NOTE: This is the (now obsolete) Mark I gadget. The latest and greatest
gadget in this saga can be found
here.
I got a Virgin Mobile phone to see how the plan would work, and it
quickly became apparent there was a flaw. If I don't actually tote the phone
around with me everywhere, I can't hear the ring tone (especially with the
television on).
I started a search for a ring tone amplifier (or some such gadget) that
would work with my phone, and while there are upwards of 4 billion hits for
regular phones, it quickly became apparent that no such thing seemed to
exist for cell phones (unless you want to count all the movies where
terrorists set off bombs using cell phones, but blowing things up seemed
like a bit more notification than I wanted even though I have no doubt I can
find cell phone detonator plans on the internet more easily than cell phone
ring amplifiers - but before the FBI comes around, I hasten to add that I
didn't actually look :-).
I did find this
pen/cell phone ring detector, and started thinking about hacking one
apart to get to the signal that lights the LED. It then occurred to me that
it would be nice to keep it in working condition so I could use it more
normally sometimes, and I thought about just detecting the LED flashing. But
I soon realized, my "Flasher V7" phone (otherwise known as an Audiovox 8910)
has a front display with a backlight that comes on when there is an incoming
call, it seemed obvious that one way to meet my criteria for a ridiculous
ring detector would be to notice the phone display lighting up and forget
the pen detector. For fun, I got one of the pens anyway, but the range is
not too good. Anything more than about 2 inches away, and the pen detector
doesn't notice the phone. Even with the phone and the pen in my shirt
pocket, the phone can be too far away to set off the pen. With both the
phone and the pen next to each other on the desk, I also noticed that
sometimes the pen goes off for no apparent reason (I guess the phone is
contacting the cell tower or something). I'd definitely put the pen in the
category of a novelty, not a particularly useful item.
I found a small photocell and an LM324 op-amp chip. Even more
dangerous, I found an application note for the LM324 showing how to use it
as a photocell amplifier.
I envisioned a small box with a cradle I could leave the phone in while
I'm at home with my Rube Goldberg light detector built into it and some sort
of contraption to make lots of noise (but not an explosion) when the phone
lights up. Another advantage is that I'll always leave my phone in the same
place, so I won't forget where it is. If I keep the gadget near an
electrical outlet, I can plug in the charger too. (So I clearly need to
design the cradle to leave room for the hole in the phone where the charger
plugs in - something to remember if I get that far).
Anyhow, before I can get finished, I have to get started. I dug my trusty
breadboard rig out from under several layers of junk, started plugging in
wires, and was soon able to demonstrate that the sample op-amp circuit
really did work. I could hide the photocell under a little cap and get a few
millivolts output, or remove the cap and get 3 or 4 volts output on the
other side of the op-amp. All very nice, but I'm going to want to use this
to switch things on and off, so a nice true/false kind of signal with square
edges would probably be more desirable. A little more scrounging around the
back room came up with a 7413 schmitt trigger nand gate, and gave me the
first chunk of my circuit:
The square edges are nice, but I'm going to want to make noise long
enough to notice once the phone lights up, and I'll want to "debounce" any
effect of flickering light, not to mention that the Rube Goldberg principle
requires me to add extra circuits even if they turn out not to be
needed. Since my junk pile includes a 555 timer chip, and since the trigger
for a 555 timer is a negative going edge, and since that's what comes out of
my nand gate, the universe is practically demanding that I throw in a 555
timer circuit to stretch the signal out and invert it back to a +5 while I'm
at it (there may also be a federal statute requiring all hobbyist circuits
to include at least one 555 timer chip :-). I also happen to have some big
capacitors lying around from repairing an old motherboard a while back, so
they are the ideal candidate to give me a nice long signal from the
following 555 "monostable" timer circuit:
If I hook the output from the schmitt trigger in circuit 1 to the trigger
input of the 555 in circuit 2, I get a little less than 20 seconds of steady
+5 volt output from the output of the 555 when the photocell gets lit up
(however briefly). I didn't pick 20 seconds - it picked me. I just plugged
in random resistors together with the 1500 uF capacitor I happened to have
until what seemed like a reasonable time showed up. No doubt a smaller
capacitor and bigger resistor could achieve the same thing with less energy
drain, but this was the first combo that worked well for me.
I still don't know what I'm gonna make noise with, but all this time I've
sort of had the idea I'd like to use an old-fashioned electro-mechanical
bell and clapper (just a small one - not an 18 inch fire alarm bell or
anything like that). If I go with something like that, I'll obviously need
to trigger a relay. My junk pile failed me on relays, so I bought a small 5
volt relay at Radio Shack (part number 275-240). Perhaps the 555 could drive
the relay directly, perhaps not, but with some 2N2222 transistors lying
about back in the trusty junk pile, why not add a relay driver circuit while
I'm at it?
Adding this additional junk to my breadboard demonstrates that it also
works. I can hear the relay click (and watch the resistance go from infinite
to zero) when I light up the photocell, and 20 seconds later it clicks off.
