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Help with 555 timer circuit

Discussion in 'General Electronics Discussion' started by guskenny83, Jul 1, 2016.

  1. guskenny83

    guskenny83

    46
    5
    Jul 29, 2009
    Hi.

    Im playing around with 555 timers trying to learn how they work.

    I built a circuit very similar to the one here:

    https://encrypted-tbn3.gstatic.com/...AU87g7llTWJMHiOtBb1GbvFdqO_6r1LDFZI6uu-IhlyaB

    With the intent to just make a simple square wave oscillator, with the frequency controlled by a pot at R2, so i can try out different resistor and capacitor values to see what changing them does to the frequency ranges.

    When i hooked a speaker up to pin 3 (output) and pin 1 (ground), i got no sound. But when i hooked it up to my cheap oscilloscope, it seemed to be putting out a nice square wave, and at a good frequency range, i just couldnt hear it through the speaker.

    When i accidentally ungrounded the speaker, i got a very faint sound through it, and then when i touched the end of the speaker wire with my finger, it got louder.

    So i experimented with touching the ground speaker wire to various parts of the circuit, and when i touched it to pin 5 (control) it worked great with a nice loud square wave sound coming from the speaker.

    My question is, why did this happen? And should it? My understanding is that pin 5 is there to allow the frequency to be controlled by an external voltage, and is not normally used for anything in these simple circuits. So why does it suddenly make my speaker work when it is connected to it?

    Is it because the speaker shouldn't be connected directly to ground and there should be a resistor or something in between, and connecting it to pin 5 is allowing the speaker to ground through some resistance? Im probably completely off the mark with my reasoning, but that's the only thing i could think of.

    Any help understanding this would be greatly appreciated.

    Sorry if it is a very simple question, im just trying to learn about all this stuff..

    Thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,364
    1,901
    Nov 17, 2011
    The impedance of your speaker is probably way too low for the 555 to drive a signal through it. The 555 can drive ~200mA if I remember right (and depending on the specific type).
    At 5V that translates to R=5V/200mA = 25Ω A typical 4Ω or 8Ω speaker will overload the 555's output. By removing the ground wire and using your body as wire, the impedance increases considerably which lets the 555's output swing with full amplitude. Due to your body's resistance the current is rather small and the sound from the speaker is low.
    Pin 5 has a moderate impedance to ground, so the speaker became louder. What happened to the control voltage in this configuration will require some deeper analysis which I'm not in the mood to do :rolleyes:

    Try a 32Ω headphone speaker from the output to ground, that should work. Or add a buffer stage that is capable of driving a low impedance speaker. See e.g. here for more detailed information.
     
  3. BobK

    BobK

    7,642
    1,662
    Jan 5, 2010
    An 8 ohm speaker on the output of a 555 should certainly produce a sound, and quite loud, assuming the 555 is oscillating within the audio range. 200mA is just the max current the chip can output, when loaded higher the voltage will drop, but it will not stop completely. I cannot explain what you are seeing.

    Bob
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Just a thought but maybe his supply can't deliver 200mA or he's using a MOS 555?

    Chris
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    I forgot to mention that the speaker should be coupled to the 555 though an electrolytic cap. If it isn't it will respond to only half of a cycle.

    Chris

    Edit: This is poorly stated so I'd better correct it before I get a flogging. What meant to say is speaker (audio output) responds to a change in current flowing through the coil. A square wave switches from 0V to some voltage and back to 0V suddenly. The speaker will only respond (produce sound) during the periods that current is changing, not the flat topped high or low periods. The lower the frequency the more pronounced this will become to your ears. An electrolytic cap will attempt to keep current through the speaker coil changing over the 360 deg of the waveform. It will also give you a less raspy tone.
     
    Last edited: Jul 3, 2016
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