This post is part of the Rack box project series.

Simple unit to control three sound signals; beep, siren and buzzer. Uses an AVR ATtiny2313 microcontroller.

Table of contents


This simple unit controls three sound signals; beep, siren and buzzer. Each sound signal will only be given once; when a new alarm is triggered. The input must remain clear for at least four seconds for that signal to rearm.

A beep is given every six minutes if an alarm is active, as a reminder. The sound LED flashes when a sound alarm is active; once for signal 1 (beep), twice for signal 2 (siren) and three times for signal 3 (buzzer), this flashing pattern is repeated.

This module used to control the sound alarms in the Rack box. Modules with alarm outputs sent a signal to this sound controller, which carried out the sound alarm. If system mute was active the power to the buzzer and siren was disconnected.

The sound alarm controller functionality was later implemented in the stack lights and horn controller, eliminating the need for this module.


  1. 100 ms siren
  2. One second siren
  3. Three seconds buzzer



  1. Signal: Beep
  2. Signal: Siren pulse
  3. Signal: Buzzer pulse


  1. Siren
  2. Buzzer
  3. Sound LED
  4. Life-signal to Module heartbeat monitor

D-Sub 9-pin

  1. 0v
  2. 5v
  3. Signal: Beep
  4. Signal: Siren pulse
  5. Signal: Buzzer pulse
  6. Siren
  7. Buzzer
  8. Sound LED
  9. Life-signal

Source code


Test fitting circuit board in enclosure
Circuit board complete, without microcontroller
Testing the sound alarm controller logic
Sound alarm controller installed in the rack box
Buzzer and siren on the rack box
Ceiling of the rack box, with the sound alarm controller and other modules

Schematic drawing

Schematics for the sound alarm controller

Parts list

  • 1 × AVR ATtiny2313-20PU, DIL-20, 20 MHz, 18 I/Os
  • 1 × Capacitor, aluminium electrolytic, 10 µF, 25V
  • 1 × Capacitor, ceramic, 1 nF, 100V
  • 1 × D-sub soldering cups, 9 pin male
  • 1 × DIL socket, 20-pin, 7.62mm
  • 1 × Enclosure, plastic (1591 FL), 100x50x25mm, flange
  • 1 × LED 5mm coloured clear, Green, 2.1V, 20mA, 30mcd, 10°
  • 1 × LED 5mm coloured clear, Yellow, 2.0V, 20mA, 40mcd, 10°
  • 2 × LED holder 5mm, RTC51, black plastic
  • 32 cm2 PCB, stripboard, 100x160mm, 160cm2
  • 5 × Resistor, carbon film, 0.25W, 330 Ω, 5%
  • 5 × Resistor, carbon film, 0.25W, 4.7 kΩ, 5%
  • 3 × Resistor, carbon film, 0.25W, 10 kΩ, 5%
  • 4 × Transistor, NPN, 100 mA, 45V, 0.5W, BC547B

Last commit 2023-02-05, with message: Add series for the rack box project.