Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

555 Timer Triggering Problem

  1. Feb 11, 2009 #1
    I am having trouble with a mono-stable 555 timer circuit.

    When I connect this to a power supply (right now it is a 120VAC to 12VDC converter), the timer turns on and then off, like it would have if it was activated by the trigger. After this, when the trigger is used, everything works perfectly like it is supposed to.

    I am trying to figure out how to stop the timer from triggering right when it is connected to a 12V supply; I only want it to turn on when the trigger is used.

    I apologize for the crude schematic that isn't drawn properly:

    Things to note:
    - This will be installed in an automobile; the Remote Module is the ground connection that the remote module makes to sound the horn; right now its on a work bench
    - The charging resistor is a 100K variable resistor that only has one end and the wiper connected.

    Any help is appreciated. Thank you for your time!
  2. jcsd
  3. Feb 11, 2009 #2


    User Avatar
    Science Advisor

    You're probably getting some transient output on start-up. You could try to put a capacitor between the output and ground (in parallel with the relay) which should snub this transient. The RC constant will just be the capacitance and the coil resistance. You could also put a resistor in series with this capacitor to better control the RC time constant (if it's too short). Unfortunately, this'll also delay the remote sequence (but chances are that this transient is much shorter than the actual operating pulse).
  4. Feb 11, 2009 #3
    I found this advice,

    "When the "RESET" terminal is not going to be used it is normal practice to connect this input to the supply voltage. This is especially true of the CMOS version of these timers as the inputs of these devices are very sensitive."

    Though a transient on reset would pull the output low, wouldn't it?
  5. Feb 11, 2009 #4
    Thank you. Yes, it would pull the output to low. I will tie this to the supplied voltage to be proper but I dont think this is the problem.

    I will try the decoupling capacitor from + to ground and see if that fixes the problem. Thanks so far!
  6. Feb 11, 2009 #5


    User Avatar
    Science Advisor
    Gold Member

    Tie a capacitor to the reset pin, other cap lead to ground. Then, tie the reset pin to +12V through a resistor. This holds the device in reset during power up.

    I wouldn't advise tying a capacitor between ANY output on ANY IC and ground. I think there are more design flaws putting capacitors where they shouldn't be than not putting them where they should be.
    I also avoid using a bigger timing cap than necessary. The transistor inside the 555 has to discharge that monster. The more capacity you hang on pin 7 the more current it has to sink out of the cap when the output (pin 3) goes low. If nothing else, stick a couple of hundred ohms in series with pin 7.
    As far as transient in general go, look to where they could be coming from. The biggest is the power supply, next would be the trigger input. You should also consider transient suppression on the trigger input.
    Last edited: Feb 11, 2009
  7. Jul 29, 2011 #6
    connect the pin 4 to 12v then replace your 30 micro farad by .01 micro farad. ive tried it...
  8. Jul 29, 2011 #7
    one thing, you should put 1N001 in the output pin before connecting to a relay so that the flow of current is one directional only.
  9. Jul 30, 2011 #8


    User Avatar
    Science Advisor
    Gold Member

    Pay attention Jamers. The thread is about 2.5 years old. There is no 30 micro farad in the circuit, you didn't read my suggestion about what to do with pin 4. Really can't see the point of a 1N001 in series...
  10. Jul 31, 2011 #9
    i mean 30 nano farad capacitor in the circuit ( pin 5). the purpose of 1n001 or the extra diod is to protect the 555ic when connecting it in relay to drive higher voltage. ive been using monostable circuit in my water vending machine and i dont have any problem in false triggering.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook