555 timer help

Discussion in 'General Electronics Chat' started by ajhoward, Nov 18, 2011.

  1. ajhoward

    Thread Starter Member

    Nov 16, 2011
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    I am trying to make up a fridge door alarm, using 555 timer chips, but the 555 that provides the alarm tone from the buzzer, is just remaining high. I am still at the breadboard stage, and have just set up the astable circuit to get that working first, but it doesn't the output just stays high. The circuit is identical as the circuit here, except that the output is driving a piezo buzzer. I tried another 555 timer chip, same deal, are they both dead?
     
  2. cube01

    New Member

    Mar 10, 2011
    16
    1
    Firstly, I assume you are talking about this circuit?


    [​IMG]


    The first thing I would check is that the IC is oriented correctly - pin 1 is at upper left, where 'upper' can be found by looking for a small tab missing out of the end of the IC, or by a small dot/circle on the top. Next I would say to make sure you are seeing that pin 1 is at the upper left pin 4 is at lower left, pin 5 is at the LOWER right (not top right, which is a common mistake), and pin 8 is at the upper right.

    Also, what values are you using for R1, R2, and C1? And what is your supply voltage?

    It will also be helpful if you can post a pic of your breadboard circuit.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. ajhoward

    Thread Starter Member

    Nov 16, 2011
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    This is what done it. Thanks heaps, such a simple stuff up. Yes, that is the circuit I was referring to, R1 & R2 are 10K, C1 is 100uF. I am using a 9V battery.

    Now on to my next hurdle or should I say hurdles. :)


    • I have a timeout 555 circuit connected to trigger the astable 555 circuit after a set period of time. The timeout circuit is triggered by a NO reed switch. However I only want the timeout circuit to trigger the alarm by the reed switch, not have the alarm come on before it is triggered at power on.
    • How can I make the alarm stop, once the trigger pin has been removed from ground?
     
    Last edited: Nov 18, 2011
  5. ajhoward

    Thread Starter Member

    Nov 16, 2011
    46
    0
    I think I have it, I can use a transistor to hold the reset pin of the monostable vibrator low while the reed switch is open and release it when it is closed.

    Bill answered the other with the input signal conditioner. :)
     
  6. cube01

    New Member

    Mar 10, 2011
    16
    1
    That sounds like a good solution...

    Glad I could help, and that you got it all figured out.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If you're planning on operating the alarm on a 9v battery, you'll be replacing the battery every few days - unless you have the power completely disconnected when the door is closed.
     
  8. ajhoward

    Thread Starter Member

    Nov 16, 2011
    46
    0
    Yeah, I have been trying to keep the consumption down as much as I could. What I ended up doing is using another 555 monostable timer connected to a solid state relay ic that disconnects all chips except for itself 30 seconds after the door is opened.

    What is the best way to choose the resistor values for the transistor?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Rbase = (Vin - Vbe) / (Ic / 10)
    where:
    Vin = the voltage on the end of the resistor opposite the base; in this case your output on pin 3 of the timer.
    Vbe = ~0.7v-0.8v under light to moderate load (moderate load being ~1/4 of the transistors' rated collector current)
    Ic = the desired collector current.
     
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