The song plays while the LED flashes back in main. The example uses p26 for the speaker PWM output. The note array contains the frequency and a duration array that contains the duration of each note. The new SongPlayer class in this example program sets up the timer with the and plays a song passed to it in two arrays.
#MBED COOKBOOK WAVEPLAYER WAV FILE CODE#
If an application needs to play a sound or a long sequence of notes (i.e., a song) and continue to execute other code, a timer interrupt can be used so that the call returns immediately and the song continues to play using timer interrupts while other code can execute in main. Demo for new class to play a note on a speaker using a PWM outputSee commit byPlaying a Song with PWM using Timer InterruptsThe previous speaker class example does not return from PlayNote until the note ends. Songs can also be played, if you have the notes and durations available. The earlier euro police siren is used in the test program. To be consistent with other mbed APIs, the volume argument will range from 0.01.0. The Speaker class in this demo uses the existing PwmOut class and uses these three arguments in the PlayNote(frequency, duration, volume) member function. The obvious three arguments are the frequency, the time duration of the note, and the volume level. A new C class to play a note using PWMBased on the previous example, a new class can be developed to play a note on a speaker using PWM. If only a beep, click, or siren alarm sound is needed this may be the appropriate choice. The advantage of using the PWM hardware is that it takes minimal memory and no processor time to output an audio tone. At higher audio frequencies (above a couple thousand hertz) it also becomes harder for humans to hear the difference between a square wave and a sine wave. Not quite a pure tone, but then again most instruments also generate some harmonics. A lower duty cycle setting produces lower volume, but keep in mind that since square waves or pulses are generated there will also be a lot of higher frequency harmonics produced. Set the PWM period to 1/frequency of the desired sound.
#MBED COOKBOOK WAVEPLAYER WAV FILE DRIVER#
The black plastic case top can pull off along with the tiny speaker wires, so get underneath to the PCB a bit to pry it up.A typical driver circuit for digital outputsSpeaker driver circuit used for mbed audio demosUsing PWM hardware to generate a simple audio toneThe simplest way to generate an audio signal to play on the speaker is to use a hardware output. The first demo uses mbed p21 for the digital control signal.Be careful when removing the Sparkfun PCB mount style speaker from the breadboard. For a quick breadboard demo, you can leave out the (around 1K) resistor if you do not have one (just connect the digital control signal jumper wire directly to transistor base) and a diode is not needed. Be sure to double check the orientation of the three transistor pins in the schematic below and the images above, if it is not correct it will not work! The part number seen in the transistor photo above is on the flat side of the transistor case. The speaker is the load.įor VCC, use VU or Vout.
Next, hook up the speaker and 2N3904 transistor as shown in the below. With the small PCB mount speaker from Sparkfun seen earlier, soldering is not required as its pins will plug into a breadboard. If you pull it out hard using the top black metal speaker cover, the PCB on the back cover can pull off and break the tiny speaker wires soldered to the pins inside the back cover.WiringSolder one end of two long jumper wires to the speaker's terminals for easy breadboard hookup. WarningThe small Sparkfun PCB speaker works great in a breadboard and is very handy, but when removing it from a breadboard be careful to pry the bottom up first with something flat like a screwdriver. A lot of this volume difference is due to the fact the square waves sound louder to humans than sine waves at low frequencies. The volume is even lower on the AnalogOut pin without a driver. PWM output pins have a bit more drive current than the AnalogOut pin. The demo code will use a PWM output pin and then the AnalogOut pin. Small will respond to a very limited audio frequency range without a driver, but the volume is still relatively low and is often only of use at the resonant frequency listed in the datasheet. Hooking up the speaker directly to an mbed output pin results in very low volume and a driver circuit is needed to boost the current levels to drive the speaker to provide adequate volume levels. A recent CBS News story on engineers designing car feedback soundsWith just a small low cost speaker and a 2N3904 driver transistor is it possible to generate simple sound effects on mbed for under $2.
0 kommentar(er)
