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Schematic Design Help

  1. Jan 29, 2014 #1
    I am trying to build a BLE device and have run into some obstacles. I did create a basic breadboard circuit using an Arduino Uno and the Redbear BLE Module and am trying to shrink it down to a much smaller PCB. The schematic is attached as well as the datasheets for the microcontroller and the BLE Module.

    The device is meant to heat a wire controlled using PWM


    1. I do not know if an n-type MosFet is appropriate to amplify the current from 40 μA to 2A. If it is appropriate, I need help finding which specific mosfet to use.

    2. In general, I would really appreciate it if someone with experience could give any suggestions to optimize this circuit.

    3. The BLE Module says it has its own microcontroller. Can this be used instead of my microcontroller for PWM?

    4. I have inserted decoupling and bypass capacitors that are polarized. From what I know, these are fine and non-polarized are not necessary. Is this true?

    Appreciate all your help, This is my first PCB! :shy: :approve:

    Attached Files:

  2. jcsd
  3. Jan 29, 2014 #2


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    Welcome to the PF.

    Couple things... First, what's a BLE?

    Second, the FET is connected incorrectly. And to amplify from 40uA up to 2A will take a multi-stage amplifier. And are you planning on getting that 2A from the battery? Through those resistors?

    And the decoupling caps go in parallel to the power supply connections, not in series (see your Vcc cap in series with pin 15).
  4. Jan 29, 2014 #3
    BLE is bluetooth low energy.
    Are the source/drain switched? Yes I was hoping to be able to get the 2A from the battery, is there a problem with that? I am not really familiar with multi-stage amps, will have to look into it.

    The VCC cap in series with pin 15 is supposed to be a bypass cap, is that supposed to be in parallel?

    Thank You!
  5. Jan 29, 2014 #4


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    You have to have a DC path to and from the supply!
    If you need to decouple the power supply to the processor circuitry, could you not supply the mosfet R4 directly from the battery?
  6. Jan 29, 2014 #5

    I could do that, but why shouldnt i decouple the power supply to the mosfet?
  7. Jan 30, 2014 #6


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    You need to ask yourself what 'de-coupling' is for. It is to de couple what one part of a circuit does from another part of a circuit. If a part of a circuit is impervious to power supply variation then it doesn't need protection. If the mosfet takes great gouts of current and it is connected downstream of those two decoupling resistors in the (low pass filter) decoupling circuit the the volts will dip accordingly. If you connect the mosfet right to the battery, there will be less of a voltage dip (less series R in its circuit) and the decoupling circuit will, by virtue of the LP action, smooth off the sharp dip in Vcc volts to your fragile control circuit. When it's switched on, the mosfet can actually deliver more current when it's not decoupled from the battery.
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