Ultrasonic direct detection circuit

Direct detection method of receiving/transmitting is paired. If ultrasonic waves (with signal voltage) are received, it means that there is no measured object between the receiver/transmitter . On the contrary, when no ultrasonic waves are received (no signal voltage), there is an object to be measured in the middle.
    (1) Introduction to circuit principle The ultrasonic direct detection circuit is shown in Figure 6-21. The driving circuit for sending ultrasonic sensors uses NE555
to form a separately excited oscillation circuit. Adjust RP1 to maximize the output voltage of the ultrasonic sensor, and RP2 is used to adjust the time lag voltage.
    The LM393 comparator is used to amplify the signal, the output signal of the ultrasonic sensor MA40A3R is used as its input, and the LM393 output is a square wave
signal.
    The output of LM393 is connected to the dedicated LM2907N for tachometer. The LM2907N is equipped with a frequency/voltage conversion circuit and a comparison circuit on the chip.
The input of the LM2907N requires a signal that changes with frequency. This requirement is compatible with the output square wave signal of LM393.
    Because the output voltage of LM393 is not low enough, the UIN (pin ⑩) of LM2907N needs to be added with the forward bias voltage of the diode
(0.6v). Added to the voltage of LM393.
    The conversion voltage of LM2907N is compared with the on-chip comparator voltage and output through the comparator. When _,★-40kHz, the full-scale conversion
output voltage is 12V. If the comparator (pin ⑩) is added with a comparison voltage of Ucc/2=12v/2=6V, then when it is higher than 20kHz, the comparator
outputs a high level, the on-chip transistor is turned on, and the LED emits light. When there is no object blocking the ultrasonic wave, the input frequency is 40kHz.
    If there is an object blocking the ultrasonic wave , MA40A3R does not receive the signal, the LM2907N comparator outputs a low level, the on-chip transistor is turned off, and
the LED goes out.
    (2) See the schematic diagram for circuit component parameters.