Wake up microcontroller using ultrasonic sensor

Question

I need to design wake up system for microcontroller based on ultrasound. I have no documentation and two sensors, marked as T and R, they look exactly like those in hc-sr04.

I have transmitter circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

And receiver like this:

^{simulate this circuit}

The problem is that I got output from lm393p only when sensors are couple of centimeters apart (where I need couple of meters), so really small range, and output is 40kHz pwm signal, but I really would prefer to have high when pwm is detected and low when no signal is present. Also, I need to use as few components as I can. I don't have access to an osciloscope, just simple multimeter. I would appreciate any sugestions how I can make this system better.

Answer 1

That simple transmitter circuit seems OK. You might reduce R1's value a tad.

Receiver circuit has problems.
A variable threshold level might be in order via R5 (1k trimpot) below. It is easiest to arrange the comparator so output pin goes low when input is excited with 40 kHz. Output is at +12V when idle: R1 (10k) is absolutely necessary as a pull-up to +12V.

A 1nf input capacitor high-pass input filter helps to keep audio from triggering...these ultrasonic transducers still have some response below 20kHz.

^{simulate this circuit – Schematic created using CircuitLab}