Question

For what purpose LEDs are used in the circuit?
(A) LEDs are used to provide high feedback current.
(B) LEDs are used to give visual feedback from the circuit.
(C) LEDs are used to control the current.
(D) LEDs are used to control the voltage.

Answer 1

Hint- LEDs are p-n junction diodes that emit radiations under forward bias. Energy is released during these radiations. The energy is released in the form of photons. We then utilise this energy in the form of light.

Complete Solution
Light Emitting Diode (LED) is a heavily doped p-n junction which under forward bias emits radiation. When the diode is forward biased, electrons are sent from n->p and holes (positive vacancies ) are sent from p->n. At the junction boundary, the concentration of minority charge carriers increases as compared to the equilibrium concentration i.e. when there is no bias.

At the junction boundary, on either side, excess minority charge carriers are present, which recombine with majority charge carriers near the junction. On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted.

When the forward biased current of the diode is small, the intensity of light emitted is small.

As the forward biased current increases, intensity of light also increases and reaches to a maximum value. However, any further increase in the forward biased current results in decrease of light intensity.

It is this light intensity (that is being emitted )that we measure and that gives us the visual feedback of the circuit.

Hence, option (B) is correct.

Note- There are many types of LEDs according to colour of light. The energy gap is different for different colours. Over the years, LEDs have been replacing the use of various light equipment.