Photo diode

Photo diode is used to capture reflected light of IR LED.

A semiconductor diode that, when exposed to light, generates a potential difference or changes its electrical resistance.

A Photo diode is a reverse biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light.

When no light is incident on the pn junction of photo diode, the reverse current is extremely small. This is called DARK CURRENT.

When light is incident on the pn junction of the photo diode there is a transfer of energy from the incident light (photons) to the atoms in the junction .this will create more free electrons (and more holes) these additional free electrons will increase the reversecurrent.This electrical energy can be recorded as voltage drop fluctuations by using a series resistor in the outer circuit and taking voltage readings across it.

IR LED

An Infrared light-emitting diode (IR LED) is a type of electronic device that emits infrared light not visible to the naked eye.

The wavelength and colour of the light produced depend on the material used in the diode. Infrared LEDs use material that produces light in the infrared part of the spectrum, that is, just below what the human eye can see. Different infrared LEDs may produce

infrared light of differing wavelengths, just like different LEDs produce light of different colours.

Since the human eye cannot see the infrared radiations, it is not possible for a person to identify whether the IR LED is working or not, unlike a common LED. To overcome this problem, the camera on a cell phone can be used. The camera can show us the IR rays being emanated from the IR LED in a circuit.