Two tone alarm generators of the frequency modulated variety are also quite successful.
Here, the output tone's frequency is changed in a certain way. For example, by using a square wave modulating signal, the tone may be changed between two tones at a rate equal to the frequency of the modulation oscillator.
The tone of the output signal changes linearly up and down with the modulation oscillator's frequency of operation whenever a triangle modulation signal is employed.
A weak connection between the modulation astable's output and the tone generator astable's input allows the modulation oscillator's output to switch the operating frequency of the latter up and down as it changes from one logic state to another.
The tone generator fails to operate at its normal frequency and instead switches between about 500 Hertz and 1.2 kHz.