Pins 4 and 5 on a 555 are weird

Discussion in 'General Electronics Chat' started by OldSkoolEffects, Jan 27, 2013.

  1. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    68
    1
    I wasn't able to find anything that covers this, so I thought I'd ask. I have a board design with a 555 in astable mode that drives a pair of 4015s and a 4017. I've actually wirewrapped this design, and had the 555 fully connected, including pins 4 and 5 with no issues. Fast forward to today, and I had a test PCB made, and the 555 will not work with either/both of these pins connected. A friend of mine who has a fair bit of experience messing with 555's told me he never connects them, and it never seems to cause issues. Lo and behold, I bent the leads out so they weren't connected, and everything worked.

    Are there certain limiting factors that will make pins 4 and 5 not allow the overall chip to function? I'd like to hear some opinions before I rework this board in EAGLE too much.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for the 555 tells you all about it.

    Pin 4 is the reset pin and is connected to the supply voltage at pin 8 to make the 555 work. The 555 is reset when pin 4 is lower than the supply voltage.

    Pin 5 is at 2/3rds of the supply voltage. Usually it has a filter capacitor to ground. The pin 5 voltage can be raised and lowered a little to make Pulse Width Modulation.
     
  3. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    68
    1
    I realize that the datasheet and every source I can find says that these pins MUST be connected, but I've found that to not be the case. I just don't know how I have seen several circuits that are fully functioning astable timers that have 4 and 5 removed from the circuit.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    It depends on the specific circuit and application. One circuit may require pins 4 and 5, another may not. You cannot apply a hard and fast rule as to where to connect these pins without considering the actual circuit.
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Leaving pin 5 open often works fine. The reset pin goes to the base of a transistor, if unused it is recommended to connect it to Vcc. Leaving it open may lead to unexpected behaviour.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Connected to what?

    Pin 4 should always be tied high if it is not in use; otherwise it is floating and can cause random resets. It costs nothing to tie it high, so why not?

    When pin 5 is not used, it is often tied to ground through a .01μF cap; the datasheet shows it that way, but then says it is optional. For the cost of a 15 cent cap, I think it is worth putting it in.

    You are correct in that there are many schematics on the web that don't reflect these two directions. In both cases, you can sometimes get by without it, but why would you, given the effort you put into building a circuit?
     
  7. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    68
    1
    Whelp, as usual, you guys are right. I popped on a new 555, with 4 connected and 5 not, and it's working. That wasn't the case before, but after looking at the first 555, it may have been damaged. So 4 doesn't necessarily need connected, but it's good practice to do so, if only to keep the high/low from floating?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Many Instructables are "designed" wrongly by little kids as young as 10 years old.
    They can't read yet so they ignor the manufacturers datasheets.
     
    jaygatsby likes this.
  9. KnRele

    New Member

    Jan 7, 2013
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    Well... if you want an unreliable circuit, pin 4 can be left unconnected, but if you want a circuit that works all the time, every time, then pin 4, reset, has to be connected to the positive voltage (or driven by the output of some other device). Some of the older transistor-ones might tend to make open look like High, perhaps similar to what TTL does, but for CMOS variants, all bets are off.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Several times you have talked about pin 4 and pin 5 being "connected" but, not once, have you given any indication of HOW you are connecting them! (or, if you have, I missed it). In general, when asking a question it helps A LOT if you provide a reasonably complete description of what you are asking about. In this case, we have no idea if by "pins 4 and 5 connected" you meant you were connecting them both to ground!
     
  11. Brownout

    Well-Known Member

    Jan 10, 2012
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    Maybe he connected them to each other?
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I asked in post 6, but never got an answer.
     
  13. sheldons

    Active Member

    Oct 26, 2011
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    heres how pin 4 and 5 can be connected.....iv not shown all the timing components and all other connections needed....pin 4 is the 555 reset pin which if not used should always be connected to pin 8(supply),if it is grounded the 555 will be held reset...pin 5(control voltage) is best decoupled with a suitable capacitor to ground.
     
    Last edited: Jan 28, 2013
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