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Why does speaker on output click twice on 555 timer output?

Discussion in 'General Electronics Discussion' started by Enigma, Aug 3, 2012.

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  1. Enigma


    Jul 15, 2012
    Hi all..
    Maybe Im missing something but can someone please explain why a speaker tied to the output of the 555 clicks when output is on (when output goes high) and then clicks again when output is off (when output goes low)?
    An led turns on when output is high and off when low so why does the speaker make a click sound on both on and off??

    I bet its something so simple but im a bit lost :confused:

    Thanks Guys
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The speaker issues a click tone every time the output of the 555 changes because:
    1) when the output changes from low to high, the coil of the speaker is energized, thus moving the membrane in one direction -> click
    2) when the output changes from high to low, the coil of the speaker isde. enrgized, thus moving the membrane in the reverse direction -> click
    Assuming the speaker is connected to GND - if the speaker speaker is connected to VCC energizing and de-energizing change roles.

  3. BobK


    Jan 5, 2010
    When the output goes high, the speaker cone is pushed out -- click. When the output then goes low, the speaker cone returns -- click.

  4. BobK


    Jan 5, 2010
    1 minute too late.

  5. Enigma


    Jul 15, 2012
    thanks for the quick replies

    I can understand how the speaker clicks when the output goes high but I dont understand why it clicks when the power is cut off?
    what exactly is causing the click?
    The thing about studying electronics is you get to a point where you think it makes sense, then it just crumbles and you have to go over everything all over again! lol
  6. CocaCola


    Apr 7, 2012
    Bob, explained it...

    Forward or backward movement of the cone both produce sound... When you send the high signal the cone goes forward creating a click, and stays at that forward position until the power is removed, aka goes low, at that point it moves back to standing position creating another click...
    Last edited: Aug 3, 2012
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I have two answers. First, why a sudden movement in a speaker cone "sounds like" a click. Second, how a series capacitor affects the movement of the speaker.

    Assuming your speaker is directly connected to the output of the 555 (without a coupling capacitor) (which I hope is not the case), each time the 555 output changes state, the speaker cone moves rapidly from one steady position to another steady position, as others have described already in this thread. A positive voltage makes it jump outwards, and when the voltage disappears, it jumps inwards. (Or the opposite, depending on which way the speaker is connected.) Each one of those changes in the cone's position sounds like a click.

    Why does it sound like a click? Why does it seem to have a certain duration? It's because of a general phenomenon called high-pass filtering, where high frequencies are passed, and low frequencies are attenuated.

    When you listen to a loudspeaker, the sound is passed from the cone to your ear by movement of air molecules. There is an inherent high-pass effect in this coupling. If a speaker cone is reproducing a 1 kHz tone, it makes the neighbouring air molecules vibrate forwards and backwards quickly, and this vibration is coupled into other air molecules, and the sound tends to travel outwards from the speaker, until it reaches your ear and you hear it as a tone.

    But if that speaker cone was moving smoothly at, say, 1 Hz (that's once in and out every second), smoothly with no jumps, the air that moves in response to it is moving comparatively slowly, and the movement dissipates easily into the surrounding air. If you're a metre away from the speaker, for example, little if any air movement is present at your ear - it has all dissipated into the surrounding air, because the air molecules were moving so slowly.

    (Your ear doesn't respond well to very frequencies below around 20 Hz and your brain doesn't interpret them as sound, but that's beside the point for this explanation.)

    At the instant when the speaker cone jumps from its rest position to a new position, air moves quickly and that movement travels to your ear, where you hear it. But the cone stops moving, so effectively there is just one little burst of air pressure, and the pressure evens out again after a short time. This sounds like a click or a thud, depending on how long it takes for the air pressure to flatten out again.

    That is essentially why a speaker cone jumping from one position to another sounds like a click.

    You may have used headphones that have direct coupling into your ear canal, or at least sit very close to it. These can have amazingly strong low-frequency response because they are so tightly coupled to your eardrum. If you connect one of those to a battery, through a resistor to limit the current to avoid damage to the headphone and to your eardrum, you will hear a "click" in its true glory. It sounds more like a thud or a bang, and is accompanied by a feeling of physical pressure on your eardrum. (It's probably unwise to do this too many times!)

