light activated led help LDR

[jimitrader]

I have built this for an idea i am working on ..but it seems to work backwards? when i block the light in the LDR (darkness) it turns on the LED? it is supposed to come on when i shine a light on it and dark when no light? it seems backwards because it does nothing when i shine the light on it..but everywhere i look it shows same circuit? anyone have any thoughts how it could be reversed so it works like its supposed too?? or did i do something wrong? its in the breadboard so its possible that i made a mistake..





https://circuits-diy.com/light-activated-led-using-photoresistor-ldr/

[antonis]

Interchange LDR and 50k+1k..

P.S.1
Is LDR "isolated" from LEDs light..??

P.S2
Your photoresistor is supposed to work like a NTC thermistor so measure its resistance in dark and bright enviroment..

[jimitrader]

ok i changed out the ldr to a different manufacturer and its working as originally intended? so I am learning that there are different types LDR's? is this correct? when checking on MM there was a difference in readouts..so i changed it out and it works...hmmm what do you think Antonis? i looked online and its kind of confusing.

[GibsonGM]

Oh yes, LDRs will vary all over the place. Unless you got one from a manufacturer that provides a data sheet, it's hard to know what you're getting, and you often have to adjust your circuit to take into account the specific LDR you got. 

You can test the LDR...get it into as dark a place as you can and cover the 'lens' with your thumb, read its dark resistance. Then read resistance right up against a bright light.  Those are your 'dark' and 'light' resistances.

An even better way to do this is to put it in an isolation box...and old film canister works great for this. You can even read the R when it's dark, and then turn on an LED you wired in, which you'll use in the circuit, inside the film can to get light resistance!   This removes the unknowns for your circuit, and you can then adjust what you need to accordingly.

You will often find circuits calling for "At least 1 meg dark resistance", and this is what they mean by that...1 Megohm when in total darkness...

[antonis]

What Sir Mike said..

On your paricular circuit, LDR values between 500k & 10k (dependent on 50k pot setting) can turn BJT on hence activate LED..
So, it's always good to know the extremes of your particular device (full darkness - full brightness resistance) to properly set BJT bias configuration..

[Kipper4]

Link to the gl55xx series ldrs

https://www.kth.se/social/files/54ef17dbf27654753f437c56/GL5537.pdf

[duck_arse]

like those blokes said, put the old not-working ldr in and measure the base voltage, dark and light. then refit the workable ldr, and measure the base voltage when the led turns off.

[anotherjim]

We know LDR specs vary but common CdS are all pretty much all some Megohms in the dark and some Kohms in the light. Different specs only affect response range and linearity. It doesn't explain a reverse response, High R in light and Low R in the dark that the OP states. I can't answer that.

[antonis]

It doesn't explain a reverse response, High R in light and Low R in the dark that the OP states.

OP never stated that..
Just a "reverse" operation of the whole configuration which may as weel work for a p-n-p BJT..

@jimitrader: Are you sure about your particular transistor type..??

[anotherjim]

Huh?

..but it seems to work backwards? when i block the light in the LDR (darkness) it turns on the LED? it is supposed to come on when i shine a light on it and dark when no light? it seems backwards because it does nothing when i shine the light on it..

Then...

ok i changed out the ldr to a different manufacturer and its working as originally intended?

Doesn't say anything about changing transistor type. If he had a PNP it would invert operation for sure, but it wouldn't then un-invert with a different LDR.

[antonis]

Maybe the use of question marks instead of periods or exclamation marks by OP baffles things a bit..

[anotherjim]

He went outside.
He went outside!
He went outside? (Grammar wants "Did he go outside?")
English, nowadays, is normally informal. You are allowed to write as you would speak. You can't write inflexions - so look for the punctuation.
Actually, I've written lots in posts here with statements terminating with a "?" indicating puzzlement/uncertainty and some replies (or lack of) suggest the "?" has gone unnoticed.

So anyway, it worked backwards?

[jimitrader]

Quote from: antonis on November 29, 2021, 02:29:58 PM<blockquote>Quote from: anotherjim on November 29, 2021, 11:10:11 AM<blockquote>It doesn't explain a reverse response, High R in light and Low R in the dark that the OP states. </blockquote>
OP never stated that..
Just a "reverse" operation of the whole configuration which may as weel work for a p-n-p BJT..

@jimitrader: Are you sure about your particular transistor type..??
</blockquote>2n2222a 

[jimitrader]

Sorry about the question marks.  I will try harder to use proper grammar and spelling DOH!

[antonis]

2n222a

We could sake hands if only another "2" was added..

[jimitrader]

2n222a

We could sake hands if only another "2" was added..

dang missed again..lol

[anotherjim]

2n222a

We could sake hands if only another "2" was added..

A waste of good rice wine IMHO.

[duck_arse]

2n222a

We could sake hands if only another "2" was added..

he'd already gone outside.