This is a special circuit with very interesting effects, which can be added to electronic games, warning systems, alarms and other applications. In the figure we have an oscillator with variable frequency triggered by a trigger that, in this case, is a pressure switch of the Normally Open type. When you press S1, capacitor C1 charges and powers the relaxation oscillator via the frequency adjustment pot. The tone produced by this oscillator then has its maximum frequency, which excites the power stage with the Power-MOSFET. When the switch is released, capacitor C1 is discharged by the relaxation oscillator and, as this occurs, the frequency decreases until it stops. Then we have the production of a tone with decreasing frequency until it stops. The duration of the tone is given by the value of the capacitor C1, and the frequency range of the sound generated is determined by the adjustment of the trimpot and the capacitor C2. The reader can modify the values of these components in a wide range to obtain the desired effect. The trigger can be replaced by the contacts of a relay, or even by some type of automatic activation, for example, a common transistor.
