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What resistors and caps to use?

Discussion in 'General Electronics Discussion' started by subplay, Jun 29, 2013.

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  1. subplay


    Jun 2, 2013
    Hello all..

    I have always been into repairing items but these are obviously always pre-built.

    So now I want to make my own little unit..

    So my question is?

    Is there anyway in knowing what resistors I need to use whilst making a circuit.

    For instance I have a 9v battery, I connect it to a step down voltage unit so im now down to 5v

    So im going to feed this into a atmega328.. from that im goiing to have it control a series of led`s now I have looked and I need some 330uF resistors, i think they are.. But why?

    Why do I need the resistance?
    obvioulsy P1 to LED would work without any reistance...

    And also when would I know when to use a capacitor in any circuit or even a crystal..

    Im just curious, sorry about this stupid question, but these are things I need to learn about .

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    In many digital circuits, resistors are used for pullups or pull downs and the exact value is relatively unimportant. I frequently grab a couple of (say) 10k resistors and use them whenever I need something. But it also pays to know how you would select a "correct" value and what effects values larger and smaller might have.

    330uF are capacitors, not resistors. I presume these are connected across the supply rail. They smooth the power to the uC so that any transients don't mess it up. You normally have some smaller caps (say 0.1uF) directly across the power pins (or very close to them) for similar reasons.

    P1 to LED would work without resistance, but the current through the LED is only controlled by the nature of the mosfet inside the uC. This will almost certainly give you a higher current than you absolutely need, and also a higher current than Atmel would want you drawing from the pin. It will also degrade the output voltage from that pin so you couldn't (or shouldn't) use it as a logic signal to some other device. Typically you would calculate a resistor to limit the current to a safe value (for both the LED and the uC) and possibly to an even lower current to prolong battery life of because the LED is just too bright.

    You only know when to place components in a circuit if (a) the circuit diagram has them in them, or (b) you have enough knowledge to design your own circuit. You also need to know the functions of these components.

    Take a look at the datasheet for the ATMega328 and you'll find that there is a wealth of design information. Often you can pick parts of your design (like where to use a crystal) straight from these datasheets. Beware though, this datasheet is enormously complex and even I wouldn't read it as my first source of information as I might for a simpler component (say that crystal or a capacitor, or a transistor).

    A uC is a very complex beast and you need to know (if you're designing your own circuit (and presumably your own program too) how to tell the uC if you have a crystal, and if not how it should obtain the necessary clock signal.

    There is a lot to learn before you can reliable and competently start to design circuits, and starting with a microcontroller, if not thee deep end, is certainly not the wading pool. However, as I've said earlier, there are many circuits that you can look at on the internet and you can get good ideas from these, or even copy them directly before writing different software to make the circuit perform a totally different task.
  3. subplay


    Jun 2, 2013
    Hi I would just like to thnkyou with this information.. Im starting to learn again.. To think I am electronic qualifed, which I did 15 years ago and I have completely forgot about all of this stuff.. And yes uF that was a mistake, why the hell I put that there is beyond me..

    But I will take your information on board.:)
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