Using a single supply quad comparator LM339, LM239, LM2901, LM2901V, NCV2901, or MC3302, we can build a square-wave oscillator with simple circuit and formula. Here is the schematic diagram:
This circuit is suitable if you need many square-wave oscillators in one circuit, since the quad packaged comparator provide four comparators in one single integrated chip. [circuit's schematic diagram source: Motorola Application Notes] 
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