About 2 years ago, I told my father I would hook up a
circuit to his door bell that would flash a red light when he
was watching television with headphones on since he can't hear
the bell under those conditions.
You can buy light flashers for the deaf and hard of
hearing but my father already has a bell that is nice and loud
when he isn't wearing headphones so the task was to add this
circuit and leave what already worked in place.
The problem is that when there is no set deadline, it is
easy to procrastinate so this job took about 2 and a quarter
years which, considering what I needed to do, was probably about
18 months too long, but it is now done.
What it turned out to be was a recycling job with the
exception of a hand full of electronic parts, but I made use of
a 1980's vintage cordless telephone which can only do pulse or
rotary-style dialing. The telephone we get from our cable
company doesn't even recognize that kind of signal and pulse
dialing is so, so, slo-o-o-ow.
I also suspect that the hand set of the phone is partly
malfunctioning so it really was ready for the recycle bins
except that I had an idea.
The previous owner of my father's house had had the
house wired to provide Hi Fi sound in every room. There was also
an alarm system and cable TV runs going just about everywhere.
For your average middle-class house, it was amazing and
bordered on over kill but there was not one easy way to get
wires from the door bell system in to the room where the
television is without crawling up in the attic and even then, I
don't know how one could have done any wiring that wouldn't have
ended up looking down right ugly due to the places one would
have to drill the holes.
So, it would be wireless.
What I ended up with was a small box that sits on the
actual door bell box and wires in to the bell wiring. It gets a
small amount of power from the bell transformer and can tell
when someone rings the bell. This activates the old telephone
hand set for a few seconds.
By the television, is another box and the base station
for that old telephone. This setup gets its power from the part
of the telephone that originally hooked in to the telephone
line. I connected what was the phone line connection to the
input of a PIC microcontroller which just sits there, waiting
for the signal that the phone is off-hook. When it sees that, it
begins to pulse an output on the processor which is connected to
a red light which is nice and bright. It was meant for
installation on trailers and vehicles so it looks like a tail
light.
I hooked it up Saturday and it all works. My father
likes it so mission finally accomplished.
I am sorry if this message is a bit long. I could have
told you about the fun I had detecting the bell button being
pressed, but we'll save that for another time.
Any wireless transmitter and receiver capable of closing
a pair of contacts would have worked so the old telephone was
used because it was there.
Martin McCormick
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