Electronic – Arduino – Detect tone

arduino

I've got a strange setup:

Background:
I'm building a wireless binary transmitter with a couple of low powered CB radios. I've re-soldered the activator button and microphone on one; that sends two different tones (1500hz for 1, 1000hz for 0); this end of the setup runs beautifully with my Arduino.

My issue is with the receiving end: I've tried reading directly from the speaker pins using analogRead, but can't see any difference in the values from before the sounds are received, to after.

My question: how can I patch my way into the speaker on the second CB to Arduino so that I can read the tone/pitch value?

Sort of what I have:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Best Answer

Edit: skip to the bottom for a decent solution.

By default, analogRead() is far to slow to properly sample signals at 1kHz+. There are several things you can/should do if you want to use the Arduino ADC for tone detection:

Change the ADC clock prescaler. At 16MHz, I've been able to set ADPS as low as 3, although accuracy will be reduced.
Stop using analogRead(), and write your own ADC read wrapper. There is a lot of cruft in analogRead() that you don't need if you know what you're doing.
Put the ADC in free-running mode, so you don't have to wait for it to sample.

By doing all these things, it's possible to bring the sample rate of the ADC close to 10kHz or beyond, which is fast enough for your purposes. Note that unless your are quite clever with your code, this process is likely to use up almost all your cycles, leaving you with a not very useful Arduino.

However, there's an alternative: good ol' analog filters. Build two peak filters with high Q at your frequencies of interest, run them both into peak detectors, and then your Arduino need only look at two digital signals.

Edit: another way to do it. Rectify the audio, and clip it to a digital signal. Then count pulses in a constant time interval with the Arduino. This is the easiest way to do things (least hardware, doesn't require too many cycles, can run in an interrupt).

Related Solutions

Electronic – arduino – Use Arduino to detect momentary 120VAC interruption

Get a 5V transformer, put a bridge rectifier on it, connect it to a digital input, add a 10k resistor in series and a 5V zener diode across to protect the input from overvoltage as transformer will probably give you greater than 5V peaks.

In arduino firmware set up an external interrupt on said digital input. You should be getting interrupts 100 or 120 times per second. On every interrupt record a timestamp. In the main loop check if last recorded timestamp is older than 0.1 second.

Electronic – arduino – Detect car +12V using Arduino

Knowing that all sorts of weird stuff can happen in automotive power circuits, and not being especially knowledgeable in those systems, I'd err on the side of caution and use an opto-isolator.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Pin 1 = Car 12v R1
Pin 2 = Car Ground
Pin 3 = NC
Pin 4 = Arduino Ground
Pin 5 = Arduino 5v R2
Pin 6 = NC

With this scheme, your Arduino and the car aren't connected electrically at all. At worst, the optoisolator is destroyed, and you can replace it for less than a dollar. Put it in a socket and you won't even need a soldering iron to perform the repair.

R1 was selected such that input voltage transients up to 120V won't exceed the maximum forward current of U1. D1 avoids exceeding the maximum reverse voltage of U1 if the input voltage is inverted. The value of R2 isn't especially critical, so it might as well be the same value as R1.

You won't need any heat sink. Heat is the result of electrical energy being converted to heat, and power is the rate of energy conversion. Power \$P\$ in an electrical system is the product of current \$I\$ and voltage \$E\$:

$$ P = I E $$

So, the voltage itself doesn't make heat: it also depends on how much current is flowing. In both these circuits, the current is low enough that the power is small and no heatsink is required.

Best Answer

Related Solutions

Electronic – arduino – Use Arduino to detect momentary 120VAC interruption

Electronic – arduino – Detect car +12V using Arduino

Related Topic