While working on my Dog Ultrasonic Deterrent I needed or wanted a means of detecting Ultrasonic Sound.

I discovered Bat Detectors:

http://home.earthlink.net/~bat-detector/BatDetector.html

http://www.nutsvolts.com/magazine/article/june2011_berber

https://www.elektormagazine.com/labs/bat-detector-with-amplitude-recovery

https://hackaday.io/project/8353-worlds-smallest-bat-detector

This one is an ingeniously simple circuit.

http://www.micro-examples.com/articles/index.php/PicoBat

It uses the signal from an ultrasonic receiver to clock a microcontroller which is divided by 4 to toggle a little piezo speaker

For instance a 25kHz ultrasonic signal produces about 6kHz in the speaker.

So I decided to build one myself.

After some tests I decided to add an amplifier for more sensitivity

Here is my Basic main code. 
Used GCBasic, 12F1840, ultrasonic transmitter (I had lying around).
Maybe would be more sensitive if I had an ultrasonic receiver.

;Chip Settings
#chip 12F1840
#config OSC=ECL, FCMEN=OFF
'
Start:
'
set wpua5 = 0 'PU off
' ultrasonic sensor on PA5
'
dir porta.0 out ' piezo
dir porta.1 out ' piezo
dir porta.2 out ' LED for program test
'
set porta.0 0
set porta.1 1
set porta.2 0
'
Do Forever
    lata = lata xor 0b111
loop
'