The GBW of the LM386 power amplifier is 2MHz, not 300kHz.
Its cutoff frequency (-3db) is 300kHz when its gain is 20.

Wow - you're right. The table in the spec sheet mentions bandwidth of 300 kHz. It's easy to read that as gain-bandwidth, but it's not.

 

Hello everyone!

I spoken with one of my senior lecturers today (and taking some advice from Audiguru) about my progress so far and for some advice. He said that at 1m 100mV p-p is not a problem and there is no need for a 2nd stage amplifier at all.

I'm surprised but also happy to say that the range for the system is far greater than 1M.

On the digital oscilloscope it measured around 600mV p-p at 1.5 metres even though on the analog one it could sense upto 1 metre whilst showing 100mV p-p, that was basically as far as my hand could go up holding a book at the top as the object.

At this distance the sine waves accordingly reacted in relevance to the distance.

I included the comparator circuit as well and it works fine as well. It's quite sensitive I found (as you move the object up and down the period of the waveform changes fast) but it does do the job just fine.

He told me to introduce a capacitor across the 250k feedback resistor for the LM358 (it was formerly 330k, the resistor R2) to act as a low pass filter to block out less than 40kHz.

The value for that i'll need to calculate.

The thing I have to do now is see if I can determine the delay in relation to the distance to the object, I'll have to show the transmitting square wave on channel 2 tomorrow on the osciliscope.

I've included the screenshots of the output square waves. The order of upload is:
-default (untouched state)
-at 1 metre
-at 1.3 metres
-30 cm distance
-2cm distance zoomed in shot

And there's this 8th pic which I've named glitch. The reason being I've found that sometimes if you've placed your hand or any object over it for a while and remove it the waveform still appears on the oscilliscope. However it oscillates by itself side to side whilst still maintaing its shape non-stop. The only wave I've found to stop this from happening is to just move another object or just your hand over the tx/rx pair quickly. I've spent the past hour testing and recording samples and this occurence happened 2 times.

Any reason for this?

Also the output waveform is slightly slanted, I suppose this wouldn't matter much right?

Thanks everyone!