Odd duty cycle timer circuit.

Discussion in 'The Projects Forum' started by Razzle308, Mar 17, 2016.

  1. Razzle308

    Thread Starter New Member

    Mar 17, 2016
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    Hi all.
    I need what appears to be an odd duty timer.
    I have looked at many both in google and here but most seem to be for high frequency or just too complex for me. :(

    I need to build a circuit with a 1 to 3 minutes OFF and .5 to 3 seconds ON with the application of a 12vdc source, cycling continuously. The project is to run on gas tiki torches that I want to flame up every few minutes.

    (it needs to be "fail safe" - as in if it fails the connected relay/solenoid is off.)

    Any one able to help please?

    TIA.
     
  2. Picbuster

    Member

    Dec 2, 2013
    374
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    use a pic make a minute counter reset @ x (1>N) minutes
    @ N produce a high pulse use the same mechanism count @ 100mS low @ wanted time.
    make a watchdog forced halt on the pulse (wanted time +1) so when duration of pulse >wanted time + one system stop.
    This will cost you 30min programming. use a pic 12f675 and a fet to control torch.
     
  3. Razzle308

    Thread Starter New Member

    Mar 17, 2016
    22
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    Thanks picbuster but that qualifies as "to complex" especially since i have no ability or software to pic program.
    Hoping for a schematic with a 555 or so.
     
    Dodgydave likes this.
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    3 seconds to 3 minutes is a 60:1 duty cycle, and 0.5 sec to 1 minute is a 120:1 duty cycle. Both of these are within the range of an 8-bit counter driven by an oscilator. A CD4060 or CD4521 counter/oscillator and two signal diodes can produce the timing waveform you describe. What is the power source, what is the output, what is the relay/solenoid, and what is the rest of the stuff we need to know?

    ak
     
    Last edited: Mar 17, 2016
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Plus one or two months to learn PIC operation, C++, the program development environment, and the programmer.

    ak
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Use 555 timer or cd4093, or cd4060 chip,

    I would use a cd4093 one gate for the timer with two presets and steering diodes one cap, .do you need a drawing.

    How are you going to detect flame by heat or light?
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    1. Do either the on time or off time have to be adjustable?

    2. If yes, do they have to be independent? For example, 64 seconds off and 1 second off fits nicely in a 7-bit counter (no, not 63 and 1). If the counter is driven by an adjustable oscillator, then turning up the delay to 2 minutes automatically increases the output pulse to 2 seconds. Is this OK?

    Of course, non-adjustable is easier. Here is a first pass. Approximate times: on = 1 s; off = 64 s. These can be adjusted by changing R1. For example, increasing R1 to 47K increases the off time to 1 m 23 s, and the on time to 1.3 s.

    Note - this is a preliminary schematic and has no output driver, power source, power supply decoupling, on/off switch, etc. because the TS has not given up any details.

    ak
    Torch-Timer-1-c.gif
     
    Last edited: Mar 17, 2016
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    OR -

    I'm not a big fan of running a 555 timer with very high component values, but this application is on the edge of being in CMOS 555 territory.
    Calculator: http://www.ohmslawcalculator.com/555-astable-calculator
    R1 - 860 K
    R2 =15 K
    C1 = 100 uF
    Thigh = 61 s
    Tlow = 1 s

    This works if the output stuff is activated by a low signal, such as pulling down on the gate of a p-channel power MOSFET or the base of a PNP power transistor. Note that the times will not be very accurate because large electrolytic capacitors have very poor component value tolerance, and temperature coefficient, and the charging current is a significant fraction of the leakage current. With a little tweaking it probably can do the job, but its not my first choice.

    If you need the other output polarity, you can add a 1-transistor inverting stage to the 555 output, or change the 555 circuit to the 2-diode/separate resistor version for a fully-adjustable astable.

    ak
     
    Last edited: Mar 17, 2016
  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    A RC oscillator plus a diode will do it.

    However, a mcu is the way to go for accuracy, flexibility plus low part count.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Are you looking for something that appears to be random in the 1 to 3 minute range or is a consistent time between flare-ups ok (with a consistent duration of each flare-up).
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    image.jpg
     
  12. Razzle308

    Thread Starter New Member

    Mar 17, 2016
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    Hi.
    @Dodgydave. A drawing of what you suggest would be great. See if it is suitable. I have gas igniters that sense the flame.



    @AnalogeKid The 555 calculator is great, added that to my favorites.

    The high (on) time needs to be fairly finite as the time it takes to create the flame i want is going to be precise. I just don't exactly know yet what the burner will be. This is what is happening if you can open it:


    Here is a flow of what the setup will look like. Torch actuation flow chart.jpg
     
  13. Razzle308

    Thread Starter New Member

    Mar 17, 2016
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  14. #12

    Expert

    Nov 30, 2010
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    To adjust the three second part you attack the 27K resistor. Replace it with a 2.2k resistor and add a potentiometer of 50K or 100K, linear taper.
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Here's my take using a Cd4093, two complimentary variable pulsed outputs of 1-4 seconds and 2-4 mins.( drawn in a hurry soz)

    DSC_0013.JPG
    Uses two gates, a 556 timer would do the same job as posted earlier.
     
  16. Razzle308

    Thread Starter New Member

    Mar 17, 2016
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    Thanks everyone for their input - great to get some helpful people.

    Is this correct Dodgydave... Love the simplicity :)
    Torch timer quad nand.jpg

    @ Picbuster...you got me thinking now. I have long wanted to do pics, or audrino. There is another facet of this system (pressure pads to accuate - Tombraider style) which require a modded keypad format. But right now I don't have time to learn. I'll use staged 555's for now. Maybe when i'm finished with the landscaping I can pursue a pic do do it all. :)
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Not quite, pins 2,3 wrong have sent you the circuit by message.
     
  18. #12

    Expert

    Nov 30, 2010
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    I think Personal Messaging doesn't work for members with less than 10 posts.:(
     
    GopherT likes this.
  19. Razzle308

    Thread Starter New Member

    Mar 17, 2016
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    Thanks so much Dave for the circuit. I have it up and running now and it's perfect.
    I'll post a vid soon but it didn't want to run the automotive relay so I need to get a lower milli-amp one i think.
    Thanks again for your help :D
     
    Dodgydave likes this.
  20. Razzle308

    Thread Starter New Member

    Mar 17, 2016
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