Audio Alarm

Updated: October 23, 2025

This is a simple circuit generating audio tone and injecting it into the intercom.

For now this circuit is only connected to the stall warning. At some point I realized that sometimes I cant hear the stall warning buzzer I have in the cockpit. So I thought it would be a good idea to push a distinctive audio signal into intercom.

The unit allows for up to 3 analogue inputs. Not sure that other two are going to be but stall warning is easy to hear now and it sounds pretty cool.

The alarm is a simple board built on an ATTINY85 chip programmed to generate a BEEP-BEEP sound when tone of the inputs is “grounded” by the stall warning vain switch in the left wing of our RV-12.

CPU frequency: 8 MHz
Standby current: 13.5 mA
Current while generating the sound: 22.5 mA

Parts List

Assembly Instructions

Software

Schematics

Schematic reference	Value
C1, C2, C3, C4, C5, C6	Ceramic, SMD 0805, 100nF, 50v
C7	Ceramic, SMD 0805, 20nF, 50v
C20	Electrolytic Capacitor, 22uF, 50v
D1	SMD 0805, Red LED
R3, R6, R9	SMD 0805, 10K
R1, R2, R4, R5, R7, R8	SMD 0805, 3.3K
R40	Trim pot, 3296W, 10K
R10	SMD 0805, 0R
R11	SMD 0805, 100R
R12	SMD 0805, 1K
U1	ATTINY85
U4	Voltage reg., UA78M05IDCY

// ATtiny85 pins: 0, 1, 2, 3, 4
// We'll use pin 0 for output

const int pulsePin = 0; // Output pin
const int ledPin = 1; // Output pin
const int SensePin1 = A1; // stall warning pin
unsigned int ADCread = 0;


void setup() {
  pinMode(pulsePin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  pinMode(SensePin1, INPUT);
  digitalWrite(ledPin, LOW);   
}

void loop() {
ADCread = analogRead(SensePin1);
if (ADCread < 50) {
  digitalWrite(ledPin, HIGH);
  tone(pulsePin, 800);   // Start 800Hz square wave
  delay(80);             // Keep pulses for 80ms
  noTone(pulsePin);      // Stop pulses
  delay(80);             // Wait 80ms before repeating
} else {
  digitalWrite(ledPin, LOW);
}
}

Programming Attiny85 is a little tricky. It is not like uploading a sketch into Arduino Uno or Nano.

There are a few different methods available.

Here is the one I use:

You will require two things (prerequisites):

Arduino Uno or Nano with a breadboard to set up a bootloader on Attiny chip.
Digispark Development board with a DIP socket for 8-pin Attiny85