Use a USB-Serial Converter like the FT232RL for activating a buzzer.
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README.md

FTDI_simple_alarm

Use a USB-Serial Converter like the FT232RL for activating a buzzer. Simple and cheap (~1,75€)

Features:

  • works on GNU/Linux only
  • no drivers or other software/libs are required
  • triggerd via bash or service daemon

Limitations:

  • works safe on system with only one /dev/ttyUSB device
  • no other task can use the FTDI device for other things

Hardware:

  1. FT232RL Breakout board ~1,5€
  2. NPN Transistor like 2N2222A ~0,0€
  3. Active Buzzer 5V ~0,25€

Schematics:

FTDI EEPROM

To invert the TXD pin on the FTDI device, programming the EEPROM is needed. With the Windoofs tool "FTDI_Prog" the Template ft232rl_alert.xml is uplouded. For GNU/Linux is ftx-prog available. The only difference to default is the inverted TXD pin.

Software:

Commmon Distributions

Make sure the device is after plug in recognized as ttyUSB with dmesg.
Edit the alert.sh with your device name, like /dev/ttyUSB0.
Run bash ./alert.sh for starting the alarm.

openWrt

opkg update
opkg install kmod-usb-serial
opkg install kmod-usb-serial-ftdi

Make shure the device is recocntied as ttyUSB after plug in with dmesg.
Edit the alarm.sh with your device name, like /dev/ttyUSB0.
Run bash ./alert.sh for starting the alarm.

Service

nano /etc/init.d/alarm

#!/bin/sh /etc/rc.common

START=99

restart() {
bash /root/alert.sh > /dev/null 2>&1 &
bash /root/alert.sh start > /dev/null 2>&1 &    
}

start() {
bash /root/alert.sh > /dev/null 2>&1 &
bash /root/alert.sh start > /dev/null 2>&1 &
}

stop() {
bash /root/alert.sh > /dev/null 2>&1 &
}

chmod +x /etc/init.d/alarm

/etc/init.d/alarm start

/etc/init.d/alarm stop