When this reflected burst of pulses is received by the Ultrasonic transducer it is amplified twice by the AC Amplifiers - the gain is about -30 for each amplifier at 40KHz giving an overall gain of about 900
The output signal from the second AC Amplifier is connected to the Comparator Switch which produces a high output pulse when the burst is detected.
This pulse is connected to the Set input of the Latch sending its output high and thus ending the timing of the returned signal. The Counter can only count when the enable is low and this is controlled by the Latch.
Adjusting VR1 of the HF Astable will calibrate the reading obtained. For correct measurement set 17015Hz. This is because:
- To measure 1cm sound needs to travel 2cm (to the object and back).
- Sound travels at 340.29m/s which means it takes 2.939us to travel 1mm.
- So it will take 58.77us to travel 2cm where we require a reading of "1".
- One pulse every 58.77us = 1/0.00005877 = 17015Hz
There is a problem with this design. The reset pulse to the counters impedes into the Count time which means that the counters will be prevented from counting for 190us, in which time sound will travel 64mm. Therefore when properly calibrated, the distance will always be in error by -3.2cm...but it means that a latch is not required and simpler circuitry overall.