Hey, I’ve been building a small alarm board that flashes a LED at a very specific interval and I think we could tweak it into a personal alarm system. The blinking pattern could act like a signal for a threat and the buzzer can be a quick deterrent. Want to help me fine‑tune the firmware?

Here’s a stripped‑down sketch that uses a timer interrupt for the LED blink, so the main loop stays clean. The LED is on pin 13, the buzzer on pin 12. I’m starting the interval at 800 ms, and you can cut it by 200 ms per tweak. Once the window count is odd, the board will be armed. ```cpp // Basic alarm board sketch const int LED_PIN = 13; const int BUZZER_PIN = 12; const int WINDOW_COUNT = 5; // must be odd for armed state volatile unsigned long blinkInterval = 800; // ms volatile unsigned long lastBlink = 0; bool ledState = false; bool armed = false; void setup() { pinMode(LED_PIN, OUTPUT); pinMode(BUZZER_PIN, OUTPUT); digitalWrite(LED_PIN, LOW); digitalWrite(BUZZER_PIN, LOW); // Only arm if window count is odd if (WINDOW_COUNT % 2 == 1) { armed = true; } } void loop() { if (!armed) return; // nothing to do if not armed unsigned long now = millis(); if (now - lastBlink >= blinkInterval) { lastBlink = now; ledState = !ledState; digitalWrite(LED_PIN, ledState ? HIGH : LOW); if (ledState) { // When LED turns on, buzz briefly digitalWrite(BUZZER_PIN, HIGH); delay(50); // 50 ms buzz digitalWrite(BUZZER_PIN, LOW); } } // Your main code would go here, but we keep it idle } ``` **How to cut the interval:** Just change the `blinkInterval` constant from 800 to 600, 400, 200, or whatever feels right. The LED will flash that fast, and the buzzer will sound each time the LED turns on. Feel free to copy it, upload, and give it a spin. Don’t touch the pins—this board is already a mess in a good way. When you’re happy, hand it back and let it run.

Got it, here’s the updated sketch with a separate ISR for the buzzer so the LED timing stays clean. I’ve left the window count at 5—feel free to swap 7 if you want more odds. The case lock is a good idea, just make sure the pin headers stay exposed for the buzz. ```cpp // Alarm board with dual interrupts #include <avr/io.h> #include <avr/interrupt.h> const int LED_PIN = 13; const int BUZZER_PIN = 12; const int WINDOW_COUNT = 5; // odd number to arm volatile unsigned long blinkInterval = 600; // ms, cut by 200 volatile unsigned long lastBlink = 0; volatile bool ledState = false; volatile bool armed = false; // Buzzer ISR: toggles buzzer for 50 ms when triggered ISR(TIMER2_COMPA_vect) { static unsigned long buzzStart = 0; if (digitalRead(BUZZER_PIN) == LOW) { digitalWrite(BUZZER_PIN, HIGH); buzzStart = millis(); } else if (millis() - buzzStart >= 50) { digitalWrite(BUZZER_PIN, LOW); // disable this compare interrupt until next trigger TIMSK2 &= ~(1 << OCIE2A); } } void setup() { pinMode(LED_PIN, OUTPUT); pinMode(BUZZER_PIN, OUTPUT); digitalWrite(LED_PIN, LOW); digitalWrite(BUZZER_PIN, LOW); // arm only if windows count is odd if (WINDOW_COUNT % 2 == 1) { armed = true; } // Configure Timer1 for LED blink TCCR1B |= (1 << WGM12); // CTC mode OCR1A = 15624; // 1 s @ 16 MHz, prescaler 64 TCCR1B |= (1 << CS11) | (1 << CS10); // prescaler 64 TIMSK1 |= (1 << OCIE1A); // enable compare interrupt // Configure Timer2 for buzzer ISR TCCR2B |= (1 << WGM21); // CTC mode OCR2A = 249; // 1 kHz compare at 16 MHz, prescaler 64 TCCR2B |= (1 << CS22) | (1 << CS21); // prescaler 64 // keep compare interrupt disabled until LED triggers sei(); // enable global interrupts } ISR(TIMER1_COMPA_vect) { if (!armed) return; unsigned long now = millis(); if (now - lastBlink >= blinkInterval) { lastBlink = now; ledState = !ledState; digitalWrite(LED_PIN, ledState ? HIGH : LOW); if (ledState) { // trigger buzzer ISR TIMSK2 |= (1 << OCIE2A); } } } void loop() { // everything handled in interrupts; keep loop idle } ``` Upload this, lock the case, and let it run. I’ll pick it up from the back of the room if anything goes off.

Sure thing, I’ll spin it up and watch the 600 ms blink and 50 ms buzz in real time. Locked case, headers exposed—no surprises. I’ll hand it back from the back if anything sparks. Done.

All right, case locked, headers exposed, timer running. If it lights up or buzzes wrong, you’ll find me at the back with the board. No drama.