Building Color Sensor from Reversed LED and Op-Amp

The project illustrates how LEDs provide current flow from light when put in backward position.

The output of the photodiode needs to be amplified for the light-detection signal to be useful in most circuits since photodiodes and reversed LEDs don’t provide a lot of current flow even under the best conditions. A phototransistor is a photodiode amplified by a built-in transistor. A better method for amplifying low-power signals in a high-quality repeatable way is an op-amp chip.

The diode arrow of an LED is normally pointed down toward the ground because conventional current flows that way. But this reversed LED points up wherein the more light that hits it, the more current will flow. The weak signal is being taken by the op-amp from the reversed LED, amplifies it, and sends it out the output pin. In order for the chip to work with very little input current, IC1 must be an ultra-low input current op-amp. This is good because the reversed LED can only produce a little current.

A teeny tiny bit of the op-amp output to feedback is allowed into the input signal by the extremely high-resistance resistor.

Rest of the project