The time base circuit 555 forms an extremely low frequency oscillator that outputs a short pulse that causes the speaker to emit a sound similar to a raindrop (see figure). The speaker is a 2-inch, 8-ohm small moving coil. The speed of the raindrops can be adjusted to the appropriate level by a 100K potentiometer. If a simple time switch is added to the power supply terminal, the power can be cut off in time after the user enters the dream zone.
The 555 timer is mainly composed of a resistor and a capacitor to form a charge and discharge circuit, and two comparators are used to detect the voltage on the capacitor to determine the level of the output level and the on/off of the discharge switch. This makes it convenient to construct a delay circuit from microseconds to tens of minutes, which can conveniently constitute a monostable flip-flop, a multivibrator, a Schmitt trigger, or the like, or a waveform transform circuit.
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