resistor value for LDR help

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dazzer7

Joined May 7, 2016
26
If any one can help me am doing a small project to make a night light sensor with the parts 5v power supply - LED - bc547 transistor - LDR Photoresistor , i am trying to work out what resistor value to use with the LDR , I found a datasheet online but i am still not sure what to look for they seems to be lots of different values , the transistor is from a Arduino kit so i found a datasheet online i thinks it the right one.. 20211213_124142.jpg
 

BobaMosfet

Joined Jul 1, 2009
1,979
@dazzer7
Can you post a couple of clear/clean shots of the LDR so we can make sure it matches? We may have a better datasheet. This looks like a VT90N1, which is a Perkin Elmer LDR- (going by the datasheet you already found). Here is the actual datasheet if it is what you have.

1639412426399.png
 

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k1ng 1337

Joined Sep 11, 2020
528
If I'm not mistaken, the value chosen is dependent on what amount of ambient light you want to turn the LED to turn on. R1 and LDR are forming a voltage divider where LDR is acting as a variable resistor. You can swap R1 and/or LDR for a trimpot of the same value range then use a test light (vs total dark) and measure voltages. When the base of the transitor sees 0.7v it will turn on. This is one way that I did it and have used your exact circuit. I suggest using an op amp as you gain more control.
 
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Dodgydave

Joined Jun 22, 2012
10,297
The problem with this circuit is as soon as the ldr gets 0.7V across it the transistor starts to conduct and the led will be dim the slowly get brighter as light fades.
You're better using an op amp comparator like the LM397..
 

sparky 1

Joined Nov 3, 2018
716
Measure the resistance when it gets dark.
Find a fixed resistor with that resistance and build a voltage divider. (and circuit)
When it works replace the fixed resistor with the LDR.
 

k1ng 1337

Joined Sep 11, 2020
528
The problem with this circuit is as soon as the ldr gets 0.7V across it the transistor starts to conduct and the led will be dim the slowly get brighter as light fades.
You're better using an op amp comparator like the LM397
The problem with this circuit is as soon as the ldr gets 0.7V across it the transistor starts to conduct and the led will be dim the slowly get brighter as light fades.
You're better using an op amp comparator like the LM397..
My circuit exhibited this behavior in daylight which I thought was pretty cool as I could then see the IV curve of the transitor as daylight faded. Circuit works fine when going from indoor lights to darkness when the light switch is turned on or off. Of course a comparator allows for either full on or full off over the light intensity range where a definite voltage can be selected. I suggest trying both. TS's circuit was the first I ever built and I was mighty impressed at my own prowess and the miracles of technology!
 
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