The sound of metal clanging was particularly jarring at night.
Chen Hao jumped up from his chair with a start, his water glass falling to the ground. Without bothering to pick it up, he rushed to the window and saw the newly installed alarm bracket on the north side leaning precariously in mid-air, one of its supporting rods broken and dangling from the power line.
“The wind was too strong last night.” Susan also arrived at the scene, still holding her toolbox which she hadn’t had time to close. “The spare support fell over and hit the main power line.”
Nana crouched down to inspect the interface box: "A momentary current fluctuation was detected. The system misinterpreted it as an intrusion signal and triggered a level one alarm."
"So that sound just now wasn't a test?" Chen Hao asked.
“No,” Susan shook her head. “It’s a malfunction.”
They waited ten minutes before the indicator lights returned to normal. Just as dawn broke, the four of them gathered on the small platform outside the control room, around the newly assembled alarm control panel.
"Let's try the real thing first," Chen Hao said. "This time, let's make sure nothing gets blown over by the wind."
Susan turned on the power switch. The main unit's screen lit up, and after a few seconds displayed "System self-test in progress." Carl stood at a distant testing point, holding a piece of metal, preparing to tap the ground to simulate footsteps.
“Begin,” Nana said.
Carl knocked.
The alarm didn't go off.
He knocked twice more forcefully.
Three seconds later, the horn suddenly emitted a sharp sound, which lasted for five seconds before abruptly stopping. Then the red light flashed wildly, but the green light remained off.
"Does this count as approved?" Chen Hao asked.
“No.” Susan stared at the data panel. “The response was delayed by three seconds and interrupted twice. Under normal circumstances, we would have been knocked over by the time the wild boar charged at us.”
Nana pulled up the log: "The sensor received the signal 1.8 seconds later than the actual tap, possibly due to poor wiring contact. In addition, the last signal was lost due to unstable voltage."
“I’ll check the route.” Carl turned and left.
Susan unplugged a section of the interface module and found some rust inside: "These recycled parts are being used far too roughly."
"Something is better than nothing," Chen Hao said, leaning against the wall. "At least it made a sound once."
“If the alarm doesn’t go off, it’s useless.” Susan’s tone hardened. “If it alarms every time there’s the slightest disturbance, who will believe it after a while? When something really happens, they’ll just think it’s a false alarm.”
No one spoke.
Chen Hao looked down at the tips of his shoes. He knew she was right. That night, they were surrounded by stray dogs, but they didn't make a sound. By the time they heard a noise, the dogs had already climbed over the makeshift fence.
“Then keep modifying it,” he said. “First, make sure it rings steadily, then worry about when it rings.”
The second test began at 10:00 AM.
This time, they resoldered all the wiring, added a voltage regulator to the main unit, and even replaced the speaker. The test point was moved to a more distant location to simulate a target approaching from thirty meters away.
Carl tapped the ground.
One second later, the alarm sounded.
"It's done?" Chen Hao looked up.
The sound lasted only two seconds before stopping. Then, ten seconds later, it rang again without warning, this time lasting fifteen seconds.
“They’ve gone off on a rampage again,” Karl frowned.
Nana checked the backend: "The first response was normal, but subsequent signals were identified as repeated interference and automatically blocked. The blocking release time was set incorrectly, causing delayed re-reporting."
"In other words, it thought it had misheard, and then suddenly remembered," Chen Hao summarized.
"More or less," Nana nodded.
The third test was scheduled for the afternoon.
They modified the shielding logic and added a signal confirmation mechanism. Susan manually set the alarm to trigger only after three consecutive vibrations to avoid false alarms caused by a single disturbance.
Carl struck the door three times.
I waited for five seconds, but there was no response.
Tap three more times.
Ten seconds later, the alarm went off.
“Too slow,” Susan immediately objected. “A real threat won’t stop and wait for you to knock three times.”
“No, it will make noise even when the wind blows through the leaves.” Chen Hao scratched his head. “We’re worried that it’s too sensitive, but we’re also worried that it’s too insensitive.”
“The problem isn’t with the program,” Nana said. “It’s with the sensor itself. Existing components can’t accurately distinguish between vibration intensity and frequency. A light footstep and a falling branch look almost the same on the data.”
“Then we can only adjust the threshold,” Susan said. “We need to find a balance—one that can detect living creatures approaching without being affected by the environment.”
