Ultrasound with PIC12F629

[T.Jackson]

Hi !

I'm using a PIC12F629 with internal oscilator (4Mhz) a LM386 as amplifier, and a piezo to emmit ultrasound wave (from 28Khz to 60Khz).
All I can achieve is an annoying sound but no ultrasound at all.

I'm actually using the suroutine below, obtained from Melanie in this forum:

MakeSound:
For CounterA=1 to 1000
High gpio.0
PauseUS 10
Low gpio.0
PauseUS 10
Next CounterA
Return

Can anyone help me?

Tks in advance.

From memory I think the bandwidth of the LM386 is inversely proportional to the gain. More gain = less bandwidth. Open loop gain of the LM386 is something like 200, this will place a huge tax on the useable bandwidth. Generally above 15KHz or so, at very low output it's out of our hearing range. All told, Hi-Fi is 20Hz to 20KHz. I believe that the LM386 can go as high as about 100KHz with full feedback. i.e. No gain at all. In this configuration it's deemed a 'buffer' only.

Best Regards,
Trent Jackson

[Melanie]

Also, further to Trent's fine comments...

A piezo transducer has a resonant frequency, the further away from that frequency (or it's harmonics), and the piezo is rubbish. Use an Ultrasonic transducer (old style burglar alarm, antique TV remote, car parking sensors type), but even then, most don't go much beyond 40kHz. A pipistrelle Bat should be a quite good emitter... wire one up to a spare PIC I/O and you're in business (not checked a pipistrelle's Datasheet to see if it's 5v triggered)... trouble is you'll then have to budget for a conformal coating over the PCB to protect it from the Bat guano!

[T.Jackson]

And, Melanie’s right. You're only generating ultrasound for 20mS, 1,000 cycles of 20uS, 1 / 20mS = 50Hz. And the more I think about it with what I just wrote, if you were out of bounds with bandwidth you wouldn't hear anything at all. Completely attenuated.

Best Regards,

Trent Jackson

[T.Jackson]

Keep in mind that you're applying close to 5V to the input of the LM386.
IO ports on PIC’s are CMOS with a guaranteed minimum of 73% of VCC when set high. (You'll lose a bit because of the coupling cap) With this in mind, assuming a supply rail for the LM386 at around 9V, you really don't want a gain of much more than about 1.5 Of course, I am assuming that you don't have a volume control set in place enforcing a limitation on volume.

Best Regards,

Trent Jackson