One of the simplest square wave generator circuits, with only 4 resistors, 2 capacitors and 2 npn transistors. Can generate frequencies up to 800 KHz, maybe more.
The one shown has frequency of about 1.5 KHz. The formula is as follows
f = 1/(1.38 RC) assuming both capacitors are equal and the series resistors (ones in the mid) are equal.
