Connect with us

Op amp for RC servo audio-controlled

Discussion in 'Audio' started by seitam, Jul 28, 2013.

  1. seitam

    seitam

    4
    0
    Jul 28, 2013
    Hello,

    I am developing a project that consists in controling a micro servo motor using the audio output of a smartphone (iphone/android, etc).

    I have used this scheme: (source: http://www.gluemotor.com/how-to-make)

    [​IMG]

    I also made the control signals (50hz, 1-2ms pulse) with matlab, and exported in .wav format.

    Here is a caption of the signal (audacity):

    [​IMG]

    Right now I am able to control up to 2 servo motors (one per audio channel) with my iphone, just reproducing the audio files.

    The main problem is when I try to do this with an android-based smartphone. All the android devices that I have used so far are not able to provide enough signal to run the servomotors.
    I read about how the control signal must be. I found that it is just a voltage signal that uses the values 0-5v. (0 for no signal and 5v for the peaks).
    I have not an oscilloscope, but I am pretty sure that the android devices that I have tested do not provide the voltage required of 5v.

    So to fix this problem I need to amplify the signal in order to get 5v on the peaks.
    I though that operational amplifiers may help me. So I get some 741 op-amp's and I mouted using the non-inverting scheme.

    [​IMG]

    The problem is that the output signal do not work with the servos. I have used a 1.5 battery to test the circuit (input), and the output was 2.75V using a 2.2k and 4,6k resistor (that is close to the ideal output of 3V).

    So the circuit seems to work well with a DC signal and having trouble with the "AC" one.

    Anybody can figure out where is the problem?
    I am using a 5v cellphone transformer to feed the op amp. I am using only 5v because this motors will be a part of project thats need to be powered up with 4 1.5 batteries.
    I read that the op amps like 741 are able to manage signals with no much delay and distortion below 1mhz, so the signal generated for the servos should not be a problem.

    I am completely lost, any help will be fantastic.

    Sorry for my bad english.

    PD: I have posted the same information on the forum allaboutcircuits, I post here the same for reaching most people possible.
     
  2. BobK

    BobK

    7,632
    1,657
    Jan 5, 2010
    A 741 requires dual supplies of at least +- 5V. Use a single supply op amp like the LM324.

    Bob
     
  3. seitam

    seitam

    4
    0
    Jul 28, 2013
    Thank you Bob,

    I will try with a low supply op amp so. I will post the results.
     
  4. BobK

    BobK

    7,632
    1,657
    Jan 5, 2010
    If you use a single supply amp, you will want to bias the + input at 1/2 the supply voltage and use a capacitor coupled (in and out) inverting amplifier configuration.

    Bob
     
  5. seitam

    seitam

    4
    0
    Jul 28, 2013
    I also forgot to mention that I removed the C1 and C2 capacitors, the servomotors just do not work using capacitors between the stereo plug and the input control signal. I send an email to the owner of this scheme (kazuhisa terasaki), because I didn't know why we need to put this capacitors. Nowadays I still thinking about his functionality. (I am so much limited in electronics knowledge).

    Why I need to bias the + input at 1/2 the supply voltage and use a capacitor coupled inverting the amplifier configuration?

    Maybe these are obvious questions but I am a newbye in electronics field, thank you for your comprension and time.
     
  6. seitam

    seitam

    4
    0
    Jul 28, 2013
    Ok, problem fixed!

    The main problem was that the Vcc of 5v was not enough voltage to power up the 741.
    I have used a LM324N with a 100uF Capacitor. With the LM324N I am able to amplify up to 4 control signals.

    Thank you very much,

    Seitam.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,191
    2,693
    Jan 21, 2010
    I'll remember that when I get a quadraphonic phone!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-