Many people who live
close to the ocean have the benefit of being lulled to sleep by the
sound of the surf. This circuit may provide a similar benefit to all
those poor unfortunates who don’t live near the seaside but who do have
the small consolation that they don’t have to worry about rust and
corrosion in a salty atmosphere. The circuit consists of four
unsynchronised oscillators which are mixed together to modulate a white
noise source to simulate the more or less random nature of surf sounds.
You won’t hear the waves crashing but the ebb and flow of the white
noise will help mask other noises which would otherwise disturb your
The four oscillators are based on four op amps in a TL074 or TL084
quad op amp package (IC1). IC1a, IC1b, IC1c & IC1d are configured as
Schmitt trigger oscillators with their operating frequencies defined by
the resistor connected between their outputs (pins 1, 7, 8 & 14)
and the respective inverting inputs (pins 2, 6, 9 & 13), as well as
the electrolytic capacitors connected between these latter pins and 0V.
The result is a triangle waveform at each of the respective inverting
inputs and square waves at the same frequencies at the op amp outputs.
We don’t use the square outputs but instead feed the four triangle
waveforms to op amp IC2a which is connected as a mixer. Its output is
used to drive and modulate a noise source based on NPN
transistor Q1. This is operated with reverse bias across its
base-emitter junction and the controlled reverse current is very noisy.
By varying the amount of reverse bias, we vary the amount of white
noise produced. Since the amount of noise produced by the transistor
varies markedly between types, the gain of IC2a can be varied over a
wide range to produce the optimum output voltage to drive Q1. From
there, the noise signal from the emitter of Q1 is fed via a 47nF
capacitor to op amp IC2b which can also have its gain varied over a wide
range to drive IC3, an LM386 power amplifier which drives the
loudspeaker. In use, first adjust trimpot VR2 to set the volume level
from the loudspeaker, then adjust trimpot VR1 to get the best range of
white noise which simulates the surf sounds. Sleep well.