PURPOSE: To obtain a detecting circuit of a liquid drop or the like without electrolytic corrosion by connecting electrodes A, B to a common load through diodes having the same polarity, comparing the applied voltage with a reference voltage and inverting the polarity of the impressed voltage in accordance with the difference of both the voltages.

CONSTITUTION: The electrodes A1, B1 are connected to terminals A, B respectively, and when no liquid drop is detected and the electrodes A1, B1 are opened, the inverted output Q' of an FF is turned to +V voltage and current is made flow through loads such as a resistor R2, a diode DB and a resistor R3. The voltage of a point C is R3/R2+R3×V, and if the voltage is set up larger than the reference voltage D, the output F of a Schmitt circuit 1 is turned to "0". When a liquid drop contacts with the electrodes A1, B1 and conductive state is formed, the voltage of the point C is reduced lower than the reference voltage D, so that the output F of the Schmitt circuit 1 is turned to "1". After the liquid drop has been passed, the voltages of the points C and F are restored and a voltage pulse output is obtained from the point F. The voltage of the electrode A1 continues to hold "1" and is turned to "0" when the succeeding liquid drops passes.