CSCI 255 — Structure, power and volume

This is the second of a two-week sequence on generating audio with a microcontroller. The first week concentrated on getting a workable program. In this second lab we are going to have three goals. I hope we satisfy at least two of them.

Getting started

You can continue using three same three platforms we used last week. This week we are providing starter programs. These are solutions to last weeks lab that don’t include the C structure enhancements.

Pointers and structure in C

So have just you had written a routine playTone to play a tone at a frequency, given in Hz, for a duration, given in milliseconds. Here’s the C/C++ prototype for that function.

void playTone(int frequency, long duration) ;

Our representation of a note has an abstraction issue. The note is a real-world entity: We shouldn’t be splitting the melody into two arrays, one for frequency and the other for duration. We should have an array of C structures that represent notes.

So add the following structure definition at the front of your program, close to the prototype you have for playTone.

struct noteInfo {
   int  frequency ;    /* frequency in Hz */
   long duration ;     /* duration in mSec */
} ;
typedef struct noteInfo Note ;

Note the slight resemblance to a Java class definition.

Right now your program should have two array definitions borrowed from Tom Igoe’s Arduino tutorial. This is how Tom Igoe wrote them:

// notes in the melody:
int melody[] = {

// note durations: 4 = quarter note, 8 = eighth note, etc.:
int noteDurations[] = {
  4, 8, 8, 4,4,4,4,4 };

Replace these two array of int definitions with a single array of Note definitions. It starts with the following definition:

Note song[] = {
  { NOTE_C4, 1000/4 },

Add the definition for Note and the initialization for song into your program.

Of course you could continue to call playTone with something like playTone(song[i].frequency,song[i].duration) but you really ought to pass a Note; or, since this is C, a pointer to a Note.

Start by updating the prototype for playTone.

void playTone(const Note *tone) ;

Modify the prototype for playTone. Also modify the calls to playTone in main to match the prototype. Expect the IDE to complain while you do this.

Now you need to turn your attention to the implementation of playTone.

Now modify playTone to accept a pointer to a Note as its single argument. It won’t be much more than changing the function header and updating references like frequency to note->frequency.

At this point you should be able to compile and test your modified program.

Power regulation

Suppose you wanted to make a battery-power computer-controlled circuit. This isn’t trivial because the voltage produced by batteries varies much more than a computer chip can tolerate.

To produce the right voltage you need a voltage regulator. Because the Arduino and PIC MicroStick contain voltage regulators in addition to their microcontrollers, there is little purpose in building a voltage regulator for them. On the Arudino Uno, if you connect a battery pack with a voltage of between 7 and 12 volts between the GND and Vin connectors the internal voltage regulator will generate 5 regulated volts for the chip. By the way, you will need more at least 6 AA batteries to generate 6 volts and, if you connect the battery in the wrong direction, you will destroy your Arduino.

So let’s build a regulated 3.3 v supply that could be used for a PIC12 chip on a breadboard.

Needed supplies

You will be using polarized electrolytic capacitors to built the power supply. It is very important that you always connect the lead closest to the minus sign (-) to ground. Otherwise the capacitor can explode! (There are many YouTube videos of exploding capacitors.)

It isn’t hard to build a 3.3 voltage regulator with a LM7833 and a couple of capacitors. Here’s a schematic for building one with an LED power-on light.
voltage regulator schematic
Use the smaller capacitor on the unregulated input and the larger capacitor on the regulated output. The capacitors are needed to deal with voltage fluctuations from the battery and current fluctuations in the circuit.

Build your voltage regulator circuit, ask the instructor to take a look at it, and then use a voltage meter to check its output voltage level.

You really do need the LED to get a reasonable value. If the voltage regulator doesn’t have a load, it won’t have anything to do.

Controlling devices that require more power

Speaking of load, we really shouldn’t be driving speakers from a microcontroller. A Google search for “drive speaker with microcontroller’ will display lots of web pages with advice for generating audio from a microcontroller. (Be use to press the Images button to get all the pictures.)

The easier solution is to get a 95¢ headphone jack for your breadboard and use it to power some of those old computer speakers that are always at hand. Be sure to use the ones with the built-in amplifiers.

However, that’s just too easy. Let’s look at some of those more interesting solutions.

The transistor

The simplest solution involves a single transistor. You can see how this is done on the Using a Speaker for Audio Output page for mbed developers.

In this implementation, it would probably be best to connect the speaker to something other than the regulated voltage source. You can even use a 9 volt battery to drive the speaker.

You can also drive a DC motor with a transistor. In the case, you definitely want to use a separate voltage source and diode.

Grab a transistor, resistor, and maybe even a diode.

use them to power a speaker or motor. If you are driving a motor, this is fun only when you vary the duty cycle of a PWM wave to control the speed at which the motors is turning.

A few years ago in CSCI 255 we were able to control a few flapping birds from a PIC. Unfortunately, the cheap plastic wings couldn’t take the stress.
cheap birds

The op amp

The LM386 is a low voltage audio power amplifier that is used in many audio devices, such as the Smokey® Amp. In fact, the Smokey Amp is little more than an LM386 with a couple of capacitors.

Take a look at the schematic in the upper left corner of page 5 of the LM386 datasheet which alos appears on the Audio Power Amplifiers with LM386 page of

Try to build this on your breadboard. It’s only one potentiometer, one resistor, and a couple of capacitors. Be sure to power the LM386 from your battery holder.

This setup won’t be any louder than the 3904 amplifier, but it can amplify very weak signals through a high resistance channel, such as the 4M Ω human body. Here’s an example of a four-digit device opening and closing the circuit. I suspect this would work even if the entire class held hands to complete the circuit, but this is a Computer Science class.
off    on
The PIC only produces 3.3 v. Do not try this at home with the speaker wires coming out of your 200 watt stereo amplifier.

If you try this one, you will need a lots of parts.

The I2C controlled class-D amp

We have a few 2.8W class D amplifiers that will fit in a breadboard. We haven’t tried them out yet.