Can a led be used as a photodetector?
The same LED may be used as a photodetector by disabling the digital output driving it and sensing its photocurrent output. If the microcontroller has dual-purpose analog input/digital I/O pins, like the ADuC7023 or the Atmel ATMega controllers used in Arduinos, this can be done with an LED and two resistors—and just one processor pin.
Can a photo detector be connected to more than one transistor?
More than one phototransitor can be connected to a single voltage comparator. This would allow transistors to be placed along a section of track to indicate when a train is anywhere in that section. As long as the train is long enough to cover two sensors the circuit will continuously detect the train.
How to make a led as a light sensor?
On your solder-less breadboard construct the LED light sensor circuit as shown in figure 1. Notice that the LED diode, D 1, is reverse biased i.e. opposite to how it would be connected as a light emitter.
Why are LEDs used as photocells in a circuit?
This makes mass-produced circuits using LEDs as photocells demanding, but they are very useful in small batch or single system designs. An elegant application is where an LED is driven by an analog microcontroller.
What happens when light shines on a photodiode?
In photoconductive mode, light shining on a reverse-biased photodiode causes a photocurrent proportional to the incident light to flow. It is best to use photoconductive mode for ac signals as the frequency response is better, but light measurement in photovoltaic mode is very simple, as shown in Photodiodes and Other Light Sensors.
Why do white LEDs not make good photocells?
White LEDs contain a phosphor to convert monochrome light to white light and do not make good photocells. Manufacturers do not characterize LEDs as photocells, so minor design changes that have minimal effect on their behavior as LEDs may cause major changes in their characteristics as photocells.