Op amp for RC servo audio-controlled

[seitam]

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)

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):

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.

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.

 

[BobK]

A 741 requires dual supplies of at least +- 5V. Use a single supply op amp like the LM324.

Bob

 

[seitam]

A 741 requires dual supplies of at least +- 5V. Use a single supply op amp like the LM324.

Bob

Thank you Bob,

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

 

[BobK]

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

 

[seitam]

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

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.

 

[seitam]

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.

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.

 

[(*steve*)]

With the LM324N I am able to amplify up to 4 control signals.

I'll remember that when I get a quadraphonic phone!