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Ultimate smoke alarm - never need to change batteries!

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Daniel Morrow

Jan 1, 1970
0
Hi, I am new to this group. As per subject I am working on a project that
would allow me to have a smoke alarm in my room and I would never need to
change the batteries. I have it partially finished now. I used a 7809
voltage regulator to regulate the voltage from 12 rechargeable nimh
batteries so the voltage going to the smoke alarm is a reliable 9 volts. I
used a .33 micro farad capacitor and a 1 micro farad capacitor on the
regulator, with a heat sink and it works beautifully. But I am not done, the
current draw from the voltage regulator even when there is no load is a
constant 5 milliamperes so the nimh batteries only last a couple of weeks
apparently, and I want a more permanent solution. So I remembered an old
article in a misplaced (I cannot find it right now) model railroader
magazine that let a nicad battery charge off of the train power supply as
the train is being used and that allowed the train's front light to stay on
even if the train is receiving no electricity. So it all boils down to me
needing you guys to at least point me in the right direction for circuits or
information on how to recharge those 12 nimh batteries (or nicad if
necessary) while the smoke alarm is working so that when the ac power shuts
off as it does in most places in the U.S. every once in a while, then the
batteries supply the electricity to the smoke alarm until the ac power comes
back on or the batteries run out of juice after a couple of weeks. This
shouldn't be too hard of a project for me so any help could speed my
progress tremendously. Thanks, and let me know? By the way - I have tried a
lithium 9 volt battery in my smoke alarm and despite the instructions saying
it lasts for up to 10 years it only lasted 9 months. I think thats because
the shelf life is 10 years but the device life is much less.
 
L

Lenroc

Jan 1, 1970
0
not good....some electrical fires that cause total distruction of the home
cause the electicity to go off long before smoke would get to the bedroom.
If you have a death wish go ahead and run the smoke detector off 110. John

But the OP already says he has batteries that last a few months, and is
just looking for a way to continually recharge them. Russel just mentioned
that the OP could just feed off the wall power until it goes away, then
switch over to batteries.

If the power went out for a few months (long enough to drain the
batteries), you'd think that the OP would have other issues (like... not
having power) to worry about.

The only thing I'd worry about is making sure that if the smoke alarm _is_
working off batteries, to have some sort of signal/alarm going off to
alert you to the fact, to eliminate the risk that only the smoke alarm
would lose power...
 
D

Daniel Morrow

Jan 1, 1970
0
Thanks for the simple solution Russell! I am a newbie compared with
you engineers though so what do you mean by a reversed biased diode -
that the diode is connected in reverse? Forward biased? Thanks again
for the simple solution! By the way - I need both a permanent power
supply for this smoke alarm and a battery supply in case the power
goes out because a fire elsewhere in the house melted the insulation
and caused the circuit breakers to trip, for example. Thanks!
 
D

Daniel Morrow

Jan 1, 1970
0
JOHN T said:
............................................................................
not good....some electrical fires that cause total distruction of the home
cause the electicity to go off long before smoke would get to the bedroom.
If you have a death wish go ahead and run the smoke detector off 110.
John

Keep reading. Russel is talking about the batteries kicking in after
the power goes out, therefore the smoke alarm should still work just
fine. On another note I would say that I am sick and tired of hearing
from fire officials that you absolutely have to buy a $05.00 battery
(my smoke alarm will only accept alkaline or lithium 9 volt batteries
and the alkalines just don't work for more than 5 months each)every 9
months or else you're doomed. My way will provide the ultimate in
safety as people often forget to change their batteries or just
disable the smoke alarm altogether because of the hassle of going to
the store to buy a battery that will probably never be truly used, at
least once a year. After all - it's such a hassle I am doing this
project aren't I?

Could someone here help me with nimh or nicad recharging specifics
(i.e. trickle charging and/or in the case of nimh smart recharging) so
I can make some headway in the project? Thanks for reading.
 
J

JeffM

Jan 1, 1970
0
The battery is a non-rechargable type and is for backup only.

Put in a diode so that when the AC is cut,
the battery passes juice to the detector
but the AC supply never tries to charge the battery.
 
D

Daniel Morrow

Jan 1, 1970
0
Even if I got a smoke detector that lasts a year on one battery is not
good enough for me. I want at least a 3 to 4 year battery life. Not
only that but me not doing this project could cost me my life because
I refuse to buy batteries up the kazoo just because the industry is
slow and monopolized. My current smoke detector has diagnostics, a low
battery warning (nudge, nudge to the other poster asking for a way for
the batteries to be checked for low power) which doesn't let me use
rechargeable 9 volt batteries because those batteries only actually
supply 7.2 volts (i.e. the smoke alarm chirps and flashes the safety
light which is a standard incandescent because it thinks the battery
power is low), and a safety light in case it goes off at night and
it's dark. Anyhow John - I am just trying to be safe to potentially
save my life but I still appreciate your comments certainly, after all
it makes me think even more. I am surprised that with all of the
diagnostics and redundancy and so on in today's electronics that my
idea hasn't become commonplace with a product like it to sell. Anyhow
later.
 
D

Daniel Morrow

Jan 1, 1970
0
Correct assumption. Thanks for the additional details.
 
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