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🛡Home Security => Home Security General => Topic started by: iceblue on November 30, 2005, 06:50:30 PM
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Brian H, when I previously wrote out the
steps for Jim's smoke detector modification,
Bob Hamilton stated that there may be a
photocell modification possibility which
would exclude the need to open up the smoke
detector. I have corresponded with Bob via
email but he is unable to find any details
of the mod he had previously encountered.
It seems as though this would be a simple
and non-intrusive mod to a smoke alarm that
features a flashing strobe (such as the
_First Alert found in _Walmart). Initially,
I though I could just solder a photocell to
the leads of the _DS10A and when the strobe
flashed the transmitter would send an RF to
the console. (The _DS10A sends 2.4V DC
though its leads to the magnetic contacts.
But the problem is that photocells are
Normally Open in the absence of light and
close (resistor) in the presence of light.
Just the opposite of what we need for the
_DS10A. Are you aware of any photocells
that perform in a Normally Closed mode
(resister) in the absence of light and open
upon the presence of light OR can a 5V DC
SPDT relay somehow be connected in to change
the action of the photocell? Thanks very
much for your consideration, and, for the
consideration of any other viewers who may
have advice as to the modification.
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Note: The idea was to solder the photocell
to the _DS10A leads and then clear silicon
the photocell to the flashing strobe lens of
the smoke detector.
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I will look around to see what I can find
out.
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Thanks Brian!
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The battery operated _First Alert model
SA304 (about $10.00 at _WalMart) has a
sustained (non-flashing)escape light
(brilliant light)that activates during a
smoke alarm--perfect for photocell sensing.
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Brian H...could this be what we need instead
of a photocell?
Light Dependent Resistor (LDR)
An LDR is an input transducer (sensor) which
converts brightness (light) to resistance.
It is made from cadmium sulphide (CdS) and
the resistance decreases as the brightness
of light falling on the LDR increases.
A multimeter can be used to find the
resistance in darkness and bright light,
these are the typical results for a standard
LDR:
Darkness: maximum resistance, about 1M.
Very bright light: minimum resistance, about
100.
For many years the standard LDR has been the
ORP12, now the NORPS12, which is about 13mm
diameter. Miniature LDRs are also available
and their diameter is about 5mm.
An LDR may be connected either way round and
no special precautions are required when
soldering.
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That is backwards. You want low resistance
[like the closed magnetic switch] when it is
dark and high when the light is on it [like
the door is open and the switch opened].
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Poor memory, but I kind of remember a mod to
make the sensor work with a normally open
switch that closed when tripped.
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Brian H...sorry...my electronics vocabulary
is limited. I am confused. Here's my
understanding from reading websites. IN
LIGHT: a photocell allows current to flow
more freely, and a light dependent resistor
allows current to flow less freely or block.
(LDR-the resistance decreases as the
brightness of light falling on the LDR
increases.) Is this correct because
sometimes the web sites talk about current
moving and others talk of resistance? IN
LIGHT I would like no current to flow (thus
an open circuit). My understanding is that
the reed switch in the magnetic contact is
composed of two strands of metal that touch
when the magnet is applied thus allowing
current to move from one strand to the other
(a closed loop-or the circling of
electricity from the transmitor back to the
transmittor without interruption) for the
2.4V DC of the _DS10A leads. Thus for the
smoke detector mod, I need a "photoresistor"
that when the smoke detector is activated
(alarm during a fire) the appearance of the
emergency exit light to the "photoresistor"
will block the _DS10A current to create an
open (as in the separation of the two metal
stransds of the reed switch). And in the
absence of light, the "photoresistor" must
allow the 2.4V DV from the sensor to flow
freely (closed circuit). Have I got this
right or am I messed up (haha-probably
messed up)? If this is correct, then I am
now under the corrected understanding thst a
photocell is needed not an LDR?
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Scratch my last sentence...an LDR is needed
not a photocell...or which is it? Somebody
heeeelllllllllpppppppp! Bbrrriiiiiaaaaan!
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Here's a webpage with animation for
LDRs...maybe I am confused: Is an LDR and
Photocell the same thing? I thought they
were different...having opposite reactions
to light.
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The magnetic switch is close to 0 Ohms when
closed which is the not triggered condition.
So you need close to 0 Ohms when the light
is off or not detecting smoke. So you need
the light being on to increase the
resistance so the current stops flowing like
an open switch.
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Thanks Brian H...I am still searching the
internet for either a way to adapt or source
a photocell that will be "normally closed (0
ohms resistance) in the absence of light.
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Still have the project in mind. If I see
something I will look into it.
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Brian H, I made a nuisance of myself and
posted the question of how to use a
photocell as a NC circuit in the absence of
light and NO in the presence of light. I
received various replys such as this one
from Soeren which were practically all the
same: QUESTION (redacted)--Connected to my
transmitter leads this would act as a
Normally Open circuit in the absence of
light--I need the photocell to do the
opposite. In darkness, I need the 2.4 V DC
passing through the wires of the wireless
transmitter to loop (closed circuit) back to
the transmitter. How do I reverse the action
of the photocell? ANSWER--By a transistor
and an extra resistor. Collector to the
positive wire, emitter to the other (ground)
wire. LDR between base and emitter and
finally a resistor between base and
collector - the size of which will depend on
the specific LDR. Brian...can you maybe
help me in sizing/spec'ing the photocell,
transistor and extra resistor. I was
thinking to clear silicon the photocell to
the lens of the escape light and use a large
heat shrink tube to dress the transistor and
extra resistor which would then be epoxied
to the exterior base of the smoke alarm. As
always, thanks for your consideration.
