Electronics Forums > LDR / photo resistor - increasing resistance with increasing light???

# LDR / photo resistor - increasing resistance with increasing light???

mike
Guest
Posts: n/a

 10-19-2004, 05:35 PM
Is there such a thing as a LDR that increases resistance with
increasing light?

Or, is there a simple circuit that will invert the resistance of a
common LDR?

Disclaimer: I have very little experience with electronic/circuits.

Any help is appreciated. Thanks!
m

Terry Pinnell
Guest
Posts: n/a

 10-19-2004, 07:22 PM
(E-Mail Removed) (mike) wrote:

>Is there such a thing as a LDR that increases resistance with
>increasing light?
>
>Or, is there a simple circuit that will invert the resistance of a
>common LDR?
>
>Disclaimer: I have very little experience with electronic/circuits.
>
>Any help is appreciated. Thanks!
>m

What is the objective? If you want to control a circuit using an LDR,
you would normally arrange the LDR with a normal resistor (or pot or
preset) as a voltage divider pair. You'd then use the output voltage
from that as input to a subsequent (analog or digital) circuit
section. So, simply reversing the position of the LDR and the other
resistor would reverse the operation of the circuit.

As an example, see the first section of my garden lamp circuit here:
http://www.terrypin.dial.pipex.com/I...GardenLamp.gif

In the unlikely event that I wanted my lights to go on at dawn rather
than dusk, I could achieve that by swapping the positions of the LDR
and the resistors.

--
Terry Pinnell
Hobbyist, West Sussex, UK

John Popelish
Guest
Posts: n/a

 10-19-2004, 10:18 PM
mike wrote:
>
> Is there such a thing as a LDR that increases resistance with
> increasing light?

I don't know of any.

> Or, is there a simple circuit that will invert the resistance of a
> common LDR?

This is fairly weary in some cases.

> Disclaimer: I have very little experience with electronic/circuits.
>
> Any help is appreciated. Thanks!
> m

Tell us all the details you can think of about what you are trying to
accomplish. This will save us a lot of useless questions.

--
John Popelish

mike
Guest
Posts: n/a

 10-20-2004, 06:50 AM
wow, thanks for the response. here is exactly what i want to do:

i live in a basement w/o windows, and i want to rig inside light so it
coincides with the outside light. i was figuring i could run an LDR
placed outside, to a dimmable ballast in my basement apt.

i have an electronic dimming ballast, made by the now defunct JRS
Technology (#C232120RS501). it has a dimming control by way of a 0 to
10v class 2 circuit, 0.5mA output. it is 0-10v out, and by connecting
a 0-100k ohm potentiometer to this circuit i can dim the connected
flourescent bulbs no problem-- the greater the resistance, the greater
the V, the greater the resulting brightness.

however, what i want to do is connect an LDR in such a way that a
greater amount light hitting the LDR makes the bulbs brighter. this
means that in order to get brighter bulbs, i need a greater voltage
accross this circuit, which means i need greater resistance connected
to this circuit (i think, right?)

i have an LDR (from a radioshack multi-pack) connected in series (with
a 0-100k trimmer/potentiometer) to this 0-10v circuit, and it does
what i want it to do, only in reverse. that is, as lighting
conditions on the LDR get brighter, the dimmer makes the bulbs get
dimmer.

this makes sense since an LDR decreases resistance as more light hits
it. what i need is increasing resistance with more light. or a
circuit that simulates this.

if this looks familiar, i posted 6 weeks ago before i had any idea
what i was talking about (thanks to those who offered suggestions):

John Popelish <(E-Mail Removed)> wrote in message news:<(E-Mail Removed)>...
> mike wrote:
> >
> > Is there such a thing as a LDR that increases resistance with
> > increasing light?

>
> I don't know of any.
>
> > Or, is there a simple circuit that will invert the resistance of a
> > common LDR?

>
> This is fairly weary in some cases.
>
> > Disclaimer: I have very little experience with electronic/circuits.
> >
> > Any help is appreciated. Thanks!
> > m

>
> Tell us all the details you can think of about what you are trying to
> accomplish. This will save us a lot of useless questions.

KM
Guest
Posts: n/a

 10-20-2004, 01:40 PM
visit this web:http://www.doctronics.co.uk/ldr_sensors.htm
hope it helps!
KM

