8V previously mentioned. i am looking for a way to make that 3.8 to 5V before passing to the next IC. now i'll also try using pull up resistor.

    Bill: i'll take ur advice but since i have most of the components to try the suggestions by beenthere, audioguru n Sgtwookie, i'll make it a last step.

    thnx yall
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Use 1k pullup resistors.

    If you go much lower than that, you'll have problems getting below 0.8v for the logic 0's.
     
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Normally, you dont need pull down resistors. if your outputs are astable is due to noise or bad connections. If your pull down resistors have a low value, then the output of the chip will source a higher amount of current than with a higher value resistor. This higher value of current causes a significant voltage drop across the internal resistance of the IC and causes the output voltage to be lower than expected.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    If the logic levels are unstable then you need some supply bypass capacitors. Each old TTL IC needs a 0.1uF ceramic disc capacitor located close to the IC's supply pins.
     
  13. Bilnigma

    Thread Starter Member

    Mar 10, 2008
    11
    0
    ok Guys, now the problem is solved. i have all the outputs giving me a value of about 4V.
    the problem origniated from the PIC, the 74LS32 and 74LS273 were just passing on the error originating from the PIC like a chain reaction.
    what i did was i inserted a pull-down resistor at the PIC's output, this made the voltage level increase from about 3.8V to 4.2V. i removed all the pull-down resistors at the outputs of 74LS32, this made the voltage to increase from about 3V to 4V. 74LS273 was no different and the final output was about 4V.
    i guess the moral here is diff ICs have diff requirements... :). thnx
     
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