not getting the voltages i expect in circuit

Discussion in 'The Projects Forum' started by aterhune1984, Jan 14, 2011.

  1. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    Alright, I am lost now, according to my calculations I should be getting 5v through the microcontroller and 3v through the accelerometer, can someone take a look at my schematic and board layout to see what I am doing wrong? These are eagle files. Thanks in advance
     
  2. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    the Voltages that I am getting on my multimeter are as follows.... Going into the microchip i am getting 3V and for the accelerometer I am getting .54V. Again, I want to be getting 5V and 3V respectively, I am using a 9Vdc source. Thanks
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You will get more replies if you post your attachments as .PNG files. Many of our members do not use Eagle.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Use Eagle's File->Export facility to create .png images. Use 100 DPI resolution.
     
  5. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    sounds good, I will do this once I get back home, on vaca now.
     
  6. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    Alright sorry for the late reply, again, the problem is I am not getting the voltage i want with the voltage dividers I am using and I do not know why. Using 9V in and 2 voltage dividers to get 5V and 3V. Could someone take a look at this circuit and tell me what I am doing wrong. Currently I am actually getting 3V and .54V respectively. Thanks in advance
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need to use voltage regulators instead of resistive dividers.
    With 9v in, you'd have at least 35mA flowing through R22 (not counting the loads the ICs will place on it), which will drain a 9v battery fairly quickly, and you will have very poor voltage regulation.

    Not sure why you're using a 9v battery when they are expensive, have low mAH ratings, and require regulation for use with uC's. You'd be better off to use a couple of AA's in series, and run both the uC and accelerometer on 3v.

    If you operate the accelerometer on 3v and the uC on 5v, you will require level translation circuits so that they can "talk" to each other. That will complicate the design considerably.

    Your schematic needs some layout help. I prefer to have the PINS layer turned off, as it adds considerable clutter to the display.
    Click View, Display/Hide Layers, and click Pins to un-highlight it, then click OK. You won't see the pins on the schematic anymore.

    Try to run wires on the schematic either vertically or horizontally. Running them at odd angles makes the schematic very hard to read. Arrange things so that you have as few wires crossing as possible, and long runs are to be avoided.

    Power and inputs come from the left, outputs towards the right.
    More positive voltages towards the top, more negative or ground towards the bottom.
     
  8. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    well part of it makes sense, I will work with 3 AAA's(1.2V) as I need to power many LEDs off of them and 3V will not be enough, but 3.6 will be enough. I will have to recalculate all my resistances but I still do not understand why a voltage divider will not work. I will be able to do 3V for both the uC and Accel so they can talk to eachother no-problem. I have picked out a voltage regulator, it just would be cheaper...parts-wise to get two resistors for a voltage divider instead of buying a few dollar regulator.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    How many LEDs do you wish to power, and for how long were you needing these LEDs to operate?

    9v batteries will become exhausted very quickly at high current loads. They're generally rated at around 500mAh; which is a constant load of 25mA for 20 hours. They're OK for transistor radios played at low volume, but not for driving arrays of LEDs for any length of time.
     
  10. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    I would be powering 18 LEDS but not all at the same time, there are 6 red, 6 green, etc. This would be based on accelerometer readings so it really depends on the length of time they would be on.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    So, you're going to show acceleration as positive or negative G's?

    How about just flashing one LED at a time, rather than an entire bar of LEDs?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    After looking at your Eagle .sch file more, discovered you have VS on the accelerometer shorted to YOUT, which is bound to cause rather large problems.

    You may have destroyed the accelerometer.

    [eta]
    Nope, I was wrong - your schematic is very confusing. Trying to fix it up a bit.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You had the VS of the accelerometer connected to AVSS, the analog ground on the uC. Did you really mean to do that?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here, I've cleaned up your schematic a bit, and put a frame around it.

    I've entered "Unconnected?" text in a couple of places. Once you figure out what you want to do with those loose ends, you might consider working on the board again.
     
  15. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    sorry, didnt see the next page of your responses. Thanks alot. as far as the AVSS and AVDD. I had planned to set up the uC to register that I was using 3V on the Accel which would have been different than the 5V running the uC. But since I can now run them both at 3V I do not need either the AVSS or AVDD connected at all. the one unconnected on the right was for USART Transmit so I can connect the thing to a computer.... I did not know how to set that up in eagle so I did my best gerry-rigging it to work.

    Since I will not have a 3V exactly battery... more likely a 3.6V battery(seriesAAA) will I be able to use a voltage divider to get to 3 V for the uC and Accel or am I forced into using a voltage regulator chip? I have found one for under 50 cents so its not a huge deal, but for future projects it would be useful to figure this out. Thanks again.
     
  16. aterhune1984

    Thread Starter New Member

    Nov 24, 2010
    25
    0
    Hey just wanted to see if someone had seen my latest post, the one above this. Appreciate all the help but still have questions. Thanks in advance
     
Loading...