problem, 7490 COUNTER

Discussion in 'The Projects Forum' started by homam, Aug 17, 2010.

  1. homam

    Thread Starter New Member

    Jul 27, 2010
    helow evry body ..
    Now i am preparing the 7490 whith, 7447 , 7_segment, and a logical state //resistor whith push potton // my problem whit the 7490 is
    when i provide the 5v by sequence //from the logical state//the 7_segment disply the following numbers{0 1 2 3 8 9 c 4 5} i connect qa whith the input b supplying 5v and gnd connecting the wire acording to data sheet diagram??
    Pleace could any one of you help me
  2. Retiredguy


    Feb 24, 2007
    It sound to me as though you have Qc and Qd of the 74590 flopped with the inputs C and D of the 7447. Check your wiring between the two
  3. homam

    Thread Starter New Member

    Jul 27, 2010
    I realy thank you because that was the problem and I solve it because of your help
    but now I am going to develope my idia to count motor cycle
    i connected a photo sensor and i got signs of it //it runs the counter//
    but how can i connect four (7-segment display and their counter)
    to disply the /rpm/
    and if i have to use another IC {that used with motors} pleace mention its name
    for me
    and thank you again..........
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    What speed of motor? How accurate did you want this rev counter to be?

    If you have a motor spinning at over over 1000 rpm you might want to run your signal through a decade counter but ignore the least digit. It might tend to change and flicker constantly unless it is some type of sync motor.

    But you can also smooth that out with your sample rate. You have to pick how often you want to load your display buffer and reset your counter. If you take 1 sample per second then you need to multiply your count by 60 before you load your display buffer. If you sample 20 times per second which is as fast as practical then you need to multiply your counter value by 1200, load the display buffer and reset your counter. At that sample rate you start to need to check your delay on recycle times. You could miss an average one or two counts while you are doing a display update, and when the count is mulitplied by 1200 that would give you an error of 1200rpm per missed count. So either you use very fast circuitry, as in less than 1% of a single revolution rate per display refresh cycle. Or just reduce your sample rate towards 1 sample per second so that you have a larger base count with lower multiplier error.

    Human perception makes the faster refresh practically useless unless you are also logging the samples to a computer, so can we assume you will go with a 1 or 2 seconds sample rate? If you go with just under 2 seconds (1.875s) you can use multiply by 32 which makes life slightly easier in a digital circuit.
  5. homam

    Thread Starter New Member

    Jul 27, 2010
    dear MR potato pudding:
    i realy dont have a certain motor to count its revolution
    but in general I want to use my device with generators.
    but I still have some questions FOR YOU COULD YOU ANSWER ME.
    1-how could i multiply the counts by 1200 or 60
    2- could I use the (ICM7217A)for displying the revolutions?
    3-how about using the (lm 555) for timing the photo sensor.?
    >>>>>>>>>>>>>and thank you so much
  6. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    With generators? Are you trying to get 50 or 60 Hz AC output by any chance?

    Are you trying to recreate Line Voltage because I would rather not help you with that.

    But to answer your questions.
    1-Your easiest way to multiply by 60 or 1200 is with a microprocessor, and that is a good way to go, because a $5 microprocessor can save a lot of components and circuit building.
    If you multiply by 32 or 64 you can easily use an adder loop. Basically add your count to itself, then add the sum to itself, and add the sum to itself...

    Each adder pass is x2 so you get x2, x4, x8, x16, x32, x64.

    Another way to do it is to to just use a lookup table which takes you back to the microprocessor more than likely.

    But all of this counting revolutions will have a high lower boundary of frequency that it can accurately count. You have to slow down the sample rate in order to get better accuracy. For example if you are taking 20SPS, then you might get the same count for 19 to 22 Hz. If you sample every two seconds then .5Hz to 1.5Hz could give you the same counts. Those are rough numbers so somebody is likely to correct those.

    The more accurate way to count low frequency is to count high speed clock signals with the sample rate being set by the revolutions. Time per cycle is as accurate as the clock signal which could be a microsecond or less with a 1Mhz or faster clock. In fact if you make the clock frequency as high as you can possibly count then this will always be more accurate.
    2-ICM7217A sort of yes. It even specifies that it can be used as a Tachometer which is a related job. But not the B or C versions which are clock chip with a max diplay of 59:59.

    The A will give you a count upto 9999 or possibly 999 999 99 if you cascade two ICs and convert a clock signal count into a value with fractional Hz.

    But what do you want to use it for? It is a counter and a display driver. it won't turn a count into an rpm output so easily.

    If you counted the revolution time with a Mhz clock your counted result could be a time in μs. That is useful and might be all you need. This IC would make that easy, Just a 1Mhz oscillator clock on the count pin, your light sensor tripping the store and (short delay to) reset, you connect your display and the circuit is basically done. You can change the clock frequency to 1 kHz to get times in milliseconds for lower frequencies or stay at 1MHz and cascade two chips to get 8 digits, almost 100 full seconds worth of μseconds. Or hours worth of milliseconds with a 1Khz clock.

    3. What do you hope to measure? The 555 is versatile with several different modes. For a precise measurement there are better clocks. I think you are suggesting counting clock cycles. Use some inverters and a 1Mhz crystal instead.

    If you wanted to use the 555 it would be in a rougher. You have a target frequency that the generator should be running at. You set the 555 up as a lightsensor triggered monostable with duration so that it should be on for half of the revolution. You send the 555 output into an RC averager, and if the generator is revving fast the off time between triggers is being shortened so the averager gives a high voltage. If the generator is revving slow, then the off time between triggers gets longer and the averager voltage drops.