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The circuit above generates a square-wave output with a control for varying the output frequency.
Square-wave Relaxation Oscillators
By Masaru Kubota
Page created on 9/22/2002

Disclaimer:  I am not responsible for any damages or injuries incurred through the use of the schematics on this page.
The circuit above converts the 60-hz sine-wave hum from your house wiring into a 60-hz square-wave signal that you can listen to with headphones.
This circuit is similar to the Generic Relaxation Oscillator, except for the suggested value of the oscillator capacitor and the configuration of the headphones.  With the headphones configured as shown, both the left and right headphone drivers/speakers are in series.  The volume output of the speakers in series is louder than when compared to the speakers in parallel and the output from one speaker is more in-phase with the other.