pulse counter using 555 and Decade counter

Discussion in 'The Projects Forum' started by ieda, Dec 25, 2008.

  1. ieda

    Thread Starter New Member

    Oct 14, 2008
    7
    0
    HI!!
    I hv some problems with my project :heart rate monitoring and alarming system ..here is the flow of my project:

    decade counter ---> binary to 7 seg decoder --> 7 segment display
    (7490) (7447A)

    these three connected together to display up to 3 digit...my problems are:

    1) how i'm going to make the counter to reset(000) every 60 seconds...and start counting again from zero....repeating... i plan to use 555 Timer in my cct....

    2) How can i activate the alarm(buzzer) when the display number on 7 seg display exceed the required no.??.... for example, 7 seg should display number below 100 in 60 seconds, and when it display 120, the buzzer will ON


    Really hope someone can help me with this project...PLEASE...
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Yes, a 555 will do. Arrange the 555 as an astable with 60 second period, or roughly 30 seconds ON time and 30 seconds OFF time. Using a capacitor of 10nF, couple the 555 output to the pin2 & 3 of every 7490s. This will reset the counts to zero on the rising edge of the 555 output every 60 seconds.

    This is a bit tricky. When the 7490 counts up, its output changes. You can use a 8-input magnitude comparator like 74LS682 to 74LS685 which has a P>Q output. You can setup the Q value as the maximum heart rate alarm value and connects the "P" inputs to the 7490s output. Then use the P>Q output pin on the IC to drive the buzzer. The upper 2 bits of the magnitude comparator should be connected to the "hundred" 7490's output, with the next 4 bits to the "tens" 7490's output. The remaining 2 bits is then connected to the most significant bits of the "unit" 7490's output. This will allow an alarm to go off on multiple of 4 beats. So say from 4, 8, 12...100, 104, 108,...200, 204....upto 398.

    Edited later: R-2R mentioned in the following can't be used as counter output is not binary weighted but BCD coded. One can use BCD weighted resistors though.

    The alternative method is to use R-2R resistors on the 7490's output to convert the count value into a DC voltage. Then use a comparator to set the voltage trip point to sound the buzzer.
     
    Last edited: Dec 26, 2008
  3. ieda

    Thread Starter New Member

    Oct 14, 2008
    7
    0
    THNX A LOT!!!... i learn something... do u have the schematic coz that can help me to understand it better... there are things in your explanation that i dun fully understand....especially,the connection...

    and yes, really hope i can get the schematics.......
     
    Last edited: Dec 25, 2008
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    If you don't understand something in the post, just ask. Many people here can answer them to your satisfaction.

    If at my time of college I was given schematics I don't think I would have learned anything. Providing schematic to you without your input is just wrong and I will not do it. Sorry.

    It is your school project so you should try making a schematic first instead of asking for one. It forms a significant part of learning electronics because a schematic is not easy to draw out as there are many errors that one can make without knowing.

    However, you can always post your schematic or part of it here so we can help you find out what is wrong and you can learn this way.

    I would start by getting all the data sheets of the ICs one wanted to use from the internet. Search the internet for "7490 counter" and you will get 1000's hits if not more. Look at the schematic done by others and try to adopt part of them for your own use.
     
  5. ieda

    Thread Starter New Member

    Oct 14, 2008
    7
    0
    i'm sorry...atually i'm not asking for the whole circuit.. just want to make sure the connection...
    by the way, hv some question

    u said " couple the 555 output to the pin2 & 3 of every 7490s. This will reset the counts to zero on the rising edge of the 555 output every 60 seconds" .

    so, u mean that i must connect the 3 TIMER OUTPUTs to 3 DECADE COUNTERS ??? is it correct... so it will count from 001-009, then reset 'unit part', 'tens part' increment by 1 (010), and then the 'unit part' will start counting again from zero till 9....is it??

     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102

    Connects all the pin2 and pin3 of three 7490 together and then add a resistor of 1K to ground. Then a capacitor from pin2 and 3 to 555 timer output. This reset operation will reset all the 7490s into "0".

    The first 7490 does not reset, it just roll over. Roll over is different from reset because reset means jump to "0" regardless of present count value. Rollover is the counter moving on to the next count and the next count after "9" is "0" for 7490. One of its outputs, QD changes only once for a complete count cycle of 0~9 so this change can be used to clock the next 7490 and so on...

    So it will count from 000 to 009, then first 7490 rollover to 0 but also clocks the second 7490 into "1". You then get 010, next is 011....until you get 019, 020,...,099. Next counter change is interesting. First 7490 rollover to "0" and clocks the second 7490 to rollover at "0" and this causes the third 7490 to count to "1" and you have 100.
     
  7. ieda

    Thread Starter New Member

    Oct 14, 2008
    7
    0
    hi, L.Chung,

    i have some stupid question to ask...if u dun mind...

    i read about astable timer...it says the this timer will generate pulse..what is that mean?? if this timer generate pulse, then what about the heart beat(pulse) that i want to count??? can u explain it....

     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The 555 astable timer in your case generates a single pulse every 60 seconds. You are using this pulse to reset your counters, every 60 seconds.

    It has nothing to do with heart beat pulses. Those pulses are input to the 7490s and what you wanted to count in the first place.
     
  9. ieda

    Thread Starter New Member

    Oct 14, 2008
    7
    0
    thanks for your explanation...
    now, i understand better.... i want to start construct my circuit...i hope u can help me if i have problems with my circuit in the future....THNX!!!
     
  10. ieda

    Thread Starter New Member

    Oct 14, 2008
    7
    0
    hi!!
    wanna ask some help... u mentioned about magnitude comparator...and u said ' set the value oF Q" . How can i set it...do i have to program it:confused:?? can u recommend web sites that can guide me to use this magnitude comparator....i searched for it but nothing can help me to find out how to set the value.:(...
     
  11. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    A magnitude comparator has two values to compare. One is "P" which is changing and you connects that to the 7490 counters outputs. Another value "Q" is sort of fixed or pre-determined which you decided before hand as part of the design parameters.

    You can set the "Q" value by direct connection of those Q input pin(s) to logic 0 or 1, or using Dip switches so you can change on what value to use.

    I would advise you to get the datasheet of the magnitude comparator and read through it.
     
  12. mortensenal

    New Member

    Feb 26, 2009
    1
    0
    Can you explain why you used a coupling capacitor instead of just tying the 555 output and pins 2 & 3 of the 7490s together with jumpers? I made a decade counter and tied my astable clock directly to the clock input on my 4017s and it is working fine. Do I get the same result from the 555 in either config?
     
  13. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Pin#2 & 3 of 7490 is the reset inputs. If you are not using a coupling capacitor, then the reset signal from 555 will hold the 7490 in the reset state while the 555' output is HIGH and thus the 7490 cannot count up.

    Of course one can arrange the 555 to only output a short high pulse every 60 seconds to get the same result, without using a capacitor.
     
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