This is a circuit of Low-Light Level Drop Detector. This circuit utilize self-biasing configuration to detect small changes in light level. This circuit usually used in monitoring very low droplets rates. The collector of the transistor gives feedback that is used to stabilize the bias of the photodarlington. The effect on optical performance is reduced by a high base-source impedance that is produced by The 10-μF capacitor and 10-MΩ resistor. Here is the schematic diagram of the circuit:
When there is a liquid drop is detected by the detector, the light that reach the chip is momentary decreased causing collector voltage to momentarily rise, then output signal is generated. Due to mechanical spacing system constraints and output power constraints on the LED, the initial light bias is small. A fraction of this initial bias is the change in light level. This fraction is caused by stray light paths and drop translucence.
When biased, the output signals level is still acceptable because the photodarlington has high sensitivity. This compares with unacceptable bias point stability and signal levels when biased conventionally, like signal output across the collector bias resistor and base open. [Circuit’s schematic diagram source: seekic.com]