The ultrasonic detector or receiver circuit will be able to detect all those high frequency sounds that are beyond the hearing capacity of humans.
The units will be able to hear all those frequencies that are perhaps only audible to some animals like dogs, cats and bats. The ultrasonic receiver circuit can be used for enhancing or extending the hearing capability of a human being to levels which are not feasible to the normal ear.
How the Circuit Works
Referring to the circuit diagram shown below, the piezo speaker, MIC1, is formed by a piezo electric transducer.
The device detects the inbound ultrasonic signal and passes it to the base terminal of the transistor Q1. The two-transistor stage using Q1 and Q2 works like a booster amplifier stage which elevates the detected ultrasonic signals to an amount which is enough to trigger the single input of a very unconventional mixer circuit.
The IC U2 which is a quad bilateral switch operates like an exceptionally clean balanced-mixer circuit intended for the superheterodyne receiver. IC U1a is configured like a variable-frequency squarewave-oscillator circuit. Resistors R5, R6, and capacitor C4 are configured to fix the frequency and tuning range of the oscillator stage.
The squarewave signal from the oscillator stage is directed across a a couple of tracks. In one track, the U1a output becomes the input for the pins12 and pin13 of U2. In the second track, the signal goes to the Q3 base, constructed like an inverter.
This inverter generates a signal output which is 180° out of phase with the input signal. This out of phase signal from of Q3 is then applied to U2 at its pins 5 and 6.
At these pinouts of the op amp the two input signals which are one ultrasonic input originating from MIC1 while the other from the oscillator output are mixed together. This mixing of the ultrasonic signal from the MIC with the oscillator's square-wave causes the generation of an audible combination which is then applied to the input of a differential amplifier UIb, which is configured to provide a voltage gain of 2.
The U1b output available at pin 7 is subsequently filtered and cleaned by the resistors R19 and C9 to eliminate the high frequency material of the mixed signal.
Since it is only the difference frequency that matters, the sum frequency (the inbound ultrasonic signal added to the oscillator frequency), that may be excessively high for our ear to hear, is eradicated by R19 and C9 to provide a thoroughly clean and filtered output signal.
This cleaned up output is finally supplied to the power amplifier U3. Resistor R21 is configured to work like the circuit's volume control.
How to Test and Use
The testing and using the proposed ultrasonic receiver circuit can be done with the help of the following steps:
Swith ON power to the circuit, adjust the knobs of the volume and tuning controls to their mid-way placement. Next with the headphones fixed over your ears, start rubbing your fingers together just near the piezo transducer speaker. If everything is fine with the circuit, the rubbing of the fingers must sound like a sandpaper being brushed over a hard surface.
For the next test procedure, get hold of some small metal screws in your, along with a few nuts, and washers, then shuffle them inside your folded fingers near the piezo transducer. You may find this sound like huge metals pipes banging across each other.
While implementing the above testing procedures, make sure to keep adjusting the tuning potentiometer until your ear starts hearing full of abnormal and strange sound effects.
The tuning range of the oscillator circuit stage could be anywhere between 15 kHz and 35 kHz. This frequency range permits the user to hear external ultrasonic sounds between 15 kHz to near and above 40 kHz.
Normally, it becomes impossible for us to hear sounds above 15 kHz, but with this ultrasonic detector circuit you would be able to hear all the sounds ranging above 15 kHz and even up to 40 kHz frequencies.