mike said:
Ak! Please don't hate me for what might be a simple question, but I
have an interesting project in mind that I need some basic guidance
with.
I live in a basement... meaning, no windows and no natural light.
Which means I sleep in til 2 or 3 most days-- great on the weekends,
but the bosses aren't too happy with my "natural clock". I want to
create artificial lighting inside as close as possible to the
lighting
outside.
I was figuring to do this I'd connect full spectrum flourecent bulbs
to a dimmable ballast, and control the dimmer on the ballast with a
circuit consisting of a photocell or photresistor placed outside,
hence the dimmer/circuit would control the light levels inside to
match those the cell sees outside.
I've already got the dimmable ballasts. The dimmer operates on a 0
to
12v signal in.
My question is, well... 2 questions:
(1) what's the difference between a photocell, phototransistor, and
a
photoresistor?
(2) does anyone have recommendations on what the circuit would look
like? Will I need an AC to DC converter? And I imagine a trimmer in
line somewhere to adjust levels? I'm hoping I wont need to get a
microcontroller into the mix.
Again, terribly sorry I'm so new to this, but I'm really excited
about
this project, and I think it can be done fairly easy with some
guidance from the old pro's lurking around here.
Thanks a ton,
mike
Here is a somewhat wacky idea, and I have no idea whether it'll work.
If you can get two CdS light dependent resistors (LDR) that are about
equal, you might be able to set things up so that at a particular spot
in the room, the inside light intensity, as measured by the LDR, is
equal to the outside light intensity.
If you have a 12V source, you can create a voltage divider where the
current flows out to the outside LDR, through it (the amount depending
on the ambient lighting) and back; into a resistor, then to ground.
The voltage across the resistor will then be an indication of how
bright it is outside.
Unfortunately, that won't give you any way to track the light outside,
unless you use another identical LDR which is INSIDE, at a point in
the room (maybe near your bed?
that you want to track the outside
lighting. The wire again goes from 12V to the LDR, back to an
identical resistor, and to ground.
If the voltage across the second resistor is identical with the
voltage across the first, then your light is the same outside and
inside, at least at the point where the LDRs are.
Fortunately, you can arrange this by using a device called an "opamp",
whose main purpose is to compare voltages, and try to set the output
so that the input voltages are equal. Thus, the output of the opamp
controls that dimmable ballast somehow (the details escape me, because
I don't know what the input is expecting. You need to post the device
specifications for more detailed advice.) When the outside LDR
changes, the opamp resets the ballast so that the inside ambient light
creates the same conditions at the inside LDR. Since in this
configuration, where the LDR is first in the chain of resistances from
12V to 0V, the voltage across the resistance will increase as it gets
brighter outside, you want to hook up the + input of the opamp to the
outside resistor, and the - input of the opamp to the inside resistor.
The opamp is powered from the same 12V supply. You then hook up the
output of the opamp to the ballast control, assuming increasing
voltage makes it brighter.
Here is a web page for info for CdS LDRs:
http://www.selcoproducts.com/CFM/photocell_toc.cfm
You should also get a coupld of medium sized (maybe 1uF) capacitors,
and put them in parallel across those resistors that are connected to
ground. That will keep stray electromagnetic radiation from causing
your lights to flash (well, maybe it will).
Another issue is that equal LDRs are difficult to get (those packs at
RadioShack don't have good matching ones, anyway) Maybe if you order
them from DigiKey or Mouser you can get ones that will be similar; I
don't really know.
The other issue, which I'm sure others will comment on, is the
possibility of oscillations with this circuit. If the response of the
ballast is slow, it might allow oscillations to happen, where the
lights flicker constantly, or possibly cause problems of other kinds.
There are ways to get around this, by using whats called a PID
controller, but that may be overkill, I don't know. They are fairly
expensive, and difficult to tune (what happens is that the PID
controller, in essence, predicts whats going to happen in a bit as the
voltages change, and tries to compensate for that. You have to tune
the prediction using pots.)
Anyway, it sounds like a fun Saturday afternoon project. You can get
LDRs, resistors, and opamps from RadioShack or whereever finer cheapo
electronics are sold. A 12V wall wart and some wire are also required.
NOTE: If you can't get CDS LDRs that are equal, you may be able to
compensate by using a variable resistor (potentiometer) instead of a
resistor for one of the resistors. Get one that is twice the value of
the resistor, and hook its wiper (the center pin) to ground. Then,
adjust the wiper so that the light is about right using the screw
control. You can get these at radio shack as well.
View with courier font:
12V
| ___
o------ OUTSIDE LDR --o--------|___|--,
| | ___ |
'------ INSIDE LDR --|---o----|___|--o
| | A |
| | | |
| | '----o
/| | | |
to /+|--' | |
variable -< | | |
ballast \-|------' |
ctl \| |
GND
opamp (LM324?)
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
For unused inputs on the LM324, tie + to 12V, - to GND, and leave
outputs unconnected.
Measure the LDR using a multimeter at a medium light intensity, and
use a similarly valued resistor, or a pot that is twice the measured
value.
A goes to the + input on the opamp, B goes to the - input. The output
of the opamp goes to the variable ballast input. The pinouts are on
the back of the package you get from RS.
more info on LDRs
http://powerelectronics.com/news/power_cds_lightdependent_resistors/
Regards,
Bob Monsen
PS: The other option would be an alarm clock, or a light that turns on
on a timer...
