The Three-Knock Device

Two Parts. That's It.

No wiring. No programming. Two parts. Done.

What You Need

• 1 piezo disc, 27mm (the flat brass kind — looks like a tiny gold coin with two wires. About the size of a quarter. About a dime.)
• 1 AAA battery (from the junk drawer, the TV remote, wherever)
• Electrical tape #1 Swamp Glow#2 Route 13#3 Three-Knock

Parts — where to buy each one →

How to Build It

The piezo disc has two wires — one red, one black. Tape one wire to one end of the battery. Touch the other wire to the other end — tick. Touch it three times: tick, tick, tick. That's the whole circuit. You complete it with your finger.

Tape the disc flat against a door, a wall, a bunk frame — any surface that carries sound. Through wood, it sounds like knocking.

Want a cleaner click? Add a tactile push button (6×6mm) between the free wire and the battery. Press the button instead of touching the wire.

ATtiny85. Three Knocks. Once a Day. Same Time. Every Night.

A tiny chip controls the pattern. Three knocks, spaced a second and a half apart. Then silence for exactly 24 hours. Then three more. Turn it on at 3:07 AM — it knocks at 3:07 AM every night.

Parts

• 1 ATtiny85 chip (8-pin DIP) #1 Swamp Glow#2 Route 13#3 Three-Knock
• 1 piezo disc, 27mm (same as Version 1)
• 1 CR2032 coin battery #1 Swamp Glow#2 Route 13#3 Three-Knock
• 1 CR2032 battery holder with leads and switch #1 Swamp Glow#2 Route 13#3 Three-Knock

Parts — where to buy each one →

Pin Orientation

Hold the chip so the small notch or dot faces left.

Bottom-left is Pin 1. Top-left is Pin 8. Pins count counter-clockwise.

Pin Reference

• Pin 1 — RESET (leave unconnected)
• Pin 2 — PB3
• Pin 3 — PB4
• Pin 4 — GND (ground)
• Pin 5 — PB0 ← piezo output
• Pin 6 — PB1
• Pin 7 — PB2
• Pin 8 — VCC (+3V power)

Step 1 — Program the Chip

Use a Tiny AVR Programmer board (~$10) — plug the ATtiny85 into the socket, plug the board into USB, and upload a short program from the Arduino IDE. One time. After that, the chip remembers.

int waitMinutes = 1440;             // minutes between knocks (1440 = 24 hours)

void setup() { pinMode(0, OUTPUT); }
void loop() {
  for (int k = 0; k < 3; k++) {    // three knocks
    digitalWrite(0, HIGH);
    delay(40);                      // knock pulse
    digitalWrite(0, LOW);
    delay(1500);                    // silence between knocks
  }
  for (long i = 0; i < (long)waitMinutes * 60; i++) {
    delay(1000);
  }
}

Step 2 — Connect It

Three connections. No soldering. Bend the wires directly onto the chip pins.

• Pin 5 (PB0) → one wire from the piezo disc.
• Pin 4 (GND) → other wire from the piezo disc and black wire from battery holder.
• Pin 8 (VCC) → red wire from battery holder.

Tape it together with electrical tape. The battery holder has a built-in on/off switch.

Step 3 — Place It

Tape the disc flat against a door, behind a bed frame, under a windowsill. Tuck the ATtiny85 and battery holder behind the door frame or inside a gap. Slide the switch at the time you want it to knock — turn it on at 3:07 AM, it knocks at 3:07 AM every night after that.

Troubleshooting

No sound? Check that the piezo wires are on Pin 5 and Pin 4. Check battery is seated. Try swapping the wires. If the knock sounds too quiet, try a different surface — wood carries sound best. If you want faster or slower knocks, change delay(1500) in the code.

Tick. ATOM Matrix. Atomic Echo Base. 3:07 AM. Every Night.

This is what Hargrave actually uses in the book. Tick devices with Atomic Echo Bases hidden behind door frames, a Wi-Fi network, and a timer set for 3:07 AM. Three knocks. Every cabin. Same time. Same spacing. Every single night.

What You Need

Parts — where to buy each one →

The Setup

The ATOM Matrix plugs into the Atomic Echo Base — a speaker and microphone board. He recorded the knock sound himself — his knuckle on a pine door, three times, slow and even. Each device connects to the camp Wi-Fi, syncs time via NTP, and waits.

At 3:07 AM every device plays the audio through the Echo Base speaker. Every cabin hears the same three knocks at the same time. The 5×5 LED grid flashes amber with each tap — but nobody sees that. The device is hidden.

That's the part that really gets them — not the knocking itself, but the fact that it's synchronized. Raccoons don't knock on eight doors at the same time.