If I wait till it is dark outside and turn off all the lights, I can even
demonstrate that the thing clicks on when I light up my cell phone and hold
it over the photocell (a good thing after doing all this work).
Unfortunately, a relay click is even less audible than the ring tone on the
phone. It is time to search for my ideal noise source.
I go online to places like homedepot.com, no old fashioned bells. I
expand my search with google and find several bells that are just what I
want if only I lived in Great Britain. I check a few local hardware
stores, but no dice. Apparently, metal clanging on metal is no longer
appreciated in the United States :-).
My first choice denied me, I just wandered about the local Home Depot
keeping my eyes open for ideas, and I spotted the collection of wireless
doorbells they sell. The kind of bell I wanted to use has sometimes been
used as a doorbell. A doorbell should be designed to be loud enough to hear,
and closing a relay is a lot like pushing a button, so I ought to be able to
hack the push button to substitute my relay (if I'm careful and don't break
it completely). The wireless doorbell has the additional advantage that I
can keep the noisemaker in one place (near the center of the house so I can
hear it from anywhere) and the phone gadget somewhere else (closer to where
I'm likely to be sitting if the phone rings). I decide to buy the model with
64 different tunes (on the off chance that at least one of them won't be
utterly insipid) and the dual buttons (for front and back door), so if I
break one, I'll have another to hack after getting practice in what not to
do.
I get it home and take apart one of the buttons, and find myself
mystified. The actual button is a little black cube with 4 leads. What kind
of push button needs 4 leads I wonder? Nevertheless, I start poking around
with a piece of wire, and determine that shorting out either pair of
diagonally opposite leads on the button results in the doorbell chime going
off. I don't even have to break anything because there is enough lead
sticking out that I can just solder a couple of small wires to one pair, and
leave everything else in the button assembly undisturbed. Having added my
wires to the button, I plugged them into the breadboard across the normally
open relay connectors, removed the cover from the photocell, and
"Dong!", my wireless doorbell went off! Once again,
I waited till dark, and tried it with the cell phone backlight, and once
again, the doorbell went off. I had successfully constructed a hack that
could tell me when my phone rings!
Since this is a camera phone, I might as well take some pictures
of the setup at this point. This is final breadboard with the doorbell
wired into the relay:
This is a close-up of the photocell with the improvised cover in place (a
small piece of tubing with some foil taped over one end:
And here is what the photocell looks like without the cover:
All sorts of silly ideas swirled through my head. One of my favorites was
getting a couple of pieces of nesting pipe that I could rotate to close up
the hole I drop the phone in, but any kind of hole I drop the phone in is
always gonna have a problem with the charging cable getting in the way. In
the end, I decided a simple form fitting depression I could lay the phone
in face down should work if I have something like a felt or soft plastic
gasket to fit around the face closely and keep out ambient light. That would
leave the tail end of the phone exposed (not to mention the antenna) so I
could plug in the charger easily. Even this simple idea leaves room for
flights of fancy. Could the phone stand the heat in a vacuum form machine to
make a mold of the face? (If I could find a vacuum form machine). Could I do
a plaster cast? How about painting it with wax to make a mold for a lost wax
casting of the perfect cradle in bronze? All very silly. In the end I
realized the simplest available low tech custom molding technique is good
old fashioned paper mache (Hey! Grammar school arts and crafts actually
comes in handy... :-). A little plastic wrap to protect the phone from wet
paper and glue, and I should be able to produce a nice tight fitting mold to
keep the light out in no time. If I glue a some cardboard ribs to it, it
should be stiff enough to support the phone.
Here's the result of five or six layers of newspaper and watered down
carpenter's glue, not yet fully dry or trimmed:
I'll give this overnight to get fully dry, but I should mention
that one of those new Glad disposable plastic containers makes a
great thing to mix paper mache paste in (as long as you have lots
and lots of newspaper around when it splashes).
Anyway, here is the shell dried and trimmed sitting on the small
plastic box I found that should hold everything after a bit more
mechanical engineering:
And here we have the box top cut out, the shell mounted, and
a felt pad added to help cut off the light. Also added a groove to
the top of the box so there is room for the charger plug when
the phone is in the shell:
Obviously it is time to see about a different detector, which perhaps I
can wedge into my existing circuit without too much trouble (since a minor
mod to the board would be a lot simpler than building a new one).
Another component I had in my junk pile when I started was a
photoresistor which has almost as big a face as the display on the
phone. Perhaps I can adapt things to use it. The first thing to do is make
some experiments to see what sort of performance I can expect. Wrapping the
phone and the resistor in several layers of black plastic trash bag with
just the resistor leads punched through reveals that the resistor does a
very impressive imitation of an open circuit in absolute darkness, and then
drops down to about 80K ohms when I turn the phone light on. That's the
best I can expect, but to avoid making the same mistake again, I should
try the resistor in the shell I already have. I just need to make a couple
of small holes for the leads to fit.
Did that, and I'm impressed. With the phone plugged tightly in the shell,
I once again get something that looks like an open circuit. Even when I
shine a powerful flashlight around at in various different directions, I
still stay at least 2 M (and for that the light has to come from under the
shell - an unlikely direction for light to leak in if the box is closed.)
