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Segment counter with IC 555 & IC 4026

Discussion in 'General Electronics Discussion' started by JBMyers16, Apr 5, 2017.

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  1. JBMyers16

    JBMyers16

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    Apr 5, 2017
    I built a single display counter (from CircuitDigest.com) using an IC 555 & IC 4026 along with a few resistors, capacitors, etc. Using a 9VDC supply it works perfectly. I used a small (.56" character height, Vf=1.8) display and I would like to substitute it with a much larger display. I chose one with a character height of 2.25" and a Vf of 6.8. But, of course, it won't work. I need help in determining what changes in the circuit design are necessary so I can use the new 7 segment, common cathode display. (The original circuit can be viewed at: Circuitdigest.com/electronics-circuits/555-timer-seven-segment-counter-circuit) Thanks to anyone who can help me on this home project.
     
  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Page not found error. Don't trust a link. Please clip and post the schematic.

    Why do you say "But, of course, it won't work."
    What are the typical segment currents of the old and new displays?

    ak
     
  3. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Found it. There are errors on the schematic. C2 should be connected across the switch to debounce it, not the output of the 555. Also, there are two problems with the 4026 driving a display directly. First, the schematic does not show any current limiting resistors between the 4026 and the display. Second, the segment outputs are standard Gen-1 CMOS outputs, rated for only 2.6 mA typ. source current. The two LED examples of display driving on the datasheet show segment drivers between the 4026 and the display.

    [​IMG]
    In fact, with C2 located as shown you can eliminate the 555 and all of its components, and simply connect the switch between the 4026 CLK input and Vcc. Decrease R4 to 10 K.

    ak
     
    Last edited: Apr 5, 2017
  4. Audioguru

    Audioguru

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    Sep 24, 2016
    The circuit is also missing a very important supply bypass capacitor mentioned on the datasheet of the 555. The datasheet of a ICM7555 Cmos 555 explains that an ordinary 555 causes a current shoot-through of 400mA (!) each time the output switches that causes the 9V battery voltage to crash and mess up the counter. Maybe use a Cmos 555 instead.
     
  5. AnalogKid

    AnalogKid

    2,415
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    Jun 10, 2015
    While I'm certainly not defending the 555, two comments. First, I've never seen the noise filter capacitor on pin 5 do anything useful, and I've been looking since the 70's. Second, the cross conduction current spike is large, but very brief. Normal power supply decoupling at the device pin is adequate suppression.

    ak
     
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    Pin 5 on a 555 needs decoupling if the supply jumps up ands down caused by another 555, and the supply is not decoupled.
     
  7. JBMyers16

    JBMyers16

    2
    0
    Apr 5, 2017
    Thanks for the quick responses to my segment counter problem. First I want to say the circuit that I initially built using the diagram shown in "Analog's" post above is the exact same one that I used. And it works perfectly. That is not to say though that there are shortcomings in the design that you both have written about. My original 7 segment display was a 0.56" character height with red segment output. It is listed with Vf = [email protected] 20mA and mcd [email protected] (I don't know what mcd refers to and it could be the culprit here.) I then tried to substitute a 2.24" character height, red output display. It is listed as having Vf = 6.8 @20mA and mcd [email protected] That display did not respond at all. I've found another display with 2.24" character height, red output that I might try. It's listed with Vf=2.0 @20mA and mcd [email protected] Before trying this new display though, I would like to address the problems you both have written about. Do you agree that I should eliminate the IC555 and components altogether? Or should I replace it with ICM7555? Even though the first (0.56") display works OK, should I install current limiting resistors on each segment and if so, what value? Should I try a 15VDC power supply or perhaps larger? Thanks again for your helpful comments as I wait for more.
     
  8. Audioguru

    Audioguru

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    673
    Sep 24, 2016
    Before it is destroyed by too much current melting the output transistors you should read the "Maximum Ratings (absolute-values)" on the datasheet of the CD4026. 500mW for the entire IC which is 500/7= 71mW for each output when all 7 are turned on. The datasheet from Texas instruments has graphs that show the output voltage with different currents and supply voltages, The datasheet also shows external transistors driving an LED display. But you can use a display with 2V LEDs and a low supply voltage producing a fairly dim display but not much heating, or a brighter display using a higher supply voltage and current-limiting resistors. I think the huge display with 6.8V segments needs a 15V supply and current-limiting resistors sharing the heat.
     
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