Making a photometer with light measuring function

Most LEDs have the function of reverse photodiodes. That is, when a reverse bias is applied to the LED, it will produce a small photovoltaic output as the light level increases. Because the shell of the LED is colored and transparent, it is particularly suitable for this function.

This allows the construction of a simple circuit to estimate the state of the ambient light, light guests of different colors, and to obtain effective optical filtering for a certain light color, with typical narrow-bandpass characteristics.

Although the example described here does not achieve the design accuracy of laboratory-grade photodetectors and transimpedance amplifiers, it can produce useful results.

Using three LEDs, experiments will indicate which device has the best sensitivity to which color, see the attached figure. When ambient light strikes these LEDs, current flows, with the current through each LED being 10 to 100 nA (depending on the intensity of the applied lighting). This current flows through the base of transistor Q1 and is amplified.

The collector current of Q1 flows between potentiometer R4 and the base of Q2, R4 is used for gain correction of the first stage. Q2 provides further amplification and drives a bridge circuit DIA and DIB diodes on the left. Note that R2/D1 and R3/D2 form a bridge. The collector current of Q2 provides a slight imbalance to this bridge, which is measured by Meter M. R5 is used to adjust the sensitivity of Meter M. R4 and R5 are set so that Meter M has a suitable deflection. R4 is useful for selecting the quiescent operating point, and R5 is useful for adjusting the sensitivity.

Before making this circuit, you should check whether these LEDs are good LEDs. You can use an ordinary digital multimeter to check the voltage across the two ends of the LED, which is about 200mV.