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LM358 Single Rail OpAmp Square Wave Oscillator

Slightly obsessed with the idea of a square wave oscillator as a timer, I have found many circuits which use an OpAmp to generate a square wave. Having tried a few, I settled on the LM358 datasheet version (p. 17, Figure 27) as follows.

There's a nice explanation of the theory and calculations for this type of circuit found on this site. Rather than the values for R1 etc,. shown in the datasheet, however, I used the following slight variation.

The changes were made mainly so that I could vary the oscillations (R4), and also I added an LED because if it's not blinking then it's not really electronics...

Here it is "in the flesh", and the video contains the all important blinking stuff.