# LDR's in GCSE physics

How do LDR's work exactly because as the light intensity increases so does the current, so can someone explain how an LDR would be useful in for example switching on lights when it gets dark because I thought a higher current would be needed to switch on the lights in the dark but resistance increases in the dark.
Original post by hinal29
How do LDR's work exactly because as the light intensity increases so does the current, so can someone explain how an LDR would be useful in for example switching on lights when it gets dark because I thought a higher current would be needed to switch on the lights in the dark but resistance increases in the dark.

http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/electricityintheory/voltagecurrentresistancerev6.shtml

UPDATE: Check my next reply, i've explained it quite well.
(edited 6 years ago)
Original post by hinal29
How do LDR's work exactly because as the light intensity increases so does the current, so can someone explain how an LDR would be useful in for example switching on lights when it gets dark because I thought a higher current would be needed to switch on the lights in the dark but resistance increases in the dark.

They are just used to detect change in light intensity. When the light intensity (going to refer to this as LI) increases, then the resistant decreases. This means that the current increases as the ions are no longer vibrating, allowing electrons to flow through. This would be useful as it can detect change in temperature, and this change can be used in automated systems to switch on the lights when it gets dark. =)
Original post by hinal29
How do LDR's work exactly because as the light intensity increases so does the current, so can someone explain how an LDR would be useful in for example switching on lights when it gets dark because I thought a higher current would be needed to switch on the lights in the dark but resistance increases in the dark.

An LDR is not used in actual circuits, in fact it is a resistor which is used inside a filament bulb. When it is dark the filament bulb should be on right?

Therefore the resistor has a high resistance because when it has a high resistance more current is trying to get through it and it gets to a point when there is so much trying to get through that the wire (LDR in the bulb/Filament) glows brightly. That light is the bulb. LDRs are not actually used to slow down cirucits but their behavioural traits are exploited in a way that its resistance causes a light. Hence low light:high resistance and high light:low resistance.
Original post by HZ.CEO
An LDR is not used in actual circuits, in fact it is a resistor which is used inside a filament bulb. When it is dark the filament bulb should be on right?

Therefore the resistor has a high resistance because when it has a high resistance more current is trying to get through it and it gets to a point when there is so much trying to get through that the wire (LDR in the bulb/Filament) glows brightly. That light is the bulb. LDRs are not actually used to slow down cirucits but their behavioural traits are exploited in a way that its resistance causes a light. Hence low light:high resistance and high light:low resistance.

you could expand by saying that they are actually sensors, along with thermistors =)
Original post by hinal29
How do LDR's work exactly because as the light intensity increases so does the current, so can someone explain how an LDR would be useful in for example switching on lights when it gets dark because I thought a higher current would be needed to switch on the lights in the dark but resistance increases in the dark.

A thing called a voltage divider can be used, as the resistance goes up in the dark and V = I . R the voltage across the LDR goes up. This voltage can then be used to switch on a transistor such as a MOSFET which if you are not familiar with is like an electrical switch. The transistor enables the small voltages and currents to modulate other higher power circuits.
Original post by Pretish
They are just used to detect change in light intensity. When the light intensity (going to refer to this as LI) increases, then the resistant decreases. This means that the current increases as the ions are no longer vibrating, allowing electrons to flow through. This would be useful as it can detect change in temperature, and this change can be used in automated systems to switch on the lights when it gets dark. =)

Thank you
Original post by HZ.CEO
An LDR is not used in actual circuits, in fact it is a resistor which is used inside a filament bulb. When it is dark the filament bulb should be on right?

Therefore the resistor has a high resistance because when it has a high resistance more current is trying to get through it and it gets to a point when there is so much trying to get through that the wire (LDR in the bulb/Filament) glows brightly. That light is the bulb. LDRs are not actually used to slow down cirucits but their behavioural traits are exploited in a way that its resistance causes a light. Hence low light:high resistance and high light:low resistance.

Thanks that has helped a lot
Original post by Casisalive
A thing called a voltage divider can be used, as the resistance goes up in the dark and V = I . R the voltage across the LDR goes up. This voltage can then be used to switch on a transistor such as a MOSFET which if you are not familiar with is like an electrical switch. The transistor enables the small voltages and currents to modulate other higher power circuits.

Thank you
completely wrong, the component that detects changes in temperature in electrical circuits is a thermistor.
Original post by HZ.CEO
An LDR is not used in actual circuits, in fact it is a resistor which is used inside a filament bulb. When it is dark the filament bulb should be on right?

Therefore the resistor has a high resistance because when it has a high resistance more current is trying to get through it and it gets to a point when there is so much trying to get through that the wire (LDR in the bulb/Filament) glows brightly. That light is the bulb. LDRs are not actually used to slow down cirucits but their behavioural traits are exploited in a way that its resistance causes a light. Hence low light:high resistance and high light:low resistance.

Thank you so much that has finally sorted my confusion thank you1
Original post by 3317752
They are just used to detect change in light intensity. When the light intensity (going to refer to this as LI) increases, then the resistant decreases. This means that the current increases as the ions are no longer vibrating, allowing electrons to flow through. This would be useful as it can detect change in temperature, and this change can be used in automated systems to switch on the lights when it gets dark. =)

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