Resistor-Diode-BJT Inverting Zero Crossing Detector

The circuit is an inverting Zero Crossing Detector using resistor, diodes, and BJT. Diode D1 and resistor Rin set the threshold of conduction of transistor Q1 at approximately Vin=0V (for more accuracy an op-amp zero crossing detector can be used). Diode D2 protects the base emitter junction of transistor Q1 from reverse breakdown when the input signal goes negative. Mismatches in voltage drops across the diodes and Vbe of the transistor will make the switching threshold deviate from zero, the variable resistor Rin can compensate for this but it should be readjusted when the value of Rb is changed.