simple photometer

Most light-emitting diodes have reverse photodiode functions. That is when a light emitting diode is reverse biased it will produce a small photovoltaic output as the light level increases. Because the housing of the light-emitting diode is colored and transparent, it is particularly suitable for performing this function.

This allows a simple circuit to be constructed to estimate the state of ambient light. Lighters of different colors can obtain light filters that are effective for a certain light color. It has 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 light-emitting diodes, experiments will indicate which device has the best sensitivity for which color, see attached figure. When ambient light strikes these LEDs causing current to flow, the current through each LED is 10 to 100 nA (depending on the intensity of the added illumination). 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 serves as gain correction for the first stage. Q2 provides further amplification and drives a bridge on the left, D1A and D1B diodes. Note that R2/D1 and R3/D2 form a bridge. The collector current of Q2 provides a slight imbalance to the bridge, and meter M measures this imbalance. R5 is used to adjust the sensitivity of meter M. Set R4 and R5 so that table M has a suitable deflection. R4 is useful for selecting the static operating point. R5 is useful for adjusting the sensitivity.

Before making this circuit, you should check whether these light-emitting diodes are good light-emitting diodes. You can use an ordinary digital multimeter to check the voltage across the light-emitting diodes, which is about 200mV.