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.