I sometimes use a small and cheap speaker (not a piezo element) with a GPIO port from an embedded controller. Typically, I use a simple driver circuit like that below.
Let me be clear that I don’t think that this circuit is ideal. It does have two compelling characteristics; it works and it is simple – containing only a few components. I understand (or at least think I understand) that this is not a class A amplifier approach and that the current taken by R1 is “wasted”. My defense is that if I were designing a product for production, I would consider a better design – but I’m not and I am keen on avoiding the headache (too stupid and lazy would be another way of looking at it).
The speakers come from reclaimed parts or old orders etc…junk box stuff. They come in different sizes and shapes, but small (1-2 inches) and cheap. They are marked 8 -32 ohms and rated .1 to maybe as high as .5 W, usually .2-.25.
R2 is easy and is chosen to conform to the drive capacity of the GPIO and to provide enough current to operate the NPN cleanly (saturated I suspect is the better term). Q1 is typically a 2n2222 or 3904 or 4401 – again, what is on hand. Then I start to “estimate” R1…maybe 82 ohm, we’ll say. I choose R1 acting as though the speaker is a resistor so that I can easily get an approximation of power delivered to the speaker to avoid cooking or overdriving while achieving near-maximum volume.
I then run a simple program like the one below (which is in Arduino C), which operates the speaker at a number of different frequencies so I can actually hear the resulting response and volume.
I realize that the speaker is not a simple resistor. I understand that it is marked as impedance, not resistance, and the value takes into account inductive reactance and all that great EE stuff. I guess my question is can I do better, using this circuit, but without taking the time and effort to actually study RLC circuits and learn all of the associated formulas (again invoking the stupid/lazy defense or, more preferable would be an asset allocation choice defense)?
Example: Given:
Vcc=5V R2=1K Q1=2N3904 SP=32 ohm. .25W what is the calculator-style formulas for calculating R1 to operate the speaker at 85% of is rated power at frequencies of 500, 1000, 2000, 4000, 8000 Hz?
A second question is that I normally do not add a flyback diode across the speaker as I would if it were a relay. I almost never see this in circuits and concluded (possibly erroneously) that it is not necessary – is that true?
Let me be clear that I don’t think that this circuit is ideal. It does have two compelling characteristics; it works and it is simple – containing only a few components. I understand (or at least think I understand) that this is not a class A amplifier approach and that the current taken by R1 is “wasted”. My defense is that if I were designing a product for production, I would consider a better design – but I’m not and I am keen on avoiding the headache (too stupid and lazy would be another way of looking at it).
The speakers come from reclaimed parts or old orders etc…junk box stuff. They come in different sizes and shapes, but small (1-2 inches) and cheap. They are marked 8 -32 ohms and rated .1 to maybe as high as .5 W, usually .2-.25.
R2 is easy and is chosen to conform to the drive capacity of the GPIO and to provide enough current to operate the NPN cleanly (saturated I suspect is the better term). Q1 is typically a 2n2222 or 3904 or 4401 – again, what is on hand. Then I start to “estimate” R1…maybe 82 ohm, we’ll say. I choose R1 acting as though the speaker is a resistor so that I can easily get an approximation of power delivered to the speaker to avoid cooking or overdriving while achieving near-maximum volume.
I then run a simple program like the one below (which is in Arduino C), which operates the speaker at a number of different frequencies so I can actually hear the resulting response and volume.
C:
int Speaker = 2;
unsigned long startTime;
unsigned long currentTime;
unsigned long interval = 2000; // milliseconds
void setup() {
pinMode(Speaker, OUTPUT);
}
void loop() {
startTime = millis();
while ( (currentTime=millis())-startTime<=interval)
{
// 500 Hz
digitalWrite(Speaker, HIGH);
delayMicroseconds(2000);
digitalWrite(Speaker, LOW);
delayMicroseconds(2000);
}
startTime = millis();
while ( (currentTime=millis())-startTime<=interval)
{
// 1000 Hz
digitalWrite(Speaker, HIGH);
delayMicroseconds(1000);
digitalWrite(Speaker, LOW);
delayMicroseconds(1000);
}
startTime = millis();
while ( (currentTime=millis())-startTime<=interval)
{
// 2000 Hz
digitalWrite(Speaker, HIGH);
delayMicroseconds(500);
digitalWrite(Speaker, LOW);
delayMicroseconds(500);
}
startTime = millis();
while ( (currentTime=millis())-startTime<=interval)
{
// 4000 Hz
digitalWrite(Speaker, HIGH);
delayMicroseconds(250);
digitalWrite(Speaker, LOW);
delayMicroseconds(250);
}
startTime = millis();
while ( (currentTime=millis())-startTime<=interval)
{
// 8000 Hz
digitalWrite(Speaker, HIGH);
delayMicroseconds(125);
digitalWrite(Speaker, LOW);
delayMicroseconds(125);
}
}
Example: Given:
Vcc=5V R2=1K Q1=2N3904 SP=32 ohm. .25W what is the calculator-style formulas for calculating R1 to operate the speaker at 85% of is rated power at frequencies of 500, 1000, 2000, 4000, 8000 Hz?
A second question is that I normally do not add a flyback diode across the speaker as I would if it were a relay. I almost never see this in circuits and concluded (possibly erroneously) that it is not necessary – is that true?