Development of the digital ultrasonic distance measuring device
Circuit composition and working principle of ultrasonic rangefinder
The ultrasonic rangefinder consists of an ultrasonic generation circuit, an ultrasonic reception amplification circuit, a counting and display circuit.
Ultrasonic generating circuit
Circuit for generating ultrasonic waves. The dual timer EN556 (U2b) forms a monostable trigger. R6 and C6 form a differential circuit, which functions as follows: when button S2 is pressed, the low level becomes a positive and negative peak pulse, which is obtained through VD1 and triggers the flip of the monostable trigger. The high-level duration of the monostable flip output is approximately 1ms, i.e. tw ≈ 1.1R5C5 ≈ 1ms. EN556 (U2n) forms a multi harmonic oscillator with oscillation frequency f1=1/T1 ≈ 1/{0.7 [(R1+R2)+2 (R3+R4)] C3 ≈ 40kHz
Generator output level control. When the monostable trigger outputs a high level, the multi harmonic oscillator oscillates, and pin 5 of EN556 outputs approximately 40 rectangular pulses with a frequency of 40kHz and a duty cycle of about 50%. Considering the instability of the starting stage of the multi harmonic oscillator, a larger number of m-pulses are designed for input. If the number of output pulses is too small, the emission intensity will be low and the measurement distance will be short. Humidity sensor probe, stainless steel electric heating tube PT100 sensor, cast aluminum heater, heating coil fluid solenoid valve, but with too many pulses and long emission duration. When the distance to the measured object is close, the pulse train has not been fully emitted, which causes the echo generated by the first emitted pulse to reach the receiving end, affecting the distance measurement results and increasing the blind spot of the distance measurement. The U1a~U1e of 74HC04 (U1) form an ultrasonic pulse driving circuit, which can increase the amplitude of the pulse voltage driving the ultrasonic transmission sensor, effectively perform electrical/acoustic conversion, enhance the ability to emit ultrasonic waves, and increase the measurement distance. One path of the 40kHz pulse train is inverted through U1a, and then inverted through an inverter connected in parallel with U1b and U1e; The other route is reversed by an inverter connected in parallel to U1c and U1d in the south.
