Waveform / Oscilloscope

I have bought a couple "C7210" ultrasound emitter / receiver.
The emitter needs a 40khz pulse, and I would like to analyze the echoed signal on the receiver.
I tried to proceed like this:
C9 => Emitter
C5 => Receiver

( the second pin of emitter and receiver is to GND ).

// wout is the waveform I want to send to the emitter
var wout = new Waveform(256);
for (var i=0;i<256;i++) wout.buffer[i] = 128+Math.sin(i*Math.PI/128)*127;

// win is the waveform I want to record from the receiver
var win = new Waveform(512);
win.on("finish", function(buf) {
   drawBuffer(buf); // ( I have a small lcd on which I draw the waveform )
   win.startInput(C5,40000,{repeat:false});­ // I want to record the next 512 bytes
});

C9.reset();
C5.reset();
// How do I output a 40khz waveform ?
analogWrite(C9,0.5,{ freq:40000}); // pwm at 40khz ?
wout.startOutput(C9, 4000, { repeat:true }); // modulates the pwm ?

// How do I record an analog signal on a pin?
win.startInput(C5,40000,{repeat:false});­

Also I am not sure if the 3.3V of the esperuino are enough to make the ultrasound emitter.

So far it looks like I've recorded only parasites. Can the espruino generate and record a 40khz signal? Am I missing something?

I'm afraid at the moment, Espruino can't handle much over 20kHz - and even 20kHz doesn't leave much time for the interpreter to do anything else.

Also, your example might not do what you expect... You're outputting a 40kHz square wave (which Espruino can do just fine - in fact it can go way beyond that). You're then changing the duty cycle of that square wave at 4kHz with wout, which is producing a 15 Hz sine wave (because wout's buffer has a single sine wave in 256 samples, and 4000/256 = 15.625).

For the input, what you're doing looks good for 40kHz (although Espruino may struggle to read the input that quickly).

But: If you want to reliably see a 40kHz sound wave, you'll have to sample at at least 80kHz (because a sine wave goes both up and down in 1/40000th of a second) - and that'll almost certainly be too fast for Espruino.

All I could suggest you do is to output a small 40kHz 'chirp' using the normal PWM:

analogWrite(C9,0.5,{ freq:40000});
setTimeout(function() { digitalRead(C9); }, 10); // stop roughly 10ms later

(I don't know if this'll power your ultrasonic transmitter - you'll have to check the datasheet)

And then you could set up an analog circuit with a diode, capacitor and resistor on the ultrasonic receiver that would convert the high frequency AC echo into DC, which you could read at a lower sample rate.

Thank you very much for these explanations.
I am trying to make it chirp then!

It works. I found out how a "pulldown resistor" would solve the noise problem, and a with a bit of testing I was able to generate the 40khz pwm and record it in a 4096 bytes waveform at 20khz.
Here is a photo of the nokia LCD on which I visualize the result.
The Chirp duration was 100ms, during which 4 spikes were recorded.
I will try to record the signal with my raspberrypi, maybe it can handle a higher frequency.

Now one of the issue I have left is the 3.3V. Is there a way - of course with an external source of power - to have the 40khz PWM at twice or thrice the voltage?

With some googling I found circuits such as:
MAX756 or LT1073

The MAX757 spec specifies an "Oper. Freq." of 500kHz. Does this mean that If I provide it with a 40khz PWMed 3.3V from the espruino it will deliver a 40khz square signal at 5.0V ?

1 Attachment

That's great!

Personally, to get a 5v signal for the output I'd just use a transistor: Rather than trying to produce a 5v signal, it's easier to use the transistor to 'pull' a signal down to 0v, and just to connect the other side of the speaker to 5v (or an even higher voltage).

There will be a bunch of stuff on it online - it looks like there's a good tutorial (for Arduino) here: http://www.instructables.com/id/Use-Ardu­ino-with-TIP120-transistor-to-control-mo­to/

You can also use the ULN2003 and solder it on the board (it's just a chip with several different channels and the resistors built-in), or even the L293D which allows you to pull both high and low.