Concentration meter circuit

The photoresistor concentration meter is used to detect the light energy passing through the substance to
understand the composition or properties of the substance. First, the transmittance intensity of the standard solution must be measured, so that
the concentration of the solution to be measured is known. The circuit is shown in Figure 5-35.
    This kind of concentration meter requires monochromatic light, so the light-emitting diode VI is used. as
a light source.
    VT1, VT2, etc. form a differential circuit . When the LED does not light up and there is no light source irradiating the photoresistors
RL1 and RL2, adjust the potentiometer RP so that the mA
meter indicates 0, then the zero calibration is completed. At this time, turn on the power of the light-emitting diode smart LED . Through the light source's illumination of the standard solution and the liquid to be measured, the concentration of the liquid to be measured can be indicated
on the milliamp meter .     Divide milliamperes into concentration grids according to relevant standards, so that the concentration value can be easily read.