Whistle Responder

The Whistle Responder beeps when it hears your whistle and this amazing invention is priced at a meager Rs 350/-. Some 20 years ago it was common to see small key-holders emitting an intermittent beep for a couple of seconds after its owner whistled. These devices contained a special purpose IC and therefore were not suited to home construction. The present circuit is designed around a general purpose hex-inverter CMos IC and, using miniature components and 9V batteries can be enclosed in a box. It is primarily a gadget, but everyone will be able to find suitable applications.
About the circuit :
This device beeps intermittently for about two seconds when a person in a range of around 5 meters emits a whistle. The first two inverters contained in IC1 are used as audio amplifiers.IC1'A' amplifies consistently the signal picked-up by the small electret-microphone and IC1'B' acts as a band-pass filter, its frequency being centered at about 1.8KHz. The filter is required in order to select a specific frequency, the whistle's one, stopping other frequencies that would cause undesired beeper operation. IC1'C' is wired as a Schmitt trigger, squaring the incoming audio signal. IC1'D' is a 2 second-delay monostable driving the astable formed by IC1'E' & IC1'F'. This oscillator generates a 3 to 5Hz square wave feeding Q1 and Buzzer (BZ), thus providing intermittent beeper operation.