The SRF01 is a popular ultrasonic sensor used mostly for range finding applications. [Jaanus] discovered that they had a few flaws, including not working after being dropped. The faulty ones began to pile up, so he made a decision to tear one apart and put his engineering skills to use.
The SRF01 is special in that it only uses a single transducer, unlike the SRF04, which uses two. using only one transducer presents a problem when measuring very close distances. The transducer emits a pulse of sound and then need to listen for the echo. The smaller the distance, the smaller the time interval between the pulse and when the echo returns. There is a fundamental limit to this time around as the transducer has to recover from what is known as ringing. [Jaanus] discovered that the SRF01 solves the ringing problem with the use of a PIC24’s ADC and its 500 ksps (kilosamples per second) rate. This allows it to measure very close distances.
Be sure to check out the teardown for a lot more details on how the SRF01 works.