(E-Mail Removed) (mike) wrote in message news:<(E-Mail Removed). com>...
> wow, thanks for the response. here is exactly what i want to do:
>
> i live in a basement w/o windows, and i want to rig inside light so it
> coincides with the outside light. i was figuring i could run an LDR
> placed outside, to a dimmable ballast in my basement apt.
>
> i have an electronic dimming ballast, made by the now defunct JRS
> Technology (#C232120RS501). it has a dimming control by way of a 0 to
> 10v class 2 circuit, 0.5mA output. it is 0-10v out, and by connecting
> a 0-100k ohm potentiometer to this circuit i can dim the connected
> flourescent bulbs no problem-- the greater the resistance, the greater
> the V, the greater the resulting brightness.
>
> however, what i want to do is connect an LDR in such a way that a
> greater amount light hitting the LDR makes the bulbs brighter. this
> means that in order to get brighter bulbs, i need a greater voltage
> accross this circuit, which means i need greater resistance connected
> to this circuit (i think, right?)
>
> i have an LDR (from a radioshack multi-pack) connected in series (with
> a 0-100k trimmer/potentiometer) to this 0-10v circuit, and it does
> what i want it to do, only in reverse. that is, as lighting
> conditions on the LDR get brighter, the dimmer makes the bulbs get
> dimmer.
>
> this makes sense since an LDR decreases resistance as more light hits
> it. what i need is increasing resistance with more light. or a
> circuit that simulates this.
>
> if this looks familiar, i posted 6 weeks ago before i had any idea
> what i was talking about (thanks to those who offered suggestions):
>
>
> John Popelish <(E-Mail Removed)> wrote in message news:<(E-Mail Removed)>...
> > mike wrote:
> > >
> > > Is there such a thing as a LDR that increases resistance with
> > > increasing light?

> >
> > I don't know of any.
> >
> > > Or, is there a simple circuit that will invert the resistance of a
> > > common LDR?

> >
> > This is fairly weary in some cases.
> >
> > > Disclaimer: I have very little experience with electronic/circuits.
> > > Please be gentile.
> > >
> > > Any help is appreciated. Thanks!
> > > m

> >
> > Tell us all the details you can think of about what you are trying to
> > accomplish. This will save us a lot of useless questions.

rayjking
Guest
Posts: n/a

 10-20-2004, 02:47 PM
Mike,

If you add a cad photocell in series with a 10k resistor that is connected
to a low current ( 10ma or more ) 10/12 volt dc power supply the signal you
wish will appear across the 10k resistor. The better circuit would
substitute the 10k for a 100k pot and a series 470 ohm ( safety resistor )

Ray

"mike" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Is there such a thing as a LDR that increases resistance with
> increasing light?
>
> Or, is there a simple circuit that will invert the resistance of a
> common LDR?
>
> Disclaimer: I have very little experience with electronic/circuits.
>
> Any help is appreciated. Thanks!
> m

John Popelish
Guest
Posts: n/a

 10-20-2004, 05:04 PM
(E-Mail Removed) (mike) wrote in message news:<(E-Mail Removed). com>...
> wow, thanks for the response. here is exactly what i want to do:
>
> i live in a basement w/o windows, and i want to rig inside light so it
> coincides with the outside light. i was figuring i could run an LDR
> placed outside, to a dimmable ballast in my basement apt.
>
> i have an electronic dimming ballast, made by the now defunct JRS
> Technology (#C232120RS501). it has a dimming control by way of a 0 to
> 10v class 2 circuit, 0.5mA output. it is 0-10v out, and by connecting
> a 0-100k ohm potentiometer to this circuit i can dim the connected
> flourescent bulbs no problem-- the greater the resistance, the greater
> the V, the greater the resulting brightness.
>
> however, what i want to do is connect an LDR in such a way that a
> greater amount light hitting the LDR makes the bulbs brighter. this
> means that in order to get brighter bulbs, i need a greater voltage
> accross this circuit, which means i need greater resistance connected
> to this circuit (i think, right?)
>
> i have an LDR (from a radioshack multi-pack) connected in series (with
> a 0-100k trimmer/potentiometer) to this 0-10v circuit, and it does
> what i want it to do, only in reverse. that is, as lighting
> conditions on the LDR get brighter, the dimmer makes the bulbs get
> dimmer.
>
> this makes sense since an LDR decreases resistance as more light hits
> it. what i need is increasing resistance with more light. or a
> circuit that simulates this.
>
> if this looks familiar, i posted 6 weeks ago before i had any idea
> what i was talking about (thanks to those who offered suggestions):

You can make a voltage divider with a light dependent resistor in
series with a fixed resistor that has the LDR in either the pull up or
pull down side of the divider. If you have a 10 volt supply available
(a wall wart, perhaps) or if this voltage is available from the
ballast, you can experiment with the LDR and fixed resistor replacing
the potentiometer. Varying the value of the fixed resistor shifts the
light level that produces about half light output.

