Two 555 timers, 2 different frequencies one and power supply

Discussion in 'General Electronics Chat' started by monkeyhead, Nov 22, 2007.

  1. monkeyhead

    Thread Starter Active Member

    Mar 5, 2007
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    Hi there,
    I'm trying to make a circuit using two 555 timers set at two different frequencies. However I've set the two timers at different frequencies and the seem to conflict with each other. For example unless both are set at the same frequencies they work fine. If i alter one value of the capacitor for example then it'll effect the frequency for the other timer!

    Is it even possible to use the same supply for somthing like this?

    Many thanks
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    555s put big current spikes on the power rail. The wiring impedance and/or the power supply impedance converts this current spike to a voltage spike, which tends to cause the oscillators to "injection lock" each other. This is actually a problem whenever you have two oscillators in close proximity if they are nearly the same frequency. With LC oscillators, electromagnetic radiation can cause injection locking, and sometimes extraordinary shielding, grounding, and supply decoupling measures are required to prevent it.
    In your circuit, I would try a 100uF cap from +12V to ground, and 0.1uF caps on each 555 from pin 8 to pin 1, as close to each IC as possible, and with cap leads as short as possible. A ground plane would also help. If that doesn't work, you might have to add a small-value resistor (10 ohms or so) between +12V and pin 8 on each IC.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
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    I've seen the caps soldered to the IC pins in production units.

    An *opps* later means some underpaid worker has to solder them on. Employed people yayz?
    :)
     
  4. GS3

    Senior Member

    Sep 21, 2007
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    Some years ago I designed a simple timer for photographic enlargers and I ran into a strange problem. The output of the 555 monostable timer governed a relay directly and everything went fine except sometimes you'd get 2X or 3X the expected time. After some investigation I found out that when the 555 cut off supply to the relay this somehow triggered the 555 again and the process started over again. The problem was resolved putting a diode in series with the output and a capacitor to ground. See attached schematic.
     
  5. monkeyhead

    Thread Starter Active Member

    Mar 5, 2007
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    Thanks for the feedback, much appreicated!

    Basically what I'm trying to do is adapt a "heads or tails" circuit, using a 555 timer ,which sends its pulses to a 7474, which in turn acts as a divide by two counter. What ever the result is, either one led will light, or the other.

    The way I want to adapt this circuit is so that say for example that the "heads" light is lit then a signal will be sent to an additonal 555 timer at a set freqency to output a tone for a few seconds. And if tails is lit, then a signal to be sent to another 555 timer at another frequency to output a tone, again for a few seconds.

    In theory this should be possible right? I was told I might hit into some problems though..

    Anyhow would I be correct in saying that I should use a monostable 555 timer for outputting the tones for only a few seconds? And connecting the trigger inputs from the 555s to the Q and G' of the 7474 right?

    Many thanks
    Matt

    See attached for the circuit im adapting so far.
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
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    Probably this is caused by a ground loop. A decoupling capacitor should solve the problem. Perhaps a 10uF cap should do. If the 555 timers are to switch substantial loads, you should use a 0.1uF cap near each timer.
     
  7. GS3

    Senior Member

    Sep 21, 2007
    408
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    Probably not. While "ground loop" is one of those terms loosely thrown around it actually has a precise meaning: http://en.wikipedia.org/wiki/Ground_loop_(electricity) I doubt that this is the cause of the problem in this case.
    Well, that's like aspirin, it never hurts and it sometimes even helps. BTW, I assume you mean a decoupling capacitor connected to the power and ground terminals. It is always important to have a smooth supply voltage.

    BUT

    as I have shown in my previous post, transients can and do enter by way of other pins and all the power supply decoupling in the world is not going to help with that. Sometimes it pays to find out what is actually causing the problem and then solve it directly rather than just assuming common causes and throwing capacitors and loose terms at them.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yes, but if you have NO supply decoupling, it's the most likely problem, especially since he apparently is not driving any inductive loads, which I believe was the cause of your problem. Remember Occam's Razor. ;)
     
  9. GS3

    Senior Member

    Sep 21, 2007
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    I agree with you but decoupling the supply (which, as I said, should always be done) has little to do with a ground loop. They are two totally different things. That was the point I was trying to make.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I'm not sure exactly what you have in mind. You need to draw a schematic, and we can critique it.
    Below is another way to do it. The monostable at lower left triggers when you release the pushbutton. It allows the astable at lower right to run until the monostable times out. I included 2 pots to set the two tones. The pots do not interact, but since the duty cycle is necessarily low so that the pots will have plenty of range, I ran the output of the astable through another toggle flip-flop, so the output duty cycle will be 50%, independent of frequency.
    Note that this only uses one output transducer (speaker), and you may need a (simple) power amplifier, as the 7474 doesn't have nearly as much output drive capability as a 555. If you were intending to use a separate speaker for each tone, you will need two astables.
    Note that I have not tested this, or even simulated it. It could have errors.

     
  11. cumesoftware

    Senior Member

    Apr 27, 2007
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    Then again, probably it is. Faulty grounds can increase the influence of spikes. Ground loop is a very broad term. Resistance in the power supply conductors may have the same effect of a ground loop, and indeed, those factors are correlated.

    Don't forget that a ground loop is not only formed by the ground conductor, it is also formed by a power or signal conductor, or else it wouldn't be called a loop.
     
  12. GS3

    Senior Member

    Sep 21, 2007
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    And bad wine can cause headaches but neither of those things is a ground loop.
    No it's not. You are using it in a broad unwarranted way. Do you know the meaning of "loop"? How can there be a "ground loop" if there is no "loop" to begin with, ground or otherwise?
    Oh man. :rolleyes: Accepting for a moment just for the sake of argument that the assertion is true, having the same effect as X is not the same as being X.
    Now you are contradicting what you said before and saying what I just said. OK, we are in agreement now.
     
  13. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Indeed. And good wine too!

    Ground has resistance, as any other conductor. Think in broader terms.

    Correlation is an implication, not an assertion. Assertion is the equivalent of equality, and I didn't prove an equality.

    No, a faulty ground is a ground where the ties are not in the same AND the ground path between them has too much resistance for the current biasing it. So, where is the contradiction? The caps solve the problem, because they do ground decoupling, but I bet my sweet potatoes that just using a big cap with the grounds tied together will solve the problem.

    Concluding: Resistance IS the cause of ground loops.
     
  14. monkeyhead

    Thread Starter Active Member

    Mar 5, 2007
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    Thanks Ron for the help!

    I'm now wondering whether i should have posted this thread I've appeared to triggered something off!
     
  15. thingmaker3

    Retired Moderator

    May 16, 2005
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    Your question was welcome Monkeyhead, as will be any you have in the future.

    You did not touch anything off. We debate among each other from time to time. There is no animosity between GS3 and Cumesoftware - only a passion for our shared discipline. And perhaps a divergence of viewpoint. We all can learn from good, polite debates.
     
  16. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Indeed. That is true. We are discussing, not fighting.

    You didn't triggered anything. Any doubt, feel free to ask.
     
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