Simple Square vave generator

Discussion in 'The Projects Forum' started by Lynton, Apr 6, 2014.

  1. Lynton

    Thread Starter New Member

    Apr 6, 2014
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    Hi this is my first post. I have the need to generate a square wave output from a constant 12v supply. The frequency needs to be about 19HZ the +12v part of the wave should be about 30ms and the 0v part of the wave should be around 23ms. I want the circuit to be as simple as possible and reliable. I was planning to use a 555 timer but all the calculations seem to require you to input the R and C values to give you the output Frequencies. I cant seen to get close to the settings I want with playing with the values.
    Any ideas or suggestions welcome.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  3. #12

    Expert

    Nov 30, 2010
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    This is entirely do-able in the math, but it's hard to find accurate capacitors. Be sure to allow for a bit of adjustment in your resistance to account for this weakness in the physical capacitors.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Bad news:
    1) The on time (duty cycle) cannot be more than 50% with that circuit.
    2) A 555 will not provide rail voltage at the output. Maybe that doesn't matter but you said 12V output. It will be more like 10V if the supply is 12V.

    Good news: You could add a PNP transistor to solve both problems. It will reverse the logic (off becomes on) and give you near to rail voltage.

    Oops, I was referring to a standard astable circuit, not the one in #2
    Double oops - I have it backwards on point (1). You cannot get LESS than 50% duty cycle with the standard astable. Point 2 still valid.
     
    Last edited: Apr 6, 2014
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Wayneh,
    You may want to give him a hint of how to turn the PNP off if is connected to the 12 volt rail and his 555 is always more than 0.6V from the 12 volt rail.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Here is a well used version of a 555 timer which allows independent control of on & off times. On time would be almost R2 X C1 except that timing starts with C1 already charged to 1/3 of VCC, so time needs to be about 70% of R X C. Same situation with off time. T = 70 % C1 X R3.
    As #12 said, caps vary from marked value, looking in bin found .033, +- 5%, .032, + 80%, -10%, so it is easier to use pots for R2, R3. R2 would be abuot 600k for 30 ms, so could be made up of a 100k pot in series with 560k resistor [ R1].
     
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  7. Lynton

    Thread Starter New Member

    Apr 6, 2014
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    many thanks a few ideas I will try later this week.
     
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