Auto on-off for amplifier

I ordered a bunch of Digispark boards - for a few dollars each and included bootloader you cannot go wrong. They are basically prototype&forget boards and go under various other names (Picoduino, Pro Nano, Olimexino-85). Banggood.com has them for ~3$.

The first project is an automatic power-on, power-off for an old Sony speaker set, formerly CD/cassette deck.

My goal is for the amplifier to turn on when audio signal is detected and turn off after 30 minutes of silence. This is not so much for power saving as it is to reduce the white noise and GSM pickup during the night.


The speakers can be turned on by connecting a 100 kOhm resistor between two of the four pins on a plug







I've made various measurements to determine the correct input level that's reliably above the silence noise floor.

In my case it was somewehere around 20mV.




The ADC of ATTiny85 can be set to a 1.1V reference, improving the detection precision. Otherwise, by default, the 3.3V reference is used.

I've tested various algorithms and found that peak detection works best. Averaging seemed better on paper but was not able to detect low levels of music.

I haven't drawn a schematic, but here's a wiring picture:

If that makes sense to you as much as it does to me, here's the description.
The Digispark is fed directly from the speakers' 16.5V supply, into the VIN pin. Power dissipation is decent.
A 4N35 optocoupler connects through pins 4 and 5 a 100k resistor across the power-on pins, like in the first picture.
The 4N35 pin 1 is driven by the Digispark pin 4, through a 10k resistor. Pin #2 of the optocoupler goes to GND.

Audio signal is taken directly from the center lead of the RCA jack, through a capacitor (a random one from the box, probably in the nanoFarad range) and goes to PIN2 on the Digispark.
A 10-20kOhm capacitor from the same pin #2 to ground reduces ambient noise. *

TODO: draw a schematic and post it here. Remind me in the comments if I forget.

* To make the circuit more sensitive, increase the resistor value from pin 2 (analog in) to ground.

Code

The input is sampled every 100ms and the peak value is retained. If that peak value is above a threshold ('1' in this case) then the code detects the audio as playing and lights up an LED and turns on the optocoupler.
If after a predefined duration (30 minutes) no audio was detected then it shuts off the optocoupler.

There are various caveats to the Digispark - limited flash space (if Serial-to-USB debugging is used), limited pins (cannot use serial debugging afterwards), some pins cannot be really used (pin 5, I think).

Code has some snippets from this, however the original code does not have reliable detection.


#include <DigiCDC.h>
#include <avr/sleep.h>

#define SIGNAL_THRESHOLD 1
// A1->P2, A2->P4, A3->P3, A0->P5
#define ANALOG_INPUT_PIN 1
#define STATE_OUTPUT_PIN 1
#define AMP_OUTPUT_PIN 4

/* Timing constants
 * maxIdleMinutes - Longer than this, amplifier will be turned off
 * sampleSleeTime - Every 2,000 mSec we sample and process
 */
const int maxIdleMinutes = 30;
const int sampleSleepTime = 100;

// the setup routine runs once when you press reset:
void setup() {
  pinMode(STATE_OUTPUT_PIN, OUTPUT); //LED on Model A  or Pro
  pinMode(AMP_OUTPUT_PIN, OUTPUT); //LED on Model A  or Pro
  analogReference(INTERNAL1V1);

  SerialUSB.begin();
}

// the loop routine runs over and over again forever:
unsigned long average = 0;
unsigned long peak = 0;
unsigned long idleTimeToTurnOff = (unsigned long)(maxIdleMinutes * 60L * 1000L);
unsigned long lastIdle; // Preserves the millis() read when idle was detected

boolean isSoundPlaying() {
  peak = 0;
  long temp_peak, reading;
  for (int i = 0; i < 100; i++) {
    reading = analogRead(ANALOG_INPUT_PIN);
    average = (9 * average + reading) / 10;
    if (reading > peak) peak = reading;
    delay(1);
  }
  SerialUSB.print(average);
  SerialUSB.print('\t');
  SerialUSB.println(peak);
  return peak > SIGNAL_THRESHOLD;
}

void loop() {
  unsigned long now = millis(); // Take current time
  unsigned long diff;
  
  if (!isSoundPlaying()) {
    digitalWrite(STATE_OUTPUT_PIN, 0);
    if (lastIdle != 0)
    {
      diff = 0;
      if (now < lastIdle) // Overflow of millis()
        diff = now + (0xFFFFFFFFL - lastIdle);
      else
        diff = now - lastIdle;
      SerialUSB.print("Idle for ");
      SerialUSB.println(diff, DEC);
      if (diff > idleTimeToTurnOff)
      {
        SerialUSB.print("Turning off");
        // Turn off the Amplifier
        digitalWrite(AMP_OUTPUT_PIN, 0);
        delay(5000);
      }
    }
    else
      lastIdle = now; // Now is first time we see idle, save the time.
  }else{
    // sound is playing
    digitalWrite(STATE_OUTPUT_PIN, 1);
    SerialUSB.print("Turning on");
    digitalWrite(AMP_OUTPUT_PIN, 1);
    lastIdle = 0;
  }

  delay(sampleSleepTime);
}