7805 Voltage Regulator + Distortion

Discussion in 'The Projects Forum' started by Ozy, Jan 9, 2011.

  1. Ozy

    Thread Starter New Member

    Jan 8, 2011
    Hi guys,

    I'm trying to use a 7805 voltage regulator to drive these ICs:

    Three CMOS logic gates (74HC04, 74HC08 & 74HC32).
    Two 555 timers.
    One 4066 (quad bilateral switch).

    The output of the logic gates has excessive noise when i used 7805 supply but the noise is reduced when I use a fixed 5V power supply.

    Any ideas on what's causing this distortion?
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009

    You won't get any help around here unless you post your schematic. :)
  3. cigarsnob

    New Member

    Feb 23, 2010
    Try using some caps on both the vin and vout pins of the 7805 regulator. 100uf for vin and say 10uf for vout pin. If the doesn't work, try adding some small caps .1uf or so on the outputs of the gates.
  4. bertus


    Apr 5, 2008

    Do NOT put capacitors at the outputs of logic IC's.

    You will need decoupling capacitors accross the powersupply lines of the logic IC's

    See this thread for more info:
    Decoupling or Bypass Capacitors, Why?

  5. Ozy

    Thread Starter New Member

    Jan 8, 2011
    1st of all thank u for the replies everyone, i really appreciate it.

    As for the schematic, i doubt it's necessary as this is more of a general question abt the 7805 voltage regulator's output.

    Abt the capacitors......i already added them; 0.33uF for the input & 0.1uF at the output.
    Actyally, i was wondering if the distortion has more to do with the CMOS logic gates as their inputs tends to be extremely sensitive n i only added 1k pull-down resistors for the floating inputs! Any ideas?
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    You need to post the complete schematic for further commentary - although it is conventional to tie unused inputs to ground or Vcc.
  7. SgtWookie


    Jul 17, 2007
    The 555 timers require a minimum of 0.1uF and 1uF capacitors across their Vcc and GND terminals.

    The remaining ICs require a minimum of an 0.1uF capacitor across their Vcc/Vdd and GND/Vss terminals.

    If you are trying to breadboard your circuit, long wires will be a problem. Try to keep them short.
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Do you have the exact same value caps on both the input and output of the 7805?

    Generally, the input should be 10x the output capacitance. If input and output are equal, the 7805 can oscillate.
  9. tom66

    Senior Member

    May 9, 2009
    For the 7805, what input supply are you using? Also, what does the noise look like - can you post a scope trace?