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Raster Scan Generator (555 timer chip)

  1. May 20, 2013 #1
    Hi all,
    I need to create a circuit for displaying something onto an analog oscilloscope screen. I have determined it will be best to use a raster scan generator (essentially a linear ramp generator). I am wondering what might be the best way to design such a circuit in order to allow for variable frequencies.

    Something like this, but capable of varying the frequency (maybe by having a pot. somewhere?) I am still unsure exactly how a 555 timer works its magic, so maybe some explanation would help too...


    Thanks so much
    Last edited: May 20, 2013
  2. jcsd
  3. May 20, 2013 #2


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  4. May 20, 2013 #3
    Thanks, but I was wondering which resistor controls the frequency and why.
  5. May 22, 2013 #4


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    Andrew, only a baby needs someone to hold his hand. Have you entered these terms into Google Search yet: "control of 555 timer frequency"?
  6. May 22, 2013 #5
    If I found what I am looking for on Google, I wouldn't be asking the question on physics forums...

    I understand that if you throw in a potentiometer you can change the slope of the raster scan, thereby changing the frequency because slope = i/c. This much is obvious to me, but unfortunately it is a macro explanation, and it does not tell me how the circuit functions. Apparently you do not understand it either, so instead of calling me a baby, how about you Google "how to be helpful on a physics forum".
  7. May 22, 2013 #6


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    Andrew, excuse me, please. I apologise for my sarcastic response to your query. It was not appropriate, and I am sorry to have been so unthinking. I do not visualize my role here on Physics Forums as a teacher. I do think that I can occasionally direct someone’s attention to a source of information whereby they may, through some effort and self-study, discover the information they are seeking.

    The 555 timer is known as the “venerable timer” because it’s been around more than forty years. Probably it is the most used timer IC on our planet. To control the operation of this venerable IC one does not “throw in a potentiometer”; an exact procedure is used to get the desired outputs.

    The frequency of oscillation of the 555 astable multivibrator depends only on the resistor-capacitor chain (RA,RB,C) and is independent of the power supply voltage Vcc.

    On charging, the external capacitor C charges through resistors RA and RB. The charging time t1 is given by

    t1 = 0.693 (RA + RB) C

    and this part of the cycle is signaled by a high level on the output (pin3).

    On discharge, the external capacitor C discharges through the resistor RB into pin 7 which is now connected internally to ground. The discharge time is given by

    t2 = 0.695 RB C

    and this part of the cycle is signaled by a low level on the output.

    The total time for one oscillation (the period T) is given by the sum of these two times

    T = t1 + t2 = 0.695(RA + 2RB) C

    The frequency F is given by the reciprocal of the period, or

    F = 1.44/(RA + 2RB)C .

    With the appropriate choices of external timing components, the period of the oscillation can range from microseconds to hours.

    The duty cycle DC is the ratio of the time the output is low as compared to the period

    DC = RB/(RA + 2RB)

    The duty cycle is always less than 50% or saying it another way, the off time t2 is always less than the on time t1. Thus the output of the 555 astable circuit is asymmetric. By making RB large compared to RA, the waveform becomes more symmetric and the 555 output approaches a square wave.

    The following seven websites describe the operation and use of the venerable 555 which may be useful in your project. All were found using the Google search engine:

    This calculator computes the resistors and capacitors for a NE555 timer chip, which has been configured as an astable multivibrator (oscillator), or square wave generator. Just enter in the duty cycle and the frequency and the calculator will compute reasonable values for the resistors and capacitors. http://www.daycounter.com/Calculators/NE555-Calculator.phtml






  8. May 22, 2013 #7
    Thanks for all the information Bobby. I appreciate your appology and would like to point out that I am not merely posting the question to have you do the research for me. I actually already built a variable frequency raster generator and have found an appropriate way to change the range of frequencies. I was posting with a hope that I could learn more about how the circuit I linked works, so that I could make it more stable with a larger bandwidth. Perhaps I could have been more precise in my posting.

    As for these calculations, I will refer to the links you posted and try to find more info online. Again, thanks for the appology. I may not be a baby, but I am a high school student, so I am still sort of new to the world of ICs...

    If you want to check out my progress, I am building an SEM and keeping a blog at Bourhiscookecoalition.blogspot.com
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