Using a Light Dependent Resistor to Control Voltage

Question 1

what is an LDR

Answer 1

• light dependent resistor
• it is a resistor that is sensitive to light
• in which its resistance depends on the intensity of it

Question 2

how does the resistance of an LDR change as the light intensity changes

Answer 2

• as the level of light increases the resistance falls

- if the light intensity decreases the resistance increases

Question 3

what is an LDR made from

Answer 3

• a semiconductor with a large resistance

- such as cadmium sulfide

Question 4

why does that material have a very high resistance

Answer 4

• because there are very few electrons that are free and able to conduct
• as the vast majority of them are locked into the crystal lattice and unable to move

Question 5

what happens when light falls on the semiconductor part of the LDR

Answer 5

• the light photons are absorbed by the semiconductor lattice
• and their energy is transferred to the electrons

Question 6

what does this transfer of energy to the electrons cause

Answer 6

• it causes the electrons to break free from the crystal lattice as they have sufficient energy
• this means that they can then conduct electricity

Question 7

how would this affect the value of I in the nAvq equation

Answer 7

• n, the number of charge carriers per cubic meter, increases
• this means that I also increases as all the other variables dont change

Question 8

why does the increase in current of the LDR lead to a decrease in resistance

Answer 8

• as I = V / R and V is constant

- I increases by the resistance of it decreasing

Question 9

what does it mean if this process is progressive

Answer 9

• it means that as more light shines on the LDR
• more electrons will be released to conduct electricity
• which leads to the resistance falling further

Question 10

how can the resistance of an LDR vary from darkness to light

Answer 10

• it can be several M ohms in darkness

- but fall to only a few hundred ohms in bright light

Question 11

how can an LDR be used to control the potential divider circuit to control the output voltage

Answer 11

using the formula V out = V in x (R2 / R1 + R2)

• if the light incident (intensity) on the LDR falls, the resistance of it (R2) increases
• this leads to an increase in the V out

Question 12

how can an LDR circuit be used to control streetlight for example

Answer 12

• the principle of the previous circuit can be used to activate a switch to turn a light on when it gets dark
• and then turn it off when it gets bright
• depending on the amount of V the components are getting