This circuit was obtained in an old documentation and is characterized by the high sensitivity thanks to the use of a FET. The circuit provides a switching signal which must be applied to a power stage. Never power this circuit with a transformerless supply. R1 is 1M, R2 of 4M7, R3 and R4 of 1k. Q1 may be BF245 and Q2 or BC548. C1 has 10 nF and the diodes are commonly used.