--
John Popelish

CFoley1064
Guest
Posts: n/a

 10-20-2004, 10:32 PM
>Subject: Re: LDR / photo resistor - increasing resistance with increasing
>light???
>From: (E-Mail Removed) (mike)
>Date: 10/20/2004 1:50 AM Central Daylight Time
>Message-id: <(E-Mail Removed) >
>
>wow, thanks for the response. here is exactly what i want to do:
>
>i live in a basement w/o windows, and i want to rig inside light so it
>coincides with the outside light. i was figuring i could run an LDR
>placed outside, to a dimmable ballast in my basement apt.
>
>i have an electronic dimming ballast, made by the now defunct JRS
>Technology (#C232120RS501). it has a dimming control by way of a 0 to
>10v class 2 circuit, 0.5mA output. it is 0-10v out, and by connecting
>a 0-100k ohm potentiometer to this circuit i can dim the connected
>flourescent bulbs no problem-- the greater the resistance, the greater
>the V, the greater the resulting brightness.
>
>however, what i want to do is connect an LDR in such a way that a
>greater amount light hitting the LDR makes the bulbs brighter. this
>means that in order to get brighter bulbs, i need a greater voltage
>accross this circuit, which means i need greater resistance connected
>to this circuit (i think, right?)
>
>i have an LDR (from a radioshack multi-pack) connected in series (with
>a 0-100k trimmer/potentiometer) to this 0-10v circuit, and it does
>what i want it to do, only in reverse. that is, as lighting
>conditions on the LDR get brighter, the dimmer makes the bulbs get
>dimmer.
>
>this makes sense since an LDR decreases resistance as more light hits
>it. what i need is increasing resistance with more light. or a
>circuit that simulates this.
>
>if this looks familiar, i posted 6 weeks ago before i had any idea
>what i was talking about (thanks to those who offered suggestions):
>

>

Hi, Mike. I'm hearing you say you tried the RS photoresistor, but it operated
in a bass-ackwards fashion. You want resistance to increase as light
increases.

You might want to try something like this (view in fixed font or M\$ notepad):

VCC
+
|
| VCC
.-. 1/2 LM358 +
| | VCC |
1K| | |\| |
'-' .---|-\ ___ |/
| | | >-|___|--|2N3904
.----o-----|---|+/ 100 |>
| | | |/| |
| | | GND | ___ 1 H11F1 6
~~.-. .-. '-----------------o-|___|---o--. .---o--->
~~| | | | 150 ohms | |
LDR| |1K| | V ~~ ||-+
'-' '-' - ~~ || To Ballast
| | 2 | ||-+
| | .---------o--' | 4
| | | '---o--->
| | |
V | |
- | ===
| | GND
=== ===
GND GND .------------.
| |
1N4001 | |\ |
'---|-\ |
| >-----'
.---|+/
| |/
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

There are a couple of components here you'll have to purchase, and I'm afraid
they're special order at RS. You can try mouser.com for better prices and good
service for the hobbyist. You also might want to pick up a 9VDC wall wart for
the Vcc in the diagram.

The two components you need are an LM358 (dual op amp) and a H11F1 (an opto
analog output FET). The distinguishing characteristic of the H11F1 is that, as
current through the LED increases, the resistance of the FET decreases. You
can use that to advantage with your CdS photoresistor by seting up a voltage
divider, and using the CdS to shunt the lower resistor. That voltage is picked
up by the op amp, and used to drive the LED.

Notice that, as ambient light increases, the voltage decreses, so the current
througfh the LED decreases, causing the resistance the ballast sees as
increasing. Also note that this assumes your CdS LDR has less than a couple
hundred ohms "ON" resistance. Since there's such a wide variety in the RS CdS
parts, you'll probably have to tweak this circuit for your part to make it
work.

I hope this is of value, even though it's a little on the complex side. If you
need more help, feel free to email. Please put "LDR" in the heading to avoid
having the spam filter dump you.

Good luck
Chris

Rich Grise
Guest
Posts: n/a

 10-21-2004, 07:06 PM
On Tue, 19 Oct 2004 23:50:20 -0700, mike wrote:

> wow, thanks for the response. here is exactly what i want to do:
>
> i live in a basement w/o windows, and i want to rig inside light so it
> coincides with the outside light. i was figuring i could run an LDR
> placed outside, to a dimmable ballast in my basement apt.

I seem to remember this from all those months ago, and it seems like
someone had suggested another LDR inside, with a comparator/difference
amp, to make the inside lights _really_ track the outside. Then, as
long as you get it scaled right, you won't have to worry about polarity,
or even nonlinearity. :-)

Of course, on cloudy days, it could be a little dim. )-;

Have Fun!
Rich

Robert Monsen
Guest
Posts: n/a

 10-21-2004, 09:14 PM
Rich Grise wrote:
> On Tue, 19 Oct 2004 23:50:20 -0700, mike wrote:
>
>
>>wow, thanks for the response. here is exactly what i want to do:
>>
>>i live in a basement w/o windows, and i want to rig inside light so it
>>coincides with the outside light. i was figuring i could run an LDR
>>placed outside, to a dimmable ballast in my basement apt.

>
>
> I seem to remember this from all those months ago, and it seems like
> someone had suggested another LDR inside, with a comparator/difference
> amp, to make the inside lights _really_ track the outside. Then, as
> long as you get it scaled right, you won't have to worry about polarity,
> or even nonlinearity. :-)
>
> Of course, on cloudy days, it could be a little dim. )-;
>
> Have Fun!
> Rich
>

But eclipses and airplanes would be fun.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

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