5V TTL to 3.3V TTL conversion

Discussion in 'General Electronics Chat' started by Andreas, May 9, 2013.

  1. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    Hi there,

    I have just built a microphone trigger cct using a trusty old LMC555 (very low power) running from a +5V (LM2905-5) regulator from a 9V battery.
    My output pulse is 100ms long and I have used a simple series divider, namely 820 Ω and 2k2 Ω to translate the 5V output to 3.2V for a 3rd party device. I am not sure what front end electronics my 3rd party device has (guessing hi input Z) but I was wondering if a chip (IC buffer) exists which can also translate 5V to 3.3V rather than using a series divider? The reason being that when the pulse is off, the path to ground via the 555 is only through 820 Ω. Now I could increase these values (say 5 fold) but then this tends to attenuate the output amplitude.

    Thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    What is wrong with using two resistors in the form of a voltage divider?
     
  3. Gibson486

    Member

    Jul 20, 2012
    199
    12
  4. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    Nothing wrong with a voltage divider but with only 820 Ohms to ground, I was concerned about the "possibility" of overloading the input of the other device.
    It does work but I am just being cautious and looking for alternatives and maybe a better design solution.

    I should mention that there is also an LED with 1K resistor hanging off pin 3 as well. So another current path to consider.

    I'm not sure why when I try different resistor values in the divider (i.e. 1K and 2K) that I get such noticeable differences in output voltage when the 555 is capable of supplying up to 100mA.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    My preferred solution to the 5V to 3.3V AND 3.3V to 5V problem occurs with memory and other devices that have an SPI interface. A CMOS device powered from 3.3V with 5V tolerant inputs and TTL thresholds is the ticket for going from 5V to 3.3V. Turns out the same devices can also be powered from 5V and with TTL thresholds of (0.8V, 2.0V) work just fine going from 3.3V to 5.0V.

    With an active solution in single gate packages why would you fool around with anything else? I don't get it.
     
  6. Andreas

    Thread Starter Active Member

    Jan 26, 2009
    68
    4
    Thanks for the input PB.

    In fact I could just go with a 3.3V supply. After all the 555 is good down to 1.5V operation. See attached feature list. Hmm...
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    Make sure that you can still get the speed you require after lowering the voltage. When the 4000 series CMOS chips came out in the 1970's the spec sheets clearly showed you could make things faster by using a higher Vcc.
     
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