Diode-BJT Inverting Zero Crossing Detector

The circuit is an inverting Zero Crossing Detector using diodes and BJT. Diodes D1 and D2 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 D3 protects the base emitter junction of transistor Q1 from reverse breakdown when the input signal goes negative. The use of diodes D1 and D2 as level shifters allows changing Rb without any further adjustment in level shifting circuit but mismatches in voltage drops across the diodes and Vbe of the transistor will make the switching threshold deviate from zero and no component is in the circuit to compensate for this.