First homemade Arduino on perfboard- 5V to 3.3V options?

Discussion in 'The Projects Forum' started by DR1, Feb 24, 2013.

  1. DR1

    Thread Starter New Member

    Sep 13, 2012
    Project Details:
    So Im making my first DIY Arduino using an Atmega328
    Will control two 3W high powered RGB LEDs
    I also have a constant current led driver for them
    One 12V white led halo ring
    One Bluetooth Module

    I'm transferring the circuit I have on my Arduino Uno to a perfboard. I've done everything to get the chip working, now I need to bring down one of the 5V pins to 3.3V for the Bluetooth module I have, that takes 3.3V. The correct resistor would be 125 ohms, but the closest I have is a 150 ohm. I also have a really small pot set to 125ohms and one of the larger pots set to 125ohms. I looked, but I dont have a LM317.

    A few questions:

    1) Which would be the best way to go, the 150ohm resistor?
    2) Will those extra 25ohms make a big difference in output voltage?(If Im doing the math correct, to get an output of 3.3V with a 150ohm resistor, the arduino pin would have to output 22mA, which it's safely capable of)
    3) Or would using the pot at the 125ohms be better?
    4) Also, I have the small pot(100k), size of a dime. Then I have one of the bigger pots(25K). Whats the difference? Obviously putting the smaller pot on the perfboard would be much easier than trying to fit that big one on there, but thought I'd ask to make sure.

    Any help appreciated. Thanks
  2. takao21203

    Distinguished Member

    Apr 28, 2012
    I would not bother about these 25 Ohms. The value seems a bit low anyway.

    You can use red LEDs to drop off 1.5 volts.

    The only problem is when connecting from the 3V side to 5V side, and the tristate is not set correctly. Then you get large reverse currents.

    Depends on the data rate, you can safely use a 2.2K resistor. The higher the resistance, the slower the I/O.

    Not all chips have clamping diodes. So you can use LEDs here as well for instance blue or UV straw hat, will result in about 3V max.

    125 Ohms only would be needed for MHz range data rate.

    I would try with 2.2K, and maybe 2 red LEDs in series for clamping the overvoltage.

    And take care of the tristate for reverse connection. You must set it as early as possible when the program starts up (or the so called sketch).

    Also you need a high value resistor to allow some current if you use a LED to drop off 1.5 volts.

    I'd try with 2.2k and maybe rely on internal clamping inside the bluetooth.
  3. panic mode

    Senior Member

    Oct 10, 2011
    i am not clear on what you want to accomplish.

    a) do you want to produce 3.3V to power something (bluetooth module) OR

    b) do you want to reduce arduino output voltage from 5 to 3.3V to make it compatible with your (bluetooth) module.

    c) where is the 125 Ohm coming from? R1 is supposed to be 240Ohm, if R2 is 125 Ohm, you will get 1.9V output. to get 3.3V you would need R2 value of 390 Ohm.

    d) what makes you think that LM317 can drop 5V to 3.3V? actually it can but for very low output current and with poor regulation. when dropping voltage so little you need an LDO. classic voltage regulators (78xx, 79xx, LM317 etc.) are not LDOs, they need 2-3V of headroom to operate correctly.
  4. DR1

    Thread Starter New Member

    Sep 13, 2012
    Im a bit confused, where are you getting 2.2K from? Im using R= V/I. Not sure what you mean by tristate either.
  5. takao21203

    Distinguished Member

    Apr 28, 2012
    And your I is defined where or by what?

    Tristate means a floating I/O port bit (for reading). If you have it set for writing, and a logic high, you get 5 volts. Otherwise it is OK for reading from the 3.3V side.

    But you have to make sure it is set correctly and better also use a resistor here.

    If you want to power the module and it needs 22mA, use a red LED as said or a yellow one. The module won't become destroyed from 3.5 volts.

    Resistor is not relieable you get a totally different voltage if it is not 22mA.

    Or use 3x silicon diodes if you don't trust the LED option. They will drop approx. 0.7v each at that current.
  6. DR1

    Thread Starter New Member

    Sep 13, 2012
    Well the arduino's pin min current is 10mA, max 40mA. So, on my sketch, I have the pin set to high 5V. I originally used the max 40mA to make the calculation and got 125ohms for resistance. But now I think Im confusing myself lol.