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Whistle Responder

Source: RED Free Circuit Designs

Beeps when hears your whistle

A gadget suitable for key-holders, games etc.

Circuit diagram:

Whistle Responder


R1_____________22K   1/4W Resistor
R2_____________10K   1/4W Resistor
R3______________4M7  1/4W Resistor
R4,R8_________100K   1/4W Resistors
R5____________220R   1/4W Resistor
R6____________330K   1/4W Resistor
R7_____________47K   1/4W Resistor
R9______________2M2  1/4W Resistor
R10_____________1M5  1/4W Resistor

C1,C5__________47nF   63V Polyester or Ceramic Capacitors
C2,C3__________10nF   63V Polyester Capacitors
C4,C6___________1µF   63V Electrolytic Capacitors

D1,D2________1N4148  75V 150mA Diodes

IC1____________4049  Hex Inverter IC

Q1____________BC337  45V 800mA NPN Transistor

MIC1_________Miniature electret microphone

BZ1__________Piezo sounder (incorporating 3KHz oscillator)

B1___________2.8 or 3V Battery (see notes)
Device purpose:

Some 15 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 approx. two seconds when a person in a range of about 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's operation. IC1C is wired as a Schmitt trigger, squaring the incoming audio signal. IC1D is a 2 second (approx.) 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's operation.


  • 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.

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