    But when you take the earphone out and listen to it at a distance, there is no bass audible at all. That's because all the low frequencies that are present at the cone of the earphone dissipate easily into the surrounding air. If you want low frequencies (slow-moving air) to travel through the air in any significant way, you need a bigger cone to move a larger volume of air. Have you seen a 15-inch subwoofer? And they need a cabinet as well, to stop the slow-moving air from following a local path from the front of the cone to behind the cone, so it's forced to travel outwards from the speaker.

    The answers so far have assumed that the speaker is connected directly to the output of the 555. You didn't say so, but I expect your speaker is connected through a capacitor. (It should be, otherwise you might damage the 555 and/or the speaker because significant current will flow when the output is high.)

    When you have an output connected through a capacitor to a load, you get an effect that's loosely called "differentiation" (not EXACTLY the same as differentiation used in calculus) or described as a high-pass filter (i.e. a filter that attenuates low frequencies, like the natural filtering effect of the air). These circuits are normally explained with a resistor as the load, but a speaker will have a similar enough behaviour.

    The effect of the capacitor in series with the signal is to couple the fastest-moving voltage changes more strongly than slow-moving changes. This is because the capacitor can respond to slow-moving changes by charging and discharging, effectively "following" the more gradual changes, and leaving less of that slow-moving voltage on the other side. This is the same as the behaviour of air - slow-moving changes are "absorbed" by the air but fast-moving changes are coupled through it.

    When the 555 output changes from 0V to VCC, the capacitor-resistor differentiator / high-pass filter sees a positive "step change" at its input, i.e. its input changes instantaneously from one voltage to another like a step. What you get at the output (i.e. across the speaker) is a sharp rising voltage followed by a tail-off where the voltage settles back to zero again. It looks a bit like a sawtooth but the falling part that is normally straight is concave instead. It represents the charging of the capacitor. This would be easier with diagrams; if you're interested you might want to google some of the keywords I've used here.

    Once that pulse has finished, the capacitor has charged up to the supply voltage, and when the 555 output returns low, the charge on the capacitor initially pulls the speaker below ground, i.e. negative. As the capacitor discharges, the voltage at the speaker returns to zero. The waveshape is like an upside-down sawtooth with the same concave shape as before.

    If you have access to an oscilloscope, try making your 555 oscillate at, say, 20 Hz and look at the waveform across the speaker. You will see a continuous 0V line interrupted by short pulses going alternately positive and negative with the distinctive shape - a sharp initial edge then a concave decay back to zero.

    When the coupling capacitor is present, the speaker cone just jumps briefly outwards or inwards, and quickly returns to the rest position. This sounds similar to the click produced by a step change in the cone position, but generally less powerful, since the action of the capacitor removes the low-frequency content. It may sound more like a "tick" than a "click".
    Last edited: Aug 4, 2012
  8. Enigma


    Jul 15, 2012
    Wow you guys are truly awesome, thanks so much for your help..
    I've understood that the speaker will make a sound when reacting to charge and the same sound upon release of the charge. The mechanics of the speaker means it produces sound BOTH ways (back and forth)
    The speaker is a 100w max full range with no coupling on the output and a 2k2 resistor tied to it so it makes a click with a low range base note which i can imagine will be more thud like given some amplification. When I attach, say, a 47nf to the output the sound becomes more tinny. Ive tried to attached a coil of solder wire to act as an inductor but it didnt work and im not sure why..
    I get the bit about using earphones so as to avoid going through air and I notice the effect also when I put my ear close to the speaker I can hear the lower freq's..
    KrisBluNZ that is the answer I was looking for! I need to go over your answer a few more times and do some relevent research on the topics you discussed, some of which I am familier with. Really appreciate your time and effort to help out, means alot!
    What a great forum, your all stars*
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    You're welcome :)

    "Charge" is not the right word. The speaker responds to current.

    When the 555 output goes high, current flows from the 555, through the limiting resistor and through the coil in the speaker, creating a magnetic field, which interacts with the static magnetic field created by the magnet in the speaker and produces a force which forces the cone outwards (or inwards, depending on which way the speaker is connected).

    A constant direct current will cause the cone to move to a position that's proportional to the current, and stay there.

    When the 555 output returns low, the current goes to zero and the cone moves back to its rest position.

    In both cases, the cone moves suddenly from one position to another. This sudden movement causes the sudden burst of air pressure (either increased air pressure or decreased air pressure) which you hear as the click or thump.

    A coil of solder will only have a very low inductance. When you're dealing with audio frequencies you want inductance in the high millihenry range; these are usually made with a large number of turns of enamelled wire and a laminated iron core. Do a Google image search for "crossover network".
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