The ultrasonic emission circuit diagram is shown in the figure below. The applied voltage of ultrasonic transducer is an important factor to determine the detection distance. The applied voltage can affect the electric field strength of piezoelectric ceramic materials in the transducer, and then affect the diaphragm shape variable and piezoelectric conversion efficiency. At present, a common method is to use 7404 series inverter as the voltage driving chip of ultrasonic transmitting transducer. Although this scheme has simple design and low price, the driving peak voltage generated by it is low, and the maximum is only about 7V, which greatly shortens the detection distance. In view of this situation, this paper decides to use MAX232 instead of inverter to increase the transmission driving voltage of ultrasonic transmitting transducer and improve the piezoelectric conversion efficiency. The experimental results show that MAX232 can convert the TTL level of about 5V into 232 level of about 9.2v, the peak to peak value can reach 18.5v, the detection distance can reach 5m, and the duty cycle is about 50%, which overcomes the disadvantage of short detection distance, and other performance indexes fully meet the design requirements. The ultrasonic wave sent by this scheme takes 10 cycles as a sequence, and the interval between the two sequences is 32768us, that is, the overflow time of T1 timer. When T1 overflows, the system displays an error and re transmits the ultrasonic wave to the next measurement. The transmitting circuit of the system is shown in Figure 2.
Fig. system transmission circuit diagram