Whistle Responder Schematic - Circuit Diagram


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.

Circuit diagram:

Whistle Responder Schematic - Circuit Diagram-Circuit operation


  • R1 = 22K 1/4W Resistor
  • R2 = 10K 1/4W Resistor
  • R3 = 4M7 1/4W Resistor
  • R4 = 100K 1/4W Resistors
  • R5 = 220R 1/4W Resistor
  • R6 = 330K 1/4W Resistor
  • R7 = 47K 1/4W Resistor
  • R8 = 100K 1/4W Resistors
  • R9 = 2M2 1/4W Resistor
  • R10 = 1M5 1/4W Resistor
  • C1 = 47nF 63V Polyester or Ceramic Capacitors
  • C2 = 10nF 63V Polyester Capacitors
  • C3 = 10nF 63V Polyester Capacitors
  • C4 = 1µF 63V Electrolytic Capacitors
  • C6 = 1µF 63V Electrolytic Capacitors
  • C5 = 47nF 63V Polyester or Ceramic Capacitors
  • D1 = 1N4148 75V 150mA Diodes
  • D2 = 1N4148 75V 150mA Diodes
  • Q1 = BC337 45V 800mA NPN Transistor
  • B1 = 2.8 or 3V Battery (see notes)
  • IC1 = 4049 Hex Inverter IC
  • BZ1 = Piezo sounder (incorporating 3KHz oscillator)
  • MIC1 = Miniature electret microphone


  • 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.