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[iceblue]

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.

[iceblue]

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.

[Brian H]

I will look around to see what I can find
out.

[iceblue]

Thanks Brian!

[iceblue]

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.

[iceblue]

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.

[Brian H]

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].

[Brian H]

Poor memory, but I kind of remember a mod to
make the sensor work with a normally open
switch that closed when tripped.

[iceblue]

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?

[iceblue]

Scratch my last sentence...an LDR is needed
not a photocell...or which is it?  Somebody
heeeelllllllllpppppppp!  Bbrrriiiiiaaaaan!

[iceblue]

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.

[Brian H]

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.

[iceblue]

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.

[Brian H]

Still have the project in mind. If I see
something I will look into it.

[iceblue]

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.