Working principle and circuit design of digital ultrasonic distance meter

2.3 Counting and display circuit

The counting and display circuit is shown in Figure 3. CD40110B is a decimal reversible counting/latching/decoding/driver with functions such as up and down counting, counter status latch, and seven-segment display decoding output. The maximum output current of each segment is 25 mA, which can directly drive seven-segment common cathode LED digital tubes. Among them, pin CPU is the addition counting clock pulse input terminal: Qco is the addition counting carry output terminal; pin TE is the enable terminal, when TE=0, the counter works, when TE=1, the counter prohibits counting; LE is the latch Control terminal; when LE=1, the display data remains unchanged, but the internal counter still works normally. Three CD40110Bs form a 3-digit decimal adding counter. 3-digit LED digital tube displays the measurement distance. The unit of the LED digital tube corresponding to U7 is m, the unit of the LED digital tube corresponding to U6 is dm, and the unit of the LED digital tube corresponding to U5 is cm. U4a and U4d of 74HC00 form a counting circuit pulse generator. Its oscillation frequency f2≈1/[2.2xC12×(R35+W)]. When the ambient temperature is 25℃. When the measurement distance S is 1 m, the ultrasonic round-trip time T=2S/C≈5.8 ms.

At this time, the counter display should be 1.00, which is 1 m. Therefore, the counter should count to N=100 at T=5.8 ms. At this time, the pulse period of the counting circuit pulse generator is required to be T2=T/N=5.8x10-2ms, that is, the frequency of the counting circuit pulse generator is required to be f2= 1/T2=N/T≈17.2 kHz. To make the frequency of the counting circuit pulse generator 17.2 kHz, when C12 is 2 200 pF, R35+W=1/(2.2C12f2)≈12 kΩ. Since the propagation speed of ultrasonic waves in the air is also different under different ambient temperatures, the frequencies of the required counting circuit pulse generators are also different. In order to improve the accuracy of measurement under different ambient temperatures, a precision potentiometer W is used to adjust the frequency of the pulse generator of the counting circuit so that the distance meter displays 1.00. U1f, U4b, and U4c form the RS flip-flop. When transmitting ultrasonic waves, point C is high level, the transistor VQI of the ultrasonic receiving amplifying circuit is turned on, the ultrasonic receiving amplifying circuit has no signal input, point E outputs high level, and point U4c of the RS flip-flop outputs low level. When the CD40110B is in the decimal addition counting state, it counts the input pulses of the pulse generator. When the ultrasonic wave is transmitted, point C is low level. If the ultrasonic receiving amplifier circuit does not receive the returned ultrasonic wave and point E is still high, the output of point F of U4c of the RS flip-flop still remains low, and CD40110B continues counting. Once the ultrasonic receiving amplifier circuit receives the returned ultrasonic wave, point E becomes low level, the output of point F of U4c of the RS flip-flop becomes high level, CD40110B stops counting, and latches the displayed count value; point E returns to high level , the U4cF point output of the RS flip-flop still remains high, and CD40110B still stops counting and latches the displayed count value. If the measured object exceeds the maximum effective measurement distance, or there is no measured object, the ultrasonic receiving amplifier circuit has not received the returned ultrasonic wave, and point E is always high level. After the counter counts to 999, and then counts by 1, U7 A carry output is generated, which resets the three-digit counter and displays "0.00", indicating that the ranging is invalid. At the same time, transistor VQ2 is instantly saturated and turned on, the potential of point E instantly changes to low level, and the potential of point F changes to high level. C11 and R12 form a power-on reset circuit. When the power is turned on or the reset key S1 is pressed, the three-digit counter is reset and displays "0.00". At the same time, transistor VQ2 is instantly saturated and turned on, and the potential at point E becomes low level instantly. The potential of point F is high level. Figure 4 shows the ultrasonic operating waveform.