Time delay in buzzer beeps using 555 timer

Discussion in 'General Electronics Chat' started by Xufyan, Apr 27, 2011.

  1. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    Hello,
    I have designed this circuit,


    [​IMG]

    and placed a buzzer in parallel with the LED, however when i am applying input voltage through a 9volts battery the delay between beeps is quiet good and LED is also flashing fine but when i am supplying input voltage of 4.8volts (through 4.5volts adapter) then there is no time delay between the beeps of a buzzer and LED remains on.

    can you please explain why is this happening ? and how can i calculate the time delay of the circuit myself ?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    can't it be done through 4.5v input at pin 8 ??
    i cannot change the input voltage from 4.5volts because it is coming from the output of another IC
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    If you feed 4.5V you are bumping the spec for the 555 hard. To top it off the 555 can only output 3.2V, since the high side driver is a darlington. Part of the problem is when you bump into the bottom spec you are pushing limits that should not be pushed, what if the power is 4.45V? Components also vary in performance, you may have run into a chip that can't handle that low a voltage, in other words its quality is boarderline, but if you were to use 6V it may not matter.

    I have seen more than one design where the 555 is powered by the output of a chip. This is terrible design practice, made worse by the simple fact that pin 4 is a digital input for exactly this use. You could tie the pin 8 (the power supply pin) of the 555 to a power supply buss and still use pin 4 for exactly the same way.

    One last thing, there is a chance your power supply to the 555 is noisy. Did you provide filtering for the 555?

    The sticky on the Electronic Chat forum explains this...

    Decoupling or Bypass Capacitors, Why?
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
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    The OP is using 4.8V, not 4.5V.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    No guarantees as to what the OP is doing, we go by what is said. There are many 555 circuits that have trouble with 5V, but not 6V.

    The design he has shown should work all the way down to 4.5V. However, if there is a lot of noise (or ripple) on the power supply to the 555, all bets are off. The 555 is pretty immune to noise, but everything has it's limits.
     
  7. nerdegutta

    Moderator

    Dec 15, 2009
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    Could he use the 4.8v to turn on a NPN transistor, and then use a higher volt to the 555 chip?
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    He could, I thought about it, but as usual we're not sure what else is happening. If the power supply is pulsing to ground (the ultimate noise) the 555 would restart every time.

    At this point he wants to flash an LED and beep a buzzer. The circuit he has is valid. So barring wiring errors, there is something else going on.
     
  9. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    I increased the voltage from 4.5 to 6 and it is working now :)
     
  10. nerdegutta

    Moderator

    Dec 15, 2009
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    @Bill: Ok, it was just a quick thought.:)

    How did you do that?
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    It sounds like a starvation issue from here. It could still be noise, if the voltage of the power supply dropped significantly below 4.5 with transients then all sorts of unexpected things could have been happening.

    Would you like a quick and dirty two transistor driver that can be used with a CMOS 555? CMOS 555s can go down to 2V on the power supply.
     
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