Made the circuit shown in Figure 6, the metering device can be measured in reading, writing, the desktop, the light intensity measured point to check whether they meet the eye care needs. On primary and secondary school learning to read is particularly applicable.
Silicon photovoltaic cells exposed to light under the BP produce the corresponding voltage, when a certain value, respectively the transistor VT1, VT2 adequate base forward bias and conduction, so that the light-emitting diode VD2, VD3 corresponding light. VD2 first light, light to strong light only when the VD3.
As the ordinary monolithic silicon photovoltaic cells produce low voltage, typically less than 0.5V is not enough to make silicon transistors VT1, VT2 turn the base, so by the R1, R2, R3, RP1 and VD1 points form a simple voltage regulator pressure devices, and silicon photovoltaic cells generate a voltage in series BP. Regulation RP1, so that the light at the appropriate (for example, 100LX left) VT1 conduction, VD2 light. And then increase the light intensity and adjust the value of R6, so that light can VD3.
Use, the metering device flat on the desktop, if the light intensity measured points is not enough, then no light VD2 and VD3; If the light intensity measured point to meet the requirements, then VD2 VD3 light and no light (or very weakly light); when the light intensity is too strong, then the VD2 and VD3 were light.