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 whistles one, stopping other frequencies that would cause undesired beepers 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 beepers operation.
Parts: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)
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