Light dependent resistors...

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Externet
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Light dependent resistors...

Post by Externet » Sat Oct 06, 2012 9:34 am

Hi.
Is there any type of these LDR that will decrease its resistance when dark ?
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Edd
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Re: Light dependent resistors...

Post by Edd » Sat Oct 06, 2012 1:17 pm

.




Sr Externet . . . . .

With the initial implied semantics of " these LDR " I just then had to bring the mouse on in close enough proximity to see if it would then key onto those words, as further being tied into any hot linking referencing examples. It wasn't.

I know of of no chemical compositions responding in that reversed manner with the photo-resistive effect characteristics.

Since you have already mentioned the available variable photo resistance units, and with them NOT being self generating, with their dependency on a secondary voltage source passing through them.

A possibility ? ? ?
Why not use one of them feeding into the base /gate of a transistor /FET in the manner of then using IT for creating the "phase reversal" of its photo effect, with that voltage variance output then being retrieved at the collector / drain of the selected device.
If further needed, that could even supplementaly give you some lattitude in custom tailoring its voltage limit swing by selection of the designed stage gain.

If needing output reference, being to ~ OVDC as being ground.
Using an emitter follower for the second stage, with proper phased voltage input from prior staging, could keep the voltage on the proximity of close to near O VDC output when fed with a high bright light input into the drive photo-resistive cell.

Thasssitttt . . . . .


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Externet
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Re: Light dependent resistors...

Post by Externet » Mon Oct 08, 2012 11:09 am

Thanks Sir Edd. OK, no such reversed behavior LDR.

Yes, a simple inversion circuit as the 'Dark activated' here:
----> http://www.sentex.ca/~mec1995/circ/actrelay.htm

Without D1, Led1, R2, and sustituting Ry1 with a string of leds.
Then those would turn on dusk to dawn.
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Bob Scott
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Re: Light dependent resistors...

Post by Bob Scott » Mon Oct 08, 2012 6:05 pm

Externet wrote:Thanks Sir Edd. OK, no such reversed behavior LDR.

Yes, a simple inversion circuit as the 'Dark activated' here:
----> http://www.sentex.ca/~mec1995/circ/actrelay.htm

Without D1, Led1, R2, and sustituting Ry1 with a string of leds.
Then those would turn on dusk to dawn.
Oh, Sentex and Rony Van Roon (RIP). His site outlives him.

You could improve it using an op-amp with a little hysteresis so that the relay doesn't chatter during dawn and dusk.

EDIT: Perhaps the relay already provides a degree of hysteresis. I'd look inside my LED night lights but they're all glued when assembled. They use CdS LDRs.
-=VA7KOR=- My solar system includes Pluto.

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Re: Light dependent resistors...

Post by Dean Huster » Mon Oct 08, 2012 7:41 pm

In most applications, it's just where you put the LDR in a voltage divider that will reverse its resultant action .... on the supply side or on the grounded side.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Re: Light dependent resistors...

Post by Lenp » Tue Oct 09, 2012 3:17 am

Many of the outside light sensors use a cell to pass current to a resistor that acts as a heater which heats a high current bi-metal switch.
Rugged, cheap and a workable delay. Unless it is hit by lightning, it's near bullet proof!
Len
Len

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Bob Scott
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Re: Light dependent resistors...

Post by Bob Scott » Tue Oct 09, 2012 4:37 pm

Lenp wrote:..........a high current bi-metal switch.......
There's yer hysteresis right there. :grin:
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Re: Light dependent resistors...

Post by jimmy101 » Mon Dec 03, 2012 3:03 pm

never mind

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