This circuit with the 555, was obtained in a 90s American documentation. The signal must be applied to a good amplifier. The sound is similar to that of a police siren.
The S-meter is a signal strength meter used in communication receivers. This circuit was found in...
National Semiconductor suggests the circuit in the figure that uses two operational amplifiers of type...
This circuit was obtained from Linear Technology documentation from the 90s. The circuit uses...