Chapter 10 — Wind Village Robot Team

That morning, before climbing the hill, the whole team stopped by a small electronic components store in the district town, more than ten kilometers from the commune, and hitched two trips on Mr. Toan's motorbike. Khanh Dan used the money she spent herself - the only amount she allowed herself to spend, because she promised Mr. Nghia not to touch the school budget - to buy exactly two things that could not be made on her own: an H-bridge circuit L298N costing twenty-five thousand VND, and an ultrasonic sensor HC-SR04 costing fifteen thousand VND. The seller, seeing four children covered in road dust, attentively examining each chip as if choosing to buy something precious, couldn't help but curiously ask why they were buying it, and when he heard Phong passionately explain about using it to control the robot motor, the shop owner took five thousand dong off, and added that if the chip burned out, he would exchange it for half the price.

"Forty percent support left." Gia Han read aloud the percentage displayed on the phone screen, her voice tense as if she were reading test results. "If you lose signal now, you have to reload from the beginning."

All four children stood around Khanh Dan on the top of Tram Hill, at the foot of the old broadcast tower, each with one arm raised in a different direction — not to pray, but because each different angle of the phone captured different signal lines, and they tried for fifteen minutes to find the right angle to keep the most stable connection: Phong's arm was raised straight up into the sky, tilted exactly thirty degrees to the west.

"Don't move." Phong spoke, his voice muffled because he had to keep that position, his hands began to get tired. "I feel like if I move just a little bit, I'll lose the signal."

The document they are downloading is not a video, but a PDF of basic Arduino programming instructions in Vietnamese that Khanh Dan found from a STEM education forum, along with a connection diagram for a motor control circuit using the L298N chip - a type of H-bridge circuit that helps the Arduino control the direction of rotation and speed of the DC motor, which they need to make wheels for the robot to run back and forth, turn left and right according to command.

"Sixty-five percent." Gia Han continued reporting, not taking her eyes off the screen.

Uyen Nhi sat down on the ground, her notebook open on her lap, her pen in hand waiting to redraw the diagram as soon as the document finished downloading, not wanting to waste another second once she had the data in hand. Nam stood leaning against the rusty broadcast tower, looking far into the valley below, where he could clearly see the school roof and the dirt road leading to the village.

"Eighty percent." Gia Han said, and this time her voice was less tense, replaced by a rising excitement.

At that moment, Phong's arm began to tremble, unable to maintain the correct angle anymore. The signal bar on the phone flashes on and off continuously.

"Change hands, Phong." Nam jumped in, raised his hand to take his place, trying to find the right angle that Phong had just held. There was a tense moment when the wave line disappeared completely, the group held their breath, then it reappeared, faint but still there.

"Ninety-two percent." Gia Han read, her voice began to tremble with excitement.

No one said anything more, the whole group stood still like statues, only the wind still whistled through the iron frame of the broadcasting tower, creating a steady humming sound that sounded almost like music. When the progress bar reached one hundred percent, Gia Han screamed, unable to hold back, startling Nam and almost dropping the phone.

"Done!" She exclaimed, hugging Uyen Nhi standing closest, both of them laughing loudly in the middle of the empty hilltop.

Khanh Dan smiled at that scene, the feeling spreading to the group's excitement even though she was used to downloading technical documents in just a few seconds in the city. Perhaps because of the trade-off of fifteen minutes of shaking hands on a windy hilltop, this document holds a different value for them — not something available with just a click of a mouse, but something that must be earned with real effort.

"Can we study right here now?" Phong asked, rubbing his tired arm. "Or do I have to go back to the warehouse to learn?"

Khanh Dan looked at the clock, there was still more than an hour left until it was completely dark. She sat down on a flat stone slab - the same place where Mr. Toan often sat to fish for "antennas" - and opened her laptop, even though the battery was only more than half full.

"It's okay to study here too. Take advantage of it while you still remember the feeling of having just finished downloading." She said, calling the group to sit around. "First lesson: Arduino is not a computer, it is a small brain that only understands very simple commands, but runs extremely fast and extremely stable. I will learn three basic concepts today: digital signal pins, analog signal pins, and main loop."

