First time making a PCB for motor running bot.What to do with Capacitors? and basic pcb question

Discussion in 'The Projects Forum' started by May-En Liu, Apr 21, 2017.

  1. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    Hi guys,

    So this is my first time posting here. My apologies if I'm in the wrong sub forum.

    I'm making this line follower robot with the L293D IC for the motor driver and the Hex inverter IC for the logical decisions, anyways as I was making my pcb, I read online some tips that power fluctuations may happen and cause my bot to perform not as I want it to.

    Besides designing my PCB with no 90 degree angles, daisy chains, etc. I came a cross an article that said that capacitors, even just the 0.1mF ceramic ones(after filtering the power through the 5 volt regulator) would help lessen the ripples if I put one capacitor per IC. So I stuck one of those for the L293D

    I'm using twp 9 volt batteries. One is for powering the pcb itself and the other is for the L293D's 8th pin to power the motors.

    The Hex inverter itself has no capacitor.


    Here's where I'm a little lost.

    1) Should I add another 0.1mF capacitor just for the hex inverter?
    2) Should I add a capacitor from the 9 volt battery before connecting it to the L293D's 8th pin? If I should, is 16v 100mF capacitor too much for a 9 volt power supply?

    I'm trying to decide if I should up the power to 18 volts(max for pin 8 is 36volts), but I have 2x16v 100mF. Will it work if I line those 2 capacitors up?

    3)If no. 2 doesn't work, what capacitor should I get for my 9volt battery?
    4)Here's my PCB design(is it alright or am I doing something that could lead to poor connection/results?)
    4 23.png 32.png
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    In a motor application, I would include decoupling capacitors on every IC. Hence, you'll need one for your hex inverter.

    A large capacitor across your power supply is not a bad idea. 47μF for example.

    Secondly, a 7404 Hex Invertor IC is a TTL chip. It's maximum supply voltage is 5VDC. You are mentioning much higher voltages. How are you reconciling this?

    Thirdly, all unused inputs should not be left floating. Tie them to ground. Note that this is not as critical with TTL, but I don't think you want to use TTL.
     
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Rule #1 - Read the IC data sheets carefully first, they often include PCB layout and decoupling capacitor recommendations.

    Rule #2 - Ground is very important- here I see a thin, meandering ground trace where it could be much bigger and heavier.

    Rule #3 - Place a 0.1uf ceramic cap close and tight to each chip's power leads- as short as possible.
     
  4. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    As I stated above, I'm using 2 batteries, one for the ICs and one for the motors. (Pin 8 of the 293D IC)

    I'm using a the 7805 as a voltage regulator for the 9v battery connecting to the ICs. I could use 4 1.5 batteries but space is sort of an issue in my build so I'm sticking with the 9v.

    As for the 2nd 9v battery, that's my issue. I have two 100mF 16v capacitors. Will they work with this build or do I have to buy another one?
     
  5. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    Oh thank you, but what do you mean for rule # 2
     
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    For rule #2, after all other connections are made, the entire remaining space is allocated to ground. You don't route ground like you do power and signal traces.

    In fact, if you can make dual sided boards, I often will leave the entire second side as my ground connection.
     
  7. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    Oh I see. Thank you very much.

    So the unused input pins in the 7404 IC will all be connected to ground too? No need to touch the unused output right?
     
  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Unused outputs should not be connected, unless specifically told to. Connecting unused outputs can result in short circuits and dead ICs.
     
  9. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    okay, Thank you again.


    Is this the way to do it? I don't want the wire gaps to be too close because I'm going to be drilling it without a drill press. Untitled.png
     
  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Better. I'd do something in the lower left so that there is a more substantial connection to the leftmost blob.

    For example, you could eliminate the loop around that blob with a jumper and the connect the blob with a much larger pour.
     
  11. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    okay, thanks for the tip!

    Would it be a good idea to also blob the positive lines together as well? or it isn't necessary?
     
  12. May-En Liu

    Thread Starter New Member

    Apr 20, 2017
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    Blobbed out the positives and the ground as much as I could while giving allowance for my drilling skills
     
  13. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    In general, you wouldn't "blob out" the power lines, because then you've created a capacitor with the PCB. I do tend to use slightly larger traces for power lines, but on my circuit boards, it is more a convention than a necessity.
     
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