tone alarm by NE556 Timer
The 556 consists of a pair of 555 timers in one package. The two timers work independently and only share common power supply connections.
A typical example of a 556 circuit is shown here. The circuit drives a piezo sounder to provide a pulsed bleeping sound (similar to the reversing warning on many vehicles). One half of the 556 is running at the resonant frequency of the piezo sounder, around 3-4KHz, while the other half is running at a very low frequency of about 0.5Hz. The output from the low frequency oscillator is connected to the reset pin of the high frequency oscillator to modulate the output.