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No problem. I was thinking an inverting
transistor or a simmilar circuit myself. See
what I can do.
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Brian H....thanks...here's a linkl to a
wesite which shows a circuit diagram (near
the bottom of the page) for LDRs and
transistors. Also, there are links for
sizing transistors. Even though I have read
it, along with many other similar websites,
I still can't translate it into an actual
project...I think I am electrically
challenged!
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The link:
http://www.kpsec.freeuk.com/trancirc.htm
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Ok gave it a look and will see what I think.
First impression is it may not work as the
transistor when ON to simulate the closed
switch normally used. It will have almost
zero voltage on it and turn off. Side note.
On a Google search I did find someone that
modifies the door window sensors for a
normally open contact that closed when
tripped. Also this mod costs you I believe
$79 for one modified module. I also kind of
remember reading the older original
dorr/window sensors had a jumper to pick
normally open or closed. Not in the newer
ones as far as I can see. X10 didn't release
the schematics of the sensor but I did find
the X10 made RS sensors schematic. If worse
comes to worse. I maybe able to do a mod on
that module; that will register with a
DS7000 console.
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Brian H...here is another response I
received from GBartlett at an EZBoard
(http://p199.ezboard.com/fbasicelectronicspro
jects.showMessage?topicID=889.topic) This
shows how to modify the function of the
photocell: "...A better bet is to use a
circuit such as the one on this link:-
home.cogeco.ca/~rpaisley4...ctors.html
Replace the LED and resistor with the relay,
and you're sorted. Comparators are cheap.
The only problem would be connecting one up -
the catalogue you buy it from should
provide a datasheet or pinout for which pins
are + and - inputs, which pin is output, and
which pins are Vpwr and 0V. R1 should be a
variable resistor, so that you can tweak the
circuit to get the turn-on point just right.
There actually is a circuit on there for
doing this, so you should be OK. Graham."
What do you think? Could you translate this
into a parts and to do list? Thanks.
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Went there got the date. Hope to have
sommething for you.
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Will look at the Shack tomorrow. The LM339
shown in one of the diagrams your web page
showed. Has an open collector output. Plain
talk should toggele the sensors output no
relay needed. Will need its own small
battery pack or wall wart to work. Guess
batteries are better as AC goes off.
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Got the parts to test with. Now do you think
the detector we are trying to use will be in
normal [not bright] daylight. The photocells
change mostly from real dark to room light
than from room light to real bright smoke
detector light on. Can probably compensate
so only real bright light will trip it. The
photocells come in a kit of 5 and each is
different than the rest. I may use a mini
adjustable resistor to make it easier. 3
parts unused in the chip. Maybe an added
output to light an LED when the detector is
tripped? Feedback from you would help.
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Got the parts connected in an Experimenter
Bread Board and have started some tests. The
schematic on the web page for the cad cell
detector is close to what I found. It can be
done as shown, but each cad cell may need R1
hand selected or you can change the (2)10K
resistors and have a small adjustable
resistor to the + Input of the chip and use
a 1K for R1.
Not 100% on getting it on a PC Board today,
but maybe.
One of the unused 3 sections of the same
chip can be wired as a I see light and have
opened the signal to trip the Window/Door
sensor indication with an LED if wanted.
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Brian H....thanks for looking into this. I
would like to use the First Alert smoke
detector model SA304. This model has the
escape light (ultrabright LED).
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Beian H...my thought was to clear silicon
the photocell to the clear lens of the
Escape light and then paint any remaining
exposed part of the clear lens black so that
the photocell would be in total darkness
until an ALARM--which would cause the
ultrabright LED to light.
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OK that may make it even simpler. The
diagram as shown on the web page should do
fine with a large Black to Bright White
difference.
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iceblue; I have not forgotten you. Prototype
should be worked on today. If I can give you
values and pinouts for the chip is thet
enough?
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Iceblue;
Got the circuit tested and functional. Since
you
indicated you where going to have the cell
dark.
I did not add the adjustment variable
resistor and found
a R1 value that worked for all 5 Photocells
in the assortment.
I used the Circuit #2 Cds Photocell Diagram
from Rob Paisley.
IC1 LM339 RS#276-1712
PC1 Photocell RS#276-1657 Pack of 5
R1 2.2k 1/4W RS#271-1325 Pack of 5
(2) 10K 1/4W RS#271-1335 Pack of 5
14 pin IC socket optional RS#276-1999
I used a 6V supply from a 4 AAA cells in a
holder I had.
Led and Resistor not used on the output
IC Pin #3 + Power; Pin #12 Common; Pin# 7 +
Input to 10K resistors;
Pin #6 -Input to cell and R1; Pin #1 Output
to Door/Window Sensor
Connect the + signal wire from the sensor to
Pin #1
Connect the - signal wire from the sensor to
common.
When the cell is dark the IC is ON and acts
like a closed switch
When it is brightly iluminated it turns OFF
it acts like an open
switch.
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i had make an ldr based alrm in which i had
use a laser, transistor,resistance,capacitor
and speaker now i had to give project report
on it can you help me to give it and provide
me information about ldr,cpacitor,speaker.
and transistor please help me