§ 01 Ultrasonic Array Drive

1.1 Application Background

In order to realize the conversion of sound into "laser" in the blog post, the modulation of ultrasonic waves is used to transmit sound, and the sound is sent directionally. In Ultrasonic Directional Transmission: Ultrasonic Board Fabrication an array of 20 ultrasonic transmitting transducers was fabricated. This paper makes and tests its power drive circuit.

1.1.1 Drive Scheme

In Turning Sound Into a Laser , the motor H-bridge driver chip L293 (Quadruple Half-H Drivers) is used to complete the ultrasonic drive. According to the L293 data sheet , it can be known that its operating voltage range is 4.5V ~ 36V, and the output peak current is 2A, which can meet the Require.

▲ Figure 1.1.1 Basic features of L293

▲ 图1.1.1 L293 的基本特性

I have the MX1919 purchased from Taobao at hand . According to the MX1919 data sheet , it can be seen that its operating voltage is: 2V~ 9.6V, and the peak output current is 3.5A, which has a larger output current than the L293. But its working voltage is lacking.

▲ Figure 1.1.2 Basic features of MX1919

▲ 图1.1.2 MX1919 基本特性

Therefore, the only one L293 at hand is used to build the drive circuit later.

1.2 Test plan

Build the L239, 555 oscillator circuit on the breadboard to test the output waveform and output power of the power drive circuit.

§ 02 A PPLE script

2.1 Topic introduction

Introduction	Picture
The book is connected to the last time, in order to make teaching demonstration teaching aids,
A 5×4 ultrasonic transmission array is made, and the following is to build a driving power circuit to test it
First, build the relevant experimental test circuit on the breadboard.
There are two motor driver chips at hand, one is L293 and the other is MX1919. The operating voltage range of L293 can reach 36V, while the maximum operating voltage of MX1919 is 9.6V.

2.2 Build the L293 drive circuit

Build L293 driver circuit	Picture
Let's first choose L293 chip to build the ultrasonic drive circuit,
Here I see that the L293 has four H half bridges inside, which can form two H bridges in parallel to improve the output power.
Let's start building the test circuit in the bread lab class.
The experimental circuit is basically completed here.

2.3 Build 555 circuit

Build a 555 circuit	Pictur
Let's start building the 555 oscillator circuit.
Here is the finalized L293 and 555 test circuit.

2.4 Test the output signal

Preliminary test experimental circuit	Picture
Let's start to test the initially built test circuit.
First load the 555 working voltage +5V, use a small screwdriver to adjust the 555's oscillation frequency to make it about 40kHz.
This is to measure the square wave signal output by pin 3 of the 555 chip.
Introduce this signal to the input of L293, and the output signal of L293 can be measured. The cyan in this figure is the output signal of the 293, with a peak value of 12V.
The +12V power supply current of the test L293 is about 6mA.

Test Loading the Ultrasonic Array Board	Pictur
Next, connect the ultrasonic array board to the output of L293, and test the output waveform and working current of the circuit under load.
This is the output waveform after adding the ultrasonic array. It can be seen that a small step appears after the rising and falling edges in the output voltage. What is the reason for this? It is not known yet.
The operating current of the test circuit is approximately 67mA. The power to drive the ultrasonic array is 12V×0.067, which is about 0.8W.

2.5 Building an H-bridge circuit

2.5.1 Building an Inverting Circuit

Build the inverting circuit of the 555 output signal	Pictur
Since the 555 circuit only outputs a square wave of one polarity, the L293 also has only one non-inverting input.
Therefore, it is necessary to build an inverting circuit to invert the output of the 555 circuit to form a complementary square wave output.
Let's build this triode-based inverter circuit on a circuit board.
After building the inverting circuit, test it.
Connect the base and collector signals of the triode to the oscilloscope.
This is the inverse waveform corresponding to the transistor 8050. It can be seen that there is a long delay in the transition of the transistor from on to off.
In order to speed up the transition speed of the triode from on to off, an accelerating capacitor C1 is connected in parallel with the base resistor R1 of the triode.
Measure the output signal waveform. You can see that the rising edge of the output is faster. The waveforms of the collector and base of the triode form a reverse waveform.
Changing the value of R1 from 10k to 100k reduces the saturation of T1.
Here, the T1 collector voltage waveforms under two R1 values are compared, and it can be seen that they are basically the same. Only when R11=100k ohms, the rising edge of the T1 collector waveform is slightly improved.

Test the H-bridge output signal	Pictur	Note
Connect the output of the 555 and the reverse signal of the triode to the two half-bridge input signals of the L293.
Measure the output signals of the two half bridges. At this time, the quiescent operating current of the circuit is 14mA.
Connect the front only ultrasonic array board to the H-bridge output. The circuit shown here operates at a current of 200mA.
This is the output voltage waveform.
At this time, if the pass-in peak-to-peak value is 0.5V

circuit is hot	Picture	Not
Because the output current is relatively large and the operating frequency is very high, the consumption of the L293 itself is also very large. When you touch the L293 at work with your hand, you can feel its temperature is very high.
The following uses an infrared camera to take a picture of the temperature of the L293 when it is working.
This is the temperature of the chip surface of the work tag, which is very low.
After working for 5 seconds, you can see that the L293 chip has become very high.
Whether there are other further improvements, wait for the next experiment to test it