Display Frequency input in CD4056 driver

Discussion in 'The Projects Forum' started by MrEARTHSHAcKER, Jun 23, 2013.

  1. MrEARTHSHAcKER

    Thread Starter New Member

    Jun 20, 2013
    10
    2
    Hello,

    I have mentioned my project in my last post, but I'll say again: I'm making a timer with 2 displays presenting seconds up to 99.
    An user should be able to set it to some number and let it count down.

    Here is the question, I've bought 7-segment display, anode.
    But my driver CD4056 outputs logical 1 if a pin is being active, which can't launch a corresponding segment of the display.

    However, the documentation says something about DF (Display Frequency) input.
    As I understood, if it is on logical 1, the output pins will be low when active, and if 0, they will be high.

    I'm unsure about validity of my conclusion, but in case it is true, does it mean that this driver can suit both types of 7 segment displays, only by bringing appropriate voltage to DF?

    Thank you.

    *EDIT: It has been left unexplained: why don't I simply connect bought elements and try this out? The problem is I have been experimenting this in the school on protoboard, however the school is over, and I am still waiting for mine to be delivered. Multisim doesn't have this one, so I used 7447N, but it has no such inputs.
     
    Last edited: Jun 23, 2013
  2. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Could you post a schematic of what you have thus far?

    Before diving into the DF operation, what are the specs of your display? You said anode, so I assume you are using a 7-segment common anode LED display? Or a 7-segment LCD?

    What is the forward voltage and current of each segment?

    If the display is small and doesn't require much current, you can get away with connecting the display directly to the CD4056. If not, you'll need to use a driver between the CD4056 and the display. A ULN2004 will work if the circuit voltage is 6-15VDC. A ULN2003 will work if the circuit voltage is 5VDC.
     
  3. MrEARTHSHAcKER

    Thread Starter New Member

    Jun 20, 2013
    10
    2
    I don't have schematics in the digital form since I couldn't find this driver in Multisim.

    This is the documentation of my display.

    While testing on the protoboard in school, I was using 9VDC source, and applied the 330R resistor.

    The actual behaviour of the display connected to 4056 was: for number 0, only the G segment is on, for number 8, nothing is on etc. ( to summarize, when a pin is on logical zero, the corresponding segment is on ).

    Does this mean I wouldn't need the additional driver?
    I am still slightly confused, have I understood the meaning of DF correctly?
     
  4. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    This is a common cathode display. In order to power it, you need to "source" current to each of the segments. In other words, you need to be able to supply a positive voltage to each segment to get it to light.

    According to the datasheet, the CD4056 is capable of sourcing just under 1mA while your display requires up to 30mA. You may still be able to see something if you connect the display directly to the CD4056, but it's going to be dim. I'd suggest using a driver of some sort. Unfortunately sourcing ICs are a little harder to come by, but you can easily use a cheap transistor for each segment. I can throw together a simple schematic if you'd like.

    Sounds like the output logic needs to be reversed. Since you have a common cathode display, you need a logic 1 to turn on a segment. In most cases, you'd have to use a different IC or invert the output, but as you discovered the CD4056 has a pin that apparently reverses this - I haven't used this IC before, but reading the datasheet, I agree with you that changing the state of the DF will change the states of the output pins.

    Your circuit should work if you tie the DF pin to GND. This should cause the outputs to be a logic 0 when turned on. Again, it may be dim since the CD4056 can't source much current.
     
    MrEARTHSHAcKER likes this.
  5. MrEARTHSHAcKER

    Thread Starter New Member

    Jun 20, 2013
    10
    2
    Just to leave the last reply, if someone one day needs help about this.

    I got my breadboard finally, and tested DF input of 4056, and yeah, it works the way I described it above :D
     
    elec_mech likes this.
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