Build Your Own 1.8KHz Whistle Responder Circuit

Description

Approximately twenty years ago, it was a common sight to observe small key-holders producing a brief, sporadic beep following a person's whistling. These devices relied on a specialized integrated circuit and therefore were unsuitable for DIY construction projects. The present circuit is designed around a general-purpose hex-inverter CMOS IC and, utilizing miniature components and button clock-type batteries, can be enclosed within a matchbox. It is primarily a novelty item, but anyone can likely discover suitable applications for it.

4049 Hex Inverter IC

The 4049 is a versatile CMOS inverter IC that provides six independent inverting input stages. Its low power consumption and high input impedance make it well-suited for a wide range of digital logic circuits. This particular IC is frequently utilized in educational and hobbyist projects due to its simplicity and readily available documentation. The IC operates reliably over a voltage range of 2.6 to 3.6V, offering flexibility in power supply options.

Circuit operation:

This device emits a beep intermittently for approximately two seconds when an individual within a radius of around 10 meters produces a whistle. The initial two inverters contained within IC1 are employed as audio amplifiers. IC1A consistently amplifies the signal captured by the small electret microphone, and IC1B functions as a band-pass filter, its center frequency centered around 1.8KHz. This filter is necessary to select a specific frequency, that of the whistle, while suppressing other frequencies that could trigger unintended beeping operation. IC1C is configured as a Schmitt trigger, squaring the incoming audio signal. IC1D serves as a 2-second-delay monostable, driving the astable formed by IC1E & IC1F. This oscillator generates a 3 to 5Hz square wave, which is fed to Q1 and BZ1, thereby providing intermittent beeper operation.

Circuit diagram:

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