A liquid level detecting system applied in the completely automatic biochemical analyzer is introduced.This system uses double-layer needle-plate capacitor as the liquid level detecting probe.Detecting signal generated by double-layer needle go to hysteresis voltage comparator to form square wave.The square wave is used as the pulse source of the single-chip counter.The master controller is LPC916.In the program

through comparing the differential value of pulse number with fiducial value in the same time to judge whether the probe head contacting the liquid level or not.The system eliminates the effect of external interference signal on the measuring result by using the capacitance matching in the hardware and the self-adaptive algorithm in the software.This system has already applied in the completely automatic biochemical analyzer.The experimental result is correct

which proves the usefulness and stability of this detecting system.