changing on/off ratio of 555 timer cycle

Discussion in 'The Projects Forum' started by lowrise4, Jul 11, 2013.

  1. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    In Make: Electronics, on page 169 you are shown how to chain two 555 timers and use the first one to modulate the tone of the second one. I constructed this circuit on a breadboard and it worked good - a decent siren sound.

    It also suggests several component value changes to modify the sound. My schematic shows the top half of this circuit only since my question pertains to the first 555 timer. One of the suggestions - to modify the sound of the first 555 timer - is puzzling me:

    "Halve the value of R5 while doubling the value of C4, so that the cycle time of IC1 stays about the same, but the On time becomes significantly longer than the Off time."

    But wouldn't this result in the charging time of C4 (the 'output-on' time) staying about the same, and also the discharging time (the 'output-off' time) staying about the same? (since the cap charges through both resistors, but discharges through only the bottom one). Thanks.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    With R4 much smaller than R5, the duty cycle approaches 50% - equal on and off times - since the timing of charge and discharge is controlled mostly by R5.

    Halving R5 makes R4 relatively more important, although not nearly as much as it could be.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Try this format for 50/50 duty cycle, make both resistors the same value

    555
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think there was a mistake in the text.

    If R4 = 1k, R5 = 10k, and C4 = 68μF, the frequency will be 1.0084 Hz and the duty cycle will be 52.38% high.

    If R4 = 1k, R5 = 5k, and C4 = 136μF, the frequency will be .9626 Hz and the duty cycle will be 54.55% high. Hardly a noticeable change.

    But if R4 = 500Ω, R5 = 10k, and C4 = 136μF, the frequency will be .5165 Hz and the duty cycle will be 51.22% high. Here, the frequency is halved, but the duty cycle is hardly changed.

    But, if R4 = 10k, R5 = 5k, and C4 = 68μF, the frequency will be 1.0588 Hz and the duty cycle will be 75.00% high. That accomplishes what the author wanted, but may not be the values he/she intended.
     
    Last edited: Jul 11, 2013
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  5. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    Thank you! I tried it and it worked - finally the on-time is significanly longer than the off-time, and the frequency is about the same. Since this was not listed in the errata of this book, I figured maybe my understanding was wrong. Thanks also to others who replied.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Be careful to keep R4 at least 100 Ohms per volt of Vcc (your positive supply), as otherwise you risk burning up the 555 timer due to excessive power dissipation.

    If you place a diode (like a 1N914 or 1N4148) across R5, cathode (end with the band) towards the timing cap, then R4 affects the charge time, and R5 affects the discharge time; they are then independent.
     
    Last edited: Jul 12, 2013
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  7. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
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    I'll take note of that - thank you. The only thing I remember seeing in this book is to ensure you don't reverse the polarity on the power supply pins. And I was wondering if there were any other precautions to observe.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You should read the datasheet for the 555 timer from several different manufacturers. I suggest starting with National Semiconductor's datasheet (now merged into Texas Instruments) :
    http://www.ti.com/lit/ds/symlink/lm555.pdf

    You'll discover that it's recommended to use a 0.1uF poly or ceramic capacitor and a 1uF or larger aluminum electrolytic capacitor across the Vcc and GND pins. If you omit them, you can have difficult-to-solve problems, particularly if you have additional ICs in the circuit. When the 555 timer changes states, it momentarily shorts across Vcc and GND. Without having sufficiently large caps across Vcc and GND, nearby circuitry can behave unpredictably.
     
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