Whistle Responder

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 button clock-type batteries can be enclosed in a matchbox. It is primarily a gadget, but everyone will be able to find suitable applications.

Circuit operation:

This device beeps intermittently for about two seconds when a person in a range of around 10 meters emits a whistle.
The first two inverters contained in IC1 are used as audio amplifiers. IC1A amplifies consistently the signal picked-up by the small electret-microphone and IC1B 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. IC1C is wired as a Schmitt trigger, squaring the incoming audio signal. IC1D is a 2 second-delay monostable driving the astable formed by IC1E & IC1F. This oscillator generates a 3 to 5Hz square wave feeding Q1 and BZ1, thus providing intermittent beeper operation.

Notes:
Power supply range: 2.6 to 3.6 Volts.
Standing current: 150µA.
Depending on dimensions of your box, you can choose from a wide variety of battery types:
2 x 1.5 V batteries type: AA, AAA, AAAA, button clock-type, photo-camera type & others.
2 x 1.4 V mercury batteries, button clock-type.
1 x 3 V or 1 x 3.6 V Lithium cells.