Divide by Two Problem

Discussion in 'The Projects Forum' started by apprenticemart2, May 28, 2014.

  1. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7
    Hi All,

    I'm trying to put this circuit inside my drum machine and steal power internally so that I don't need any external power but it's pulling the voltage down to 4.052V from an initial 5.717V.
    [​IMG]

    This circuit takes 24 pulses and spits out 12 as is required by the drum machine.
    Is it correct to have unused inputs and outputs connected to GND?
    Is there a lower power/better way of doing this?
    Have I just reached the limit of power that the internal supply can provide?

    I'm using an M74HC74B1 as I don't have an HCT like the example given.
    I think that the 74HC should be fine running straight off the main power which is 6V?
    The drum machine is a Boss DR-110 and runs off a 9v DC adapter or 4xAA batteries.

    The power supply schematic is attached.
    Thanks.
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,793
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    You should NOT connect unused outputs to anything. Leave then disconnected, otherwise you can cause a short. Connecting unused inputs is recommended.
     
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  3. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7
    Thanks for your reply. I imagine that's the problem I'm having. The schematic in the first picture has pins 7, 8, 9, 10, 11, 12, 13 connected to GND and some of those pins are outputs?

    EDIT: I have just tried a 74LS74 without any inputs or outputs connected to anything just using +V and GND only to power it and it's still pulling the voltage down?
     
    Last edited: May 28, 2014
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Yes, exactly. I didn't look up the data sheet but you should have it and identify the output pins, Q and Q'.

    Ok, I did look up the data sheet. It appears that pins 8 and 9 should be left unconnected. They are the outputs of the second (unused) flip flop, 2Q and 2Q'.
     
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  5. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7
    I will try that (leave pins 8 and 9 unconnected on the 74HC74 I already have set up) and see if there is any change. Thanks.
     
  6. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7
    OK. So the voltage only drops to 4.425 now instead of 4.052.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    what is the value of the resistor connected to vcc? the vcc supply for the 74hct74 is speced at 7 volts maximum.
     
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  8. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    If your supply voltage is greater than 5V you may want to choose a CMOS version of a flip-flop, 4013 or 4027, for example.
     
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  9. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7
    I don't know how to work out what value of resistor to use so I put a 4148 diode in there to drop the voltage instead just in case but the 74HC I used should be OK at 6V without it?, and another 74LS from Ti that I have tested with should be OK too according to the datasheets.
     
  10. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7

    I could try a CD4013 instead, Thanks.
     
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the 74hct series is ttl compatable cmos.
     
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  12. apprenticemart2

    Thread Starter Member

    May 7, 2013
    67
    7
    OK folks. So thanks for your help so far.

    I have built this circuit using a 74HC74.
    [​IMG]
    If I leave all inputs and outputs of the unused second flip-flop unconnected to anything it works fine with no noticeable voltage drop in the main circuits.
    If I connect the unused inputs to each other only and not GND, or I connect the unused inputs to GND, the voltage drops by 1.6V. What is going on here? (all IC's brand new before use and tested so no shorts etc and all anti-static precautions taken)

    As for the circuit itself. I now have it working I think with an addition.

    At first the circuit was highly unstable so I tried a 10K resistor to +V on pin 3 clock input but this stopped the clock being divided down. It was still at 24 clocks and I need 12. I used my ears to tell me it was running too fast.I don't know how the 10K resistor would make it do that.

    I then moved the 10K resistor to +V and pin 1 reset that my run/stop signal goes to. It now runs at 12 clocks and seems much more stable but as the run/stop signal goes 0-5V am I missing a clock cycle doing it this way as I think it would be triggering on the down slope back to GND?

    I also changed the power supply input cap to 100uf and I may put a 100uf cap near to where I'm taking power for my modification.

    I just got a couple of 74 HCT in the post. I may try one of these now to see if the circuit works without the 10K resistor.
    Cheers.
     
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