TS555 Circuit Question

Discussion in 'The Projects Forum' started by tracecom, May 13, 2010.

  1. tracecom

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    I am building the TS555 version of the circuit shown on the third page of the attached datasheet. The schematic does not show pin 1 of the 555 attached to ground. Is that an oversight or is it correct?

    Also, what component(s) in this circuit will radiate heat? I assume the 555 will, and if so, is there any area of the 555 that will radiate less heat? In other words, what pin(s) on the 555 will likely be the hottest and which will likely be the coolest?

    Thanks.
     
  2. oidium45

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    Apr 24, 2010
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    I would not worry about heat. Both the 555 and the resistors should dissipate a small amount of energy through heat but it should not be enough to require a heat-sink.
    FYI, Not sure if you noticed but there seem to be a few different setups between R1/R2 (pg 3) depending upon which manufacturer made your 555.
     
  3. SgtWookie

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    Tracecom,

    You are correct; pin 1 needs to be connected to ground.
    Also, pin 4 should be connected to Vcc/pin 8, and there must be an 0.1uF capacitor between Vcc and GND near pin 8.

    I have re-drawn the schematic and attached it with a simulation of it operating.
     
  4. tracecom

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    Thanks SgtWookie. I had already added a .1μf cap to the Vcc pin (based on previous advice you had given me.)

    My question about heat wasn't clear. I am not worried about the components in the schematic overheating; rather, I need to place a thermistor on the same PCB and wanted to put it as far from the hot spots (if any) as possible.
     
  5. SgtWookie

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    The "hot spot" will be the 49k resistor. It'll dissipate about 1.5mW.
     
  6. tracecom

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    Here's the layout I am planning; it is the best compromise of electrical necessity and physical convenience that I have come up with.
     
  7. SgtWookie

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    That looks neat.

    In case you need a local source for TLC555's, Radio Shack usually carries them.
     
  8. oidium45

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    it looks clean
     
  9. tracecom

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    Here is the assembly wired. I moved the 49.9k resistor a little further from the thermistor.

    Now, if I can figure how to test it without the microcontroller.
     
  10. SgtWookie

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    You could actually test it using the sound card in your PC, Sound Recorder, and LTSpice to examine the output waveform.

    You would need to build a voltage divider circuit that has protection diodes to prevent excessive signal levels from zapping your sound card line-in port.

    See the attached schematic.


    You'll need a 1/8" stereo mini-jack to connect to your microphone in port. The mic port is pink in color. If you connect to the wrong port, you won't get any signal.

    CAUTION: if you wire the circuit incorrectly, you risk burning up your sound card.

    The "ToSndCard" signal gets connected to the tip of the 1/8" stereo jack. The ground is connected to the shield.

    Adjust VR1 to the point where the Sound Recorder waveform is just below maximum. You may need to enable Microphone input using your Mixer application. If you can't find your Mixer application, you need to remove and reinstall your sound card drivers.

    Use Windows Sound Recorder to record a brief sound clip; just a couple of seconds' worth.

    Sound Recorder is normally located in Programs -> Accessories -> Entertainment.

    You can then use LTSpice to read the .wav file, and produce various plots to analyze the signal content, including FFT. The FFT plot will show you what the output frequency spectrum looks like.

    LTSpice is a free Spice simulator, available from Linear Technology. Google "LTSpice Download" for the link.

    I've attached waveanalysis3.src - it reads in phone1.wav as V1, and writes phoneout.wav after processing it through a filter. You don't need the filter or the .wave portion.
     
  11. Wendy

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  12. tracecom

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    Thanks Bill and Sgt for the tip on using a pc as an o'scope. I have an old laptop too old to run any Windows later than 98SE, so I put Ubuntu on it. Maybe I can put it to use.

    I spent most of the afternoon getting a breadboard set up to run a PICAXE-18X. Due to some stupid mistakes, it took much longer than it should have, but it's finally running. I need to interface it with an LCD display and then I'll be ready to test my humidity and temperature module.
     
    Last edited: May 15, 2010
  13. SgtWookie

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    I am not certain that LTSpice will run under Ubuntu, but here's a forum where a couple members say you can run it under wine:
    http://ubuntuforums.org/showthread.php?t=322499
    See posts #8, 9.

    As far as recording the .wav file under Ubuntu, you're on your own...
     
  14. retched

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    You can run LTSpice under wine. I do.
     
  15. tracecom

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    Although I still don't have an LCD readout for my PICAXE-18X circuit, I did connect my humidity sensor to the PICAXE. With my DMM, I can see the PICAXE supply Vcc to the TS555 and the 555 responds with a 2.49v output which I hope is actually a string of pulses indicating the humidity. If I knew more about the PICAXE, I might be able to somehow see how it is interpreting the output from the 555, but unfortunately, I am a raw beginner at microcontrollers.

    If I could somehow record the pulses from the 555, could someone view them and tell me the frequency?
     
  16. retched

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    Does your DMM have a frequency/duty cycle mode?
     
    Last edited: May 18, 2010
  17. tracecom

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    It's a cheap DMM, but it does have a Hz position marked 20K. However, it doesn't show an output from the 555 in the Hz position. On the DC volts position, it shows 2.49v and on the AC volts position, it also shows an output of 2.48v which seems to indicate that it's reading a pulse stream as opposed to DC (I think).
     
  18. retched

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    ....Hmm.... You can tie an led to the 3 (output) pin and swap the resistors/cap to slow the signal to see if it is ACTUALLY pulsing.

    Do you have access to a o'scope? Any friends or neighbors have one you can use for a few min?

    As for the AC/DC thing.. I dont know how much I trust the reading from your meter. If it was AC, it should read way below because it isn't swinging past 0v into the neg region. But, I dunno. Is the DC reading stable, or is it jumpy?

    As for no reading in the Hz mode.. thats odd.
     
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