In the scheme of our transmitting circuit the input is provided by PIC, which generates a square wave at a frequency that switches between 38.2 kHz and 41.5 kHz, with peak-peak amplitude of 5 V (from 0 to 5).

To supply power to the sender is however necessary a square wave symmetric with respect to ground, with a peak-peak amplitude of at least 15 V, but not exceeding 20 V.

Hence, before providing the PIC output to the integrated circuit HEF 4049B, we inserted an amplifier stage consisting of a BJT NPN transistor in common-emitter configuration, with supply voltage of 8 V, in order to increase the signal amplitude.

The output of the amplifier stage is then inverted, because the stage is inverting (the voltage gain is negative), and sent to the integrated circuit HEF 4049B, which provides the correct voltage on the ultrasonic transmitter, according to the scheme suggested by Murata Application Manual (page 9, Applications, Transmitting and Receiving Circuits).

In the scheme of the receiving circuit there is initially the ultrasonic receiver, put in parallel to a resistor; the whole is in series with a capacitor and the signal is supplied to an operational amplifier. So far we followed the suggestions contained in the Murata Application Manual (page 9, Applications, Transmitting and Receiving Circuits), then we added some processing stages necessary for our purposes and to connect to the DSP.

After a two-stage amplification, necessary because of the small amplitude of the sinusoidal signal received, there is a symmetrical clipping stage with two diodes, in order to limit to a precise value (around to the diode threshold) the signal amplitude with respect to ground, so that it is roughly the same in the following stages regardless of the distance between transmitter and receiver, and therefore regardless of the amplitude of the initially received signal.

A buffer and a further amplification stage follow. Up to this point the signal is symmetric with respect to ground, while the DSP must receive as input square waves as regular as possible, with values 0-5 V.

So it is necessary foremost inserting a precision rectifier, made with an operational amplifier and a diode, which can lead to zero voltage all negative half-waves. For all amplifiers the integrated circuit TL084 was used.

After this stage, the maximum signal amplitude is about 4 V. We use a Schmitt trigger to obtain precise square waves, with minimum value 0 V and maximum value 5 V, which can be directly provided to the DSP. Since the integrated circuit HEF 40106B, our Schmitt trigger, is inverting, we provide the output of the first Schmitt trigger stage as input to a second stage, in order to have a perfectly squared and not inverted signal to provide as input to the DSP.

Among power supply of all integrated circuits used in our transmitting and receiving circuits, we inserted some bypass capacitors, to obtain stable supply voltages and to eliminate high-frequency noise.