# Current sensing, I think??????

Discussion in 'Electronic Basics' started by JSW, Aug 23, 2003.

1. ### JSWGuest

Hi
I want to be able to connect smoke alarms to my existing house alarm. At my
local alarm shop they sell a small pcb which replaces the 9v PP3 battery and
allows the smoke alarm to be connected to the main alarm. This pcb probably
has a voltage regulator on it (12v to 9v) and some sort of current sensing
to trigger the main alarm when the smoke alarm sounds.
What I want is to be able to make this pcb myself as the shop wanted to much
for it and a want a few of them, does anybody know a circuit for such a
thing?

Cheers
John

2. ### Fred BloggsGuest

Yeah- it's called a relay. Your house alarm has inputs labeled N.O. and
N.C.- so you run a wire pair from one of those over to relay contacts
smoke alarm circuitry to activate the relay. This will be over your
head- so go buy the finished product or drop the idea entirely.

3. ### Frank RaffaeliGuest

It's a cleaver idea. The sonalert / transducer inside the remote alarm
will draw enough current to distiguish between alarm and quiescent.
Try this:

From Central Control: +12V, Sense, Return (Gnd)
Use a 3-terminal Vreg to supply remote detector.
_________
+12V ----+-----/\/\/----+-----| In |---9V
\ R1=? | | Out |
Q1 PNP |----/\/\/---+ | Det
/| R2=1K |
| +-------Gnd
Sense -<-+ |
| |
/ |
\ R3 = 10K - 22K |
/ |
| |
Gnd ----+------------------------+

Adjust R1 to sense the current. The PNP transistor's conduction is
temperature and input dependent. Also, R1 cannot be so large that the
drop between the 12V input and the regulator exceeds the regulator
and the alarm activation circuit. The sense line is pulled to the Vreg
input voltage when enough current flows in R1. You may also require
some noise filtering, like a cap between E-B of Q1.

Frank Raffaeli
http://www.aomwireless.com/

These factory smoke alarms are tested for Heat, Smoke, and whatever adverse
conditions occur during a fire! Do you realy want to take that chance that
you may get it all right!!!! Buying a proven unit gives peace of mind!

5. ### EdwardGuest

You should also check that your insurance is still valid if you "modify"

6. ### Fred BloggsGuest

Right- and there are models that run off 120VAC with battery backup, and
they are alarmed as well as alarm- meaning central alarm control can
cause smoke alarm to sound via NO or NC inputs- as well as other way
around. Generally you want latched operation requiring manual reset-

7. ### TerryGuest

It's a good idea: But suppose it misoperates, perhaps due to some
voltage glitch in the powers supply? Could a 'non standard'
modification that caused a 'false alarm' to the company
monitoring your home alarm system then cost you for a 'false
call-out'.
On other hand if your house is of average size and you are NOT
connected to an alarm monitoring company you can probably hear
your individual smoke alarms if they go off anywhere in the
house? We can.
Here; for rented accommodation there are some requirements that
the alarm be a) AC powered; so kids don't steal the batteries for
their 'toys' and b) With more than one habitable floor, alarms be
Personally I would keep your systems separate with the possible
exception of an intermediate relay.
Relays are very good at physically/electrically separating
different systems.
Then if a smoke alarm works to activate the home alarm system,
fine. If it doesn't you still got your smoke alarm/s standing
alone, not depending on anything else to power them or give the
alert! Change smoke alarm batteries yearly. Also most smoke
alarms will give a 'beep' indication if battery goes low.
If any gas operated appliances (vented or not) also use a CO
(Carbon Monoxide) detector. Could be cheapest life insurance you