“Easier said than done.” Carl put the welding torch back in the toolbox. “We don’t have standard testing equipment; we have to rely on people running around and tapping the ground. We tried seven times today, and only one time we barely passed the test.”
“Then let’s try it for the eighth time.” Chen Hao stood up. “I don’t believe we can’t fix this broken machine.”
Before evening, they tried adding a delayed judgment function. The system no longer responded immediately, but instead collected vibration data over two seconds to make a comprehensive judgment on whether it posed a threat.
The fourth field test.
Carl tapped the ground three times in rhythm.
Two-second wait.
The alarm sounds, lasts for ten seconds, and then automatically stops.
"How did it go this time?" Chen Hao asked.
"The response time has been reduced to 1.2 seconds." Nana read the data. "No false alarms or interruptions occurred. It meets the basic early warning standards."
"It's finally starting to look like something." Chen Hao breathed a sigh of relief.
Susan hadn't even laughed yet: "That was your ideal scenario. What if the rhythm were a little off?"
She took to the stage herself, randomly tapping the ground at varying intervals and with different forces.
The alarm didn't go off the first time.
The second time, it sounded after a four-second delay.
The third time, it rang but lasted for twenty seconds until it was manually turned off.
“It still doesn’t work,” she said. “It can only recognize fixed patterns; it doesn’t know how to handle slight changes.”
Carl leaned against the wall, took off his helmet, and fanned himself. "I don't think it's necessary to be so serious. As long as it makes a sound, we know there are people or things outside. As for whether it's accurate or not, who can expect a pile of scrap metal to be smarter than a human brain?"
“It’s not a question of whether it’s accurate or not,” Susan said, turning to look at him. “It’s a question of whether we believe it or not. If we ourselves don’t believe it can be accurate, then what’s the point of having it? We’d have to send people to keep watch over it during the day.”
"So what do you want?" Karl's voice rose. "To build a military-grade system using ten-year-old parts? We don't even have an oscilloscope!"
“I know the conditions are poor,” Susan said, not backing down. “But it’s precisely because they’re poor that we need to ensure basic reliability. Otherwise, what’s the point of spending so much time just listening to it bark in the middle of the night?”
The two stood there, frozen in place.
Chen Hao glanced at the sky, then at the main unit whose red light was still flashing.
“Let me tell you something,” he said. “During my first week here, I didn’t get a full night’s sleep. I would wake up several times every night, thinking I heard some noise. Later I found out it was just the tin roof being blown by the wind. Once I grabbed a stick and rushed out, and almost stepped into a trap I had dug myself.”
He paused for a moment: "Back then, if there had been something that could have told me 'It's okay, it's just the wind,' or 'Run, there's something there,' I would have gladly kowtowed to it."
The surroundings quieted down.
"What we're doing now isn't to build an impregnable fortress immediately," Chen Hao said. "It's so that we can sleep soundly in the future. Even if it only gives us ten more seconds to react, it's worth it."
Susan looked down and fiddled with the circuit board in her hand.
Carl sighed and picked up his tools again: "Alright, tell me how you want to change it."
Over the next three hours, they disassembled the main logic module again. Nana found three low-power sensing solutions in the database and compared their applicability one by one. Susan proposed a tiered response approach: slight vibrations would activate the monitoring mode, moderate vibrations would issue an internal alert, and only continuously approaching signals would trigger an external alarm.
Chen Hao was responsible for recording the parameters for each round of debugging.
The ninth test was conducted at night.
They chose different ways to approach: walking slowly, running quickly, jumping, and dragging the object.
In most cases, the system correctly identified and responded. There were still two false alarms and one missed alarm.
“It’s far from perfect,” Susan said.
“But it works.” Chen Hao looked at the gradually stabilizing signal curve on the screen. “The next step is to make it more and more accurate.”
They decided to replace the recovery sensors with a batch of more sensitive ones the next day, prioritizing the replacement of key deployment points on the north and east sides.
It's late at night.
The four people stood under the alarm tower, the main unit's indicator light flashing on and off, as if it were breathing.
Nana wrote the modified program into the spare chip and put it into the waterproof box.
Carl tied the last section of cable tightly.
Susan wrote down the locations of the nodes that needed to be checked tomorrow in her notebook.
Chen Hao looked up at the sky and saw a crack in the clouds, revealing a few stars.
He suddenly said, "Don't you guys feel like what we're doing right now is a bit like piecing together a broken radio?"
No one answered.
But he smiled and said, "It doesn't matter if it can't be fixed. As long as we can receive one station, we've won."
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