555 problems

Discussion in 'The Projects Forum' started by envytea, Feb 18, 2008.

  1. envytea

    Thread Starter Member

    Feb 18, 2008
    10
    0
    So, I am building a series of tone generators using 555s. I have 3, all are set to produce a range of freqs between 6 to 500 Hz. All three circuits are situated in parallel with 9vdc on the line. However, I have the outputs going to one common line, for I am trying to mix waves. Each tone generator has a 100K pot to vary the freq. Thus, when all are in a common line, the wave is somewhat funky- it sounds pretty awesome! Unfortunately, there is a problem....

    Not all the generators work all the time. Sometimes only one or two work, sometimes none. The problem is weird. If I touch the capacitors coming from pin5, I can often get the vibrator working. Also, if I push on the top of the IC, the circuit will sometimes operate correctly. I'd guess that either the caps are not fully generating or there is some problem with static. I am sure that everything is wired and there are no foul connections, for sometimes the dang thing works alright. I am using an 9vdc trans rated to put out 800mA.

    What do you all think? For lack of a wiring diagram I have the following:

    555:
    Pin1 to gnd; pin2 to pin6; pin3 to out; pin4 to Vcc; pin5 with 0.01uF to gnd; pin6 to pin2, 1uf to gnd, 100k pot; pin7 to 3.3k R, 100K pot; pin8 to Vcc.

    I can concoct a visual should it be necessary.

    Any help would be tremendously appreciated. Good day all!

    Peace!
    NVT
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Chances are good that things will work better if each 555 feeds a mixer op amp through a resistor isolator.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    are you shorting the outputs of the 555s together? Don't doo dat!!!
    They have very high output current capability and will destroy each other or quickly will destroy the battery unless you mix the outputs through series resistors. The value of the resistors is determined by the input resistance of whatever they drive.
    What do they drive?
     
  4. envytea

    Thread Starter Member

    Feb 18, 2008
    10
    0
    AG,

    I have a 4.2K resistor in series at each output prior to mixing the lines. The common line goes to an RCA jack which runs to an amp. The resistors seem to diminish the current enough, for it is not too loud. Thank you for the question.

    Been,

    A mixer op amp....? This sounds interesting. Can you suggest a model; I am unfamiliar with this. Cool!

    Thank you for the help! I'll futz around a bit more and let y'alls know what's up.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Envytea,
    The 4.2k resistors in series with the outputs of each 555 IC are more than sufficient to limit the current to safe levels. Even if you're running the 555 ICs at 15V, the output current will be limited to 6.25mA, well within the limits of even the CMOS versions.
    However, you may be exceeding the limits of the 555 amplifiers when your pots reach their low-end thresholds. With just a quick glance and no analysis, I strongly recommend that you add a 3k resistor in series with the 100k potentiometer that has no low-side current limiter.

    Do you have capacitors across the ground and Vcc terminals of the ICs? You should have 0.1uF caps there on each one. If your power supply is more than 6" from your 555 ICs, use a larger electrolytic cap within a few inches of your 555s in addition to the 0.1uF caps across each IC's power supply pins.

    Since you're feeding the output to an amplifier already, and the sound is loud enough, adding an amplifier stage would likely just degrade your signal-to-noise ratio.

    Without the addition of diodes, 555 ICs don't "like" to output waveforms less than 52% in duration.

    Post a schematic diagram of what you have so far. What you've posted is somewhat helpful, but not completely clear. Your connections between pin 2 and pin 6, pin 6 and pin 2 and ground don't agree with each other.
    Generating a schematic is very worthwhile.
     
  6. ajays_sar

    New Member

    Feb 19, 2008
    1
    0
    hi
    i am here new
     
  7. envytea

    Thread Starter Member

    Feb 18, 2008
    10
    0
    Sarge:

    Thanks so much for the advice. I have scribed a schem, I hope it is useful. Now, recall, I have three of these circuits in parallel. Each has a switch at the 9VDC+, so each circuit can be shut off. The GNDs all lay together, unswitched, no diodes. The outputs, labelled "<TO RCA>" all go to a common line and meet an RCA jack with the negative on the other side.

    So, as I say, each circuit is switched. Therefore, there is about 10 inches on line between the transformer and the 555. Perhaps this needs shortening. Furthermore, it might have been erroneous for me to place the line from pin4 as I have. The circuit is laid out on an etched PCB. The line from 9VDC+ to pin4 goes under the two caps. I thought it might be a good route so that I needn't have extra wires. However, the PCB is pretty thin, and the caps might be interfering with pin4 voltage? Possible? It is interesting that when I touch the caps, especially the 1uF electrolytic, the circuits work better. Is it possible that the large concentration of charge is hindering the current on that stretch?

    Thanks again. I'll let you know how this is moving along.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    By turning on and off the supply voltage to an oscillator the tones from the others will have loud POP sounds. The tones are square-waves.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Previously, you said that you had a 3.3K resistor in series with a 100K pot from pin 7 to pin 8. Now you're showing a fixed 1.2k resistor. Is that new, or is that how it always was?

    You don't show a cap between the Vcc and ground pins. The capacitor should be 0.1uF, and the leads should be as short as possible.

    The 1uF cap is really rather large for the frequency range. Consider reducing it to 0.1uF for the lower frequencies, and 10nF (0.01uF) for higher audio ranges. This will reduce your current consumption and consequent heat dissipation.

    At the same time, replace your two 100k pots with two 500k pots, with a minimum of a 3k Ohm resistor in series.

    See the attached.
     
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