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 sleep.
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.
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.
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