Line frequency square-wave generator

With only three components and a buffer, a line frequency square wave having a 1:1 duty cycle may be derived from the power supply. During the alternate half-cycle, however, A is effectively clamped to - 0.7 V by Dl in the bridge which offsets the forward voltage across D2 giving an input to IC1 of approximately 0 V.
Line frequency square-wave generator  - schematic

When A rises above + 5 V, D2 is reverse biased and remains at +5 V. Rl is needed to load the transformer secondary maintaining a distortion-free waveform at A during the time the diode bridge is not conducting. Cl although not essential may be required to remove transients.

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