Opto-Isolator For Volume Control?

Question

I'm doing an Arduino based project using the PWM hooked up to a 0.5W 8Ω speaker in order to make various sounds, I want to implement a digital volume control and I thought about using another PWM output and an Opto-Isolator....would this be possible?

And....if it is possible - could anyone recommend a suitable candidate?

Cheers Chiphackers - Let me know if this is too vague :)

Update - I've got the speaker connected directly to the PWM output without amplification - I'm using a couple of tactile switches to turn the volume up and down

Here's the circuit (In Theory) -

Here's some code too, I've got the PWM set to high speed and I'm using it to ramp the volume of the tones up and down -

/////////////////////////////////////////////////////////////////////////////////////
// Configure PWM on pins 3 and 11 to run at maximum speed, rather than the default //
/////////////////////////////////////////////////////////////////////////////////////

pinMode(3,OUTPUT); // speaker on pin 3

cli(); // disable interrupts while registers are configured

bitSet(TCCR2A, WGM20);
bitSet(TCCR2A, WGM21); // set Timer2 to fast PWM mode (doubles PWM frequency)
bitSet(TCCR2B, CS20);
bitClear(TCCR2B, CS21);
bitClear(TCCR2B, CS22);

sei(); // enable interrupts now that registers have been set


////////// Function for playing tones -

void playTone(long tone, int duration) 
{
prevMicros = micros();

  while (micros() - prevMicros < duration) 
  {
    analogWrite(3, VOLUME);
    delayMicroseconds(tone);
    analogWrite(3, 0);
    delayMicroseconds(tone);
  }
}

Answer 1

Disclaimer: This is in response to the posted circuit, not an answer to the question of control with a PWM. I just didn't have room in the comment.

Any special reason you need the optoisolator? As your circuit currently appears, I don't think that your D5 connection to pin 5 of your optoisolator phototransistor will supply the kind of current you want. In addition, synchronization between the two PWMs will likely cause you pain (What if the D5 PWM is always off when the D3 PWM is on and vice versa? No sound! You need to average your signal, and feed it into a buffer. You'll also likely want some hefty capacitance on your V+ line to the micro and buffer so that your supply voltage stays smooth.. However, the connection to ground and the connection to D5 totally eliminate the purpose of the optoisolator, which is to allow the speaker circuit to be at a different voltage than the input. This might be useful if you had the speaker at a remote location from the Arduino, and want to run a low-current differential signal on a twisted pair out to the speaker, which would be powered from a different power supply. As is, you might as well just connect pin 6 to D3, and just use the transistor, completely ignoring the "Opto" part of the circuit.

See the output circuit for the AVR335 appnote linked in my other answer for a circuit which effectively drives a speaker with a PWM. The filters smooth the output signal to something better approximating the input (With a rolling average), so that you get a smooth wave rather than a rough digital square wave. You can remove the unary gain amplifier, that's just to remove any feedback from the microphone (Which you don't have).

You really want some filtering and amplification on the output -It'll sound absolutely terrible if you don't. You know how those talking greeting cards sound? They have filtered outputs. Your speaker will sound worse than that if you just connect it to the PWM. I was previously assuming that you were using the phototransistor to isolate your output and transfer it to an amplifier/filter circuit running on a different power supply, but a straight connection is going to sound really bad.

Answer 2

I haven't done a whole lot with audio, but wouldn't a simpler way of reducing the volume be an digitally-controlled potentiometer in-series with the the PWM signal driving the speaker(i.e. you just want to reduce the signal voltage to reduce the volume, right?)

Answer 3

Wait - You want a digital volume control that is controlled by a PWM output? As in, you send the Arduino a command over the UART, or flip some switches to a binary value, and tell it to set the volume to 11? Then, your optoisolator applies a voltage/current somewhere in your output circuit? We'd need a lot more info on what kind of output amplifier you're using. It would probably be much easier to simply modify your PWM. If you increase the maximum value, or decrease the input value, you can turn the volume down entirely in software.

If you must, you can still use the analogWrite() function to control your PWM. Try something like(pseudocode):

  //File scope variable to set the volume.  
  static uint8_t volume = 0;   

 void set_volume(uint8_t new_volume) {  
    if (new_volume < 8)   // Would result in a volume of 0 
       volume = new_volume;  
 } 

 /** Divide by 2^volume:   
  *  0 = loudest,  
  *  1 = half as loud,    
  *  2 = 1/4 as loud,  
  *  3 = 1/8 as loud, etc.   
  */ 
 inline uint8_t adjust_volume(uint8_t signal) {  
    return (uint8_t) signal >> volume;  
 }  

 //Identical ISRs for each of the tactile switches  
 ISR(PCINT0_vect) {      
   setVolume(read(PORTC))  //Or however you get your digital volume signal  
}  

void main(void)  {  
   char *sound_buffer;  

   while(*sound_buffer)  
      analogWrite(adjust_volume(*sound_buffer++));  
}    

If you want to do a volume control with an optoisolated input, then yes, that's quite possible. Assuming your input device is a pot, your options are a logical, frequency (rather than pulse width) modulated signal, which you can get from a voltage to frequency converter like the LM231 (there are many other options), or from an op-amp circuit with an RC oscillator. Feed the output into any digital optoisolator, and you should be able to read the frequency on a digital input pin, and modify your speaker output PWM maximum value or modulate your input values accordingly.

Alternatively, you can use a photovoltaic output optoisolator, which outputs an analog voltage (or current) based on the input current. Be careful of linearity issues when working with these circuits. You don't really want a linear volume control (You want something closer to logarithmic, or you can go all-out and implement an equal loudness contour,) however, you don't want a digital switch. The Vishay IL300 is an example of a linear phototransistor.

EDIT: I mentioned the AVR335 app note in a recent question about programmable greeting cards with audio. That would be a good resource for your question as well.

Answer 4

Why don't you just drive the speaker with a normal transistor and alter the PWM to control the volume? You have control over the waveform that the transistor (and thus speaker) gets... no need for additional components to control it.

Also, If you're hell-bent on using the opto, drive the LED with the PWM outputs. Connect the Emitter of the transistor to ground, the collector to the speaker, and the other side of the speaker to your V+ source (more V+ = louder sound).

Answer 5

Ok.....so I've realized that an opto-isolator is not the best approach for this particular application!

And.....as Andrew Kohlsmith and reemrevnivek pointed out - I could pulse width modulate the base of a transistor instead....

So I got some silicon npn epitaxial transistors (BD139) from the electronics store and gave them a go. It actually worked better than i thought - the volume control is nice and fine :)

Here's the circuit I used in the end -

Thanks for everyones help and suggestions!