# What is the operation principle of the LDR?

## Homework Statement

Construct suitable circuit and record your finding to illustarte the operational principle of the sensors: sound, light

Results:
Distance from light source (cm) - light intensity (lx)
0 - 1000
10 - 400
20 - 260
30 - 200
40 - 190
50- 180
experiment involved moving a datalogger away from a lamp, measuring the light intesity the further we got away.
Attached constructed circuit.

## The Attempt at a Solution

An LDR resistance changes with light. As light falls on the LDR it causes the electrons within the LDR to break free of their lattice allowing them to conduct electricity, therefore decreasing the overall resistance of the LDR. When no light falls on the LDR there are no/few free electrons within the LDR which can conduct electricity there fore there is a very high resistance
A voltage of V is passed through the circuit. As light falls on the LDR at 0cm away the light intensity is very high 1000lx therefore the resistance of the LDR will be low. A current of I is displayed on the ammeter when light intensity is 1000lx because the current through the LDR is less impeded. At 50cm away light intensity on the LDR is 180lx therefore the resistance of the LDR will be higher and the current will be impeded more therefore a lower current will be shown on the ammeter.
I am not sure if the circuit I have constructed is suitable as the other circuits I have found are very complicated.

#### Attachments

• LDR.png
3.8 KB · Views: 454

phinds
Gold Member
2021 Award
A voltage of V is passed through the circuit.
Just to be clear on terminology, because poor use of terminology usually either results from or even leads to, confusion --- you don't "pass a volage through a circuit". You apply a voltage in a circuit and this causes current to pass through the circuit
I am not sure if the circuit I have constructed is suitable as the other circuits I have found are very complicated.
Seems reasonable to me, as long as you can vary the distance from bulb to LDR. I've never worked w/ LDRs so it's possible I'm missing something. For example, for all I know, the swing in resistance could be quite small so that you need to amplify the change in current through the LDR in order to get a meaningful reading. One simple way to do this would be to put another resistor in series with the LDR and amplify the voltage at the LDR. That could be one explanation for why you see more complicate circuits.

Just to be clear on terminology, because poor use of terminology usually either results from or even leads to, confusion --- you don't "pass a volage through a circuit". You apply a voltage in a circuit and this causes current to pass through the circuit
Thanks for correcting me.
Seems reasonable to me, as long as you can vary the distance from bulb to LDR
I measured light using a datalogger by moving it back 10cm each time, recording the light intenity.

phinds
Gold Member
2021 Award
Thanks for correcting me.

I measured light using a datalogger by moving it back 10cm each time, recording the light intenity.
Light intensity? How did you measure that? Don't you mean you measured the current through the LDR?

Light intensity? How did you measure that?
TBH I have no idea. like I said before, I moved a datalogger away from the light and at every 10cm I recorded the lx which was shown on the screen.
Don't you mean you measured the current through the LDR?
Yeah, thats sounds right to me

phinds
Gold Member
2021 Award
TBH I have no idea. like I said before, I moved a datalogger away from the light and at every 10cm I recorded the lx which was shown on the screen.
WHAT screen???

Yeah, thats sounds right to me
??? Sounds right? Did you measure a reading on the ammeter or not? Does IT have a screen and that's what you're talking about? Ammeters usually have meter faces, not screens so I'm confused.[/quote][/QUOTE]

??? Sounds right? Did you measure a reading on the ammeter or not? Does IT have a screen and that's what you're talking about? Ammeters usually have meter faces, not screens so I'm confused
I used a datalogger with digital screen (someting like the attached image). I didn't use an ammeter at all. There was an LDR built into the datalogger which I moved away from the lamp. I could record multiple things with the same data logger such as temperature and sound.

#### Attachments

• digital-temperature-data-logger-500x500.jpg
36.5 KB · Views: 438
phinds
Gold Member
2021 Award
AH HA. Well, that explains my confusion. So you are telling me that the circuit you drew has nothing to do with your readings (and you have no idea what the circuit inside you meter thingy actually is). I just naturally assumed that it did.

AH HA. Well, that explains my confusion. So you are telling me that the circuit you drew has nothing to do with your readings (and you have no idea what the circuit inside you meter thingy actually is). I just naturally assumed that it did.
ahaha yeah I was going to add that I didn't measure light use that circuit, sorry! No I don't I just know I measured the light that fell on the LDR with the measurment in lx.
From wiki I got: The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area.[1] It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface.
However, could I use that circuit to explain how a light sensor works?

phinds
Gold Member
2021 Award
However, could I use that circuit to explain how a light sensor works?
Yes, but again, take a look at the caveat I put in w/ post #2

Daniel2244
Yes, but again, take a look at the caveat I put in w/ post #2
Using correct terminology?

phinds
Gold Member
2021 Award
Using correct terminology?
No (well, yes, but ...) using an amplifier to get a better signal. Your circuit is fine but you might want to include the caveat that if the range of resistive change is small then an amplifier might be needed OR an unusually sensitive ammeter.

Daniel2244
No (well, yes, but ...) using an amplifier to get a better signal. Your circuit is fine but you might want to include the caveat that if the range of resistive change is small then an amplifier might be needed OR an unusually sensitive ammeter.
Ok, you've been a great help :)

phinds
Gold Member
2021 Award
Ok, you've been a great help :)
Thanks. That's what PF is all about.

Daniel2244
Merlin3189
Homework Helper
Gold Member
My Yashica YEM55 exposure meter = Button cell + CdS LDR + resistors + moving coil meter + switch
So not much more than OP's drawing. Not calibrated in Lux, but I expect it could be.

I think OP is right in creating schematics to show the principle of operation of the sensor. If amplification or other processing is needed, as phinds suggests, I think it would be fine to use blocks labelled, amplifier, rectifier, voltmeter, current meter, integrator, or whatever. If you're really going for the brown nose points, you could add modifiers like, voltage amplifier, variable resistor for calibration, 60Hz blocking filter, or whatever bells and whistles you understand.

If you fancy giving a circuit like your datalogger, have a look at the Arduino sites and find the circuits they use with different sensors. They often boil down to such simple circuits, with the meter replaced by a microcontroller. It wouldn't surprise me if your datalogger were just one of these tarted up a bit in the display SW.

CWatters and Daniel2244