She drew on a piece of paper Uyen Nhi gave her, sketching a rectangle representing the Arduino board, surrounded by numbered pins. "The digital pin, called digital, has only two states: on or off, like a light switch. The analog pin, called analog, can read a range of values, for example measuring how strong or weak the light intensity is, not just the presence or absence of light."

"So what kind of infrared sensor was the line detected yesterday?" Gia Han asked, holding a pen and taking notes quickly.

"Number type, because it only answers whether there is a black line or not, no matter how light or dark it is." Khanh Dan explains. "But the ultrasonic sensor I'm about to learn to make a robot avoid obstacles uses a time measurement method, which is a bit more complicated - it emits high-frequency sound waves, measures the time it takes for the waves to bounce back, and then calculates the distance."

Nam frowned, trying to imagine. "Like an echo in a mountain cave? Scream once, hear the echo, know how deep the cave is?"

"Exactly so." Khanh Dan nodded, surprised by his analogy. "You just explained the principle of ultrasonic sensors in just one sentence, clearer than the textbook I've ever read."

Nam grinned, seemingly satisfied, for the first time she saw him proud of something related to knowledge, not car repair.

"So what about that square circuit with those screws? What is it used for?" Uyen Nhi asked, pointing to the L298N chip in the plastic bag Khanh Dan just bought.

"This is an H-bridge circuit, so called for short because its internal diagram is arranged in an H shape." Khanh Dan picked up the chip and pointed to the four terminals. "The Arduino only emits a very weak signal, not enough power to make the motor rotate. This circuit acts like a large water valve - the Arduino only needs to give a light command, while the H-bridge circuit will take strong current from the battery, opening and closing according to that command to make the motor rotate. It also allows reversing the direction of rotation, which is why my robot can turn left and right."

"What do you mean by reversing?" Phong asked, his hand already holding a pencil and tapping it on his knee, a habit whenever he concentrates on thinking.

"In a DC electric motor, if you reverse the positive and negative poles, it rotates in reverse." Khanh Dan explained, drawing two arrows in opposite directions on the paper. "The left wheel turns forward, the right wheel turns back, the robot will rotate in place. Both wheels turn forward, the robot runs straight. Just by coordinating these four states, I can control all basic directions."

Gia Han took notes continuously, occasionally stopping to underline phrases she considered important, while Nam closed his eyes for a moment as if he were simulating in his mind the scene of two wheels spinning in opposite directions, then suddenly opened his eyes and nodded confidently.

"So it's just like the toy tank that children often play with, the two sprockets rotate in different directions so it rotates in place, right?" he asked.

"Yes, same principle." Khanh Dan nodded, secretly surprised again by Nam's ability to communicate quickly, even though he had never studied any physical concepts about torque or rotational force in a systematic way.

The lesson lasted until the sunlight turned dark orange, the laptop battery dropped to fifteen percent, forcing Khanh Dan to close the laptop halfway. But at that time, Uyen Nhi had already drawn a complete diagram in her notebook: the Arduino board in the middle, the connecting lines radiating out like tree roots, each branch clearly noted which pin number was connected to which sensor, which motor - the drawing was detailed enough that the whole team could reassemble it correctly without having to open the original document and review it again.

"Can we really start installing it tomorrow?" Nam asked when the group stood up to prepare to go down the mountain. It was already dark. They had to follow the familiar path to avoid tripping.

"I'll try tomorrow." Khanh Dan replied, putting her laptop backpack back on his shoulder. "But I told you first, it will definitely fail the first time. Not failing is new."

"If it's broken, fix it." Phong said, his voice firm, as if it were the most obvious answer in the world.

The whole group laughed, the laughter echoing down the hillside amid the howling wind that signaled a stormy night to come, and Khanh Dan, walking at the end of the line, looked back at the top of the hill one last time before disappearing around the bend - where the old broadcast tower still stood quietly in the wind, like a silent witness to the past fifteen breathless minutes.