TRANSMITTER:
The PIC clock is used as a reference for ultrasonic output frequency.
The Schmitt Trigger cleans up the clock signal and also acts as a buffer.
The Frequency Divider (7 bit ripple counter) is used to generate the o/p freq. of 39.9KHz by dividing the clock freq. by 11110 This is pretty close to the resonant freq. of the Ultrasonic transducers used in the circuit.
The Burst Gate is how the micro-controller times the
ultrasonic bursts.
A logic '1' at the gate control and the signal is transmitted.
Logic '0' disables the o/p.
The Driver is a single transistor use to drive the transducer.
Note: The unusual frequency of the PIC clock is not a prerequisite of the design, it was chosen purely because I happen to have a few 4.433619 MHz crystals in my parts box!
If you divide this frequency by 111 you get 39.945 KHz close enough to the resonant frequency of the Ultrasonic transducers (40KHz).
Therefor if you require a different output / input frequency or indeed both, the reset value (111) of the freq divider will have to be changed.
RECIEVER:
The Hi-Gain Amplifier has an adjustable gain between 50 and the open loop gain of the selected op-amp.
The gain is adjusted to give maximum possible signal whilst keeping the amplifier stable.
The Demodulator has the job of converting the 39.9KHz
amplitude modulated signal to a DC curve that follows the amplitude of the received
signal.
But where does this modulation on the signal come from I hear you ask.
The modulation on the signal is actually the echoes received back from the immediate
environment.
The Schmitt trigger has the job of cleaning the demodulated signal into crisp on / off signals ready to interface with the micro-controller.