Hi everyone, I'm designing an electronic pet door and am thinking of adding an ultrasonic transmitter to the pet's collar and an ultrasonic receiver to the door. Such that when the pet approaches the door the door will respond by automatically opening.

Regarding the ultrasonic transmitter circuit:
View attachment ultrasonic_transmitter.pdf
(Obtained from: http://www.pages.drexel.edu/~pyc23/ultrasonic_transmitter.html)

The ultrasonic transducer I have will vibrate optimally at about 40kHz. From this, I've worked out how the designer obtained the values for R1, R2 and C1 but am confused about the other components and how their values were calculated.

1. How are C2, R3 and R4 are calculated?
2. Instead of 2N2222 ('quad two' transistor, NPN BJT transistor used for general purpose low-power amplifying), can I substitute using a BD139 which is also a NPN transistor? View attachment BD139.pdf
3. Would the values of R3 and R4 change as a result of using BD139?

Any assistance will be appreciated, thank you for reading my post.

 

R4 is determined by the specifications of your transducer. R4 will limit the current to the xdcr while the transistor is off. R4 also limits the current through the xistor when it is on. With 12V and 2k2 ohms, that's about 6 ma. a very low value.
R3 is determined by the collector current when the xistor is on. Look at the data sheet for the 2n2222 and find the Vces spec. This is the voltage drop across the xistor when it is turned on hard or saturated. You will see the parameters used to find Vces, probably something like Ic=150ma, and Ib=15 ma. Beta=Ic/Ib or 150/15=10, the beta of the xistor. For your circuit Ic=6ma. 6ma/beta=Ib=600ua. The voltage across R3 will be the output voltage of the 555 minus the Vbe of the xistor. That makes it 12-0.7=11.3 volts. Using R=E/I, R3=11.3/600ua
From the looks of this circuit any NPN transistor with a reasonably low Vces should work including the BD139.
C2 is a generic bypass capacitor. It helps eliminate high frequency garbage which may affect the 555. You will see caps like this across many different IC's in thousands of different circuits. They are always (or at least they should be) mounted as close to the IC as possible.