When I turn the phone light on, the resistance drops to about 280 K (which
isn't as low as the plastic bag experiment, thus demonstrating the value of
real world tests).
If I don't plug the phone in tight (which is a little difficult since the
shell got stiffer after everything was epoxied into the box), the resistance
is about 2 - 4 M with the phone just sitting on top blocking most of the
light. When I turn the light on, I get about 290 K, so there is still a
large change in resistance with the light on or off.
Now I just need to figure out how use this resistance change to translate
into something that I can feed the op-amp without a lot of changes in the
rest of the circuit (Oh No! I need to understand a little bit about what I'm
doing!). I dredged up distant memories of a wheatstone bridge being used to
measure resistance changes from EE classes, and looked it up on google to
refresh my memory. The LM324 can be used to amplify a voltage differential,
which is what I'll have across the center of the bridge, and that voltage
will vary if one of the arms is the photoresistor, so I may have a way to
salvage most of my existing circuit board. Breadboarding up the wheatstone
bridge circuit (this time with the photoresistor actually mounted in the
shell and in the breadboard circuit so I can test under actual conditions),
I come up with the following substitute detector:
The breadboard circuit successfully generates a large enough voltage to
trigger the schmitt trigger when the light is on, and sits there at almost
zero with the light off. I even tried shining a flashlight around, and no
false alarms. Time to add the resistors to the circuit board and wire in the
new detector.
Unfortunately, when actually connected to a schmitt trigger chip, the
LM324 doesn't work (instead of about 5 V output I see about .6 V). My feeble
brain obviously doesn't understand something about the characteristics of
the 7413 chips. So, forget the 7413. I yanked it out of the socket, and ran
some wires to the breadboard (plugged into the pins of the old 7413 socket)
and hooked up a regular old 7400 nand gate chip. By golly, that actually
works right (well, "right" might be a little strong, but it does function as
desired anyway).
So, with the phone in the shell, if I turn on the light, the relay
triggers as expected. I also tried plugging in the phone charger, and
verified that the little red LED that lights up while the phone is charging
does not trigger the relay (which is a good thing, as I wouldn't want the
alarm to go off just because I am charging the phone). I think my third
variation actually works OK. Now all I need to do is desolder all the leads
to the 7413 socket and convert it to a 7400 socket by reconnecting things
appropriately. (No doubt this whole process would be much more efficient if
I knew what I was doing :-).
While I was playing with this, I also measured the current this whole
thing draws, and it appears to be around 5.5 mA normally, and 130 mA when
the relay is active. Since Duracell's web site has a
graph
of typical service hours with the lowest load being 10 mA, I guess 5.5 isn't
too bad. I'll see how long the batteries actually last in practice.
After getting everything put together, the circuit board modified,
and all pieces soldered in (testing very carefully at each stage), I
learned one more lesson: Cadmium sulfide photoresistors are really really
sensitive to heat! The very last thing I did was shorten the leads to
the photoresistor and solder them on. Apparently I killed the photoresistor
during the soldering (that probably explains why they have such long
leads on them). Fortunately, I had another resistor around, not as big, and
the resistance doesn't drop as much as the original one, but it still works
fine with the rest of the circuit. I soldered it at the very tip of the
leads, and added a big alligator clip as a heat sink while soldering.
Fortunately, I got this one in without destroying it.
So, it is all finished (until I decide I want to make more mods).
Here's the finished box:
and the box opened to see the circuit board:
and the circuit board lifted to see the battery and doorbell remote
hidden in the bottom compartments of the box:
Adjusting the 555 timer would probably be good as well. My original
plan of running a bell needed the 20 seconds, but for just simulating
pushing a doorbell, something like 1/10 of a second would make more sense
(and draw less power in the long run). If the time were shorter, I probably
wouldn't notice the arming switch problem either.
The box also needs work. It is just hinged at one edge, so if I'm not
careful, it falls open when I pick it up. I need a pin or a screw or
something to hold it closed better. It might be nice to keep the circuit
board and battery holder from rattling around so much as well.
Of course, all these adjustments reduce the fundamental Rube Glodbergness
of the device, so I'm leaving it as it is for now.
Also in case you are wondering why the circuits make no sense, I'd just
like to say I have no idea what I'm doing. These are just the results of cut
& paste operations from bits of circuits I found in books or on the
internet interpolated with dim memories of various EE classes from
long ago (though not far away - they were just down the road at FAU :-).
And, as long as I'm giving links, I obviously need to take this
opportunity to point you at my
home page where you will find earth saving political ideas as well
as ways to keep your computer's clock set right :-).
Motivation
This story started with me wanting to get rid of my landline and go with
just a prepaid cell that could easily be 3 or 4 times cheaper (since I make
about 5 minutes of calls a year :-).
Electronics
Anyway, since idle hands are the devil's workshop, and when I'm on
vacation I try to be as idle as possible, I soon found myself poking around
among the electronic junk in my back room and asking myself: "What is the
most ridiculous possible way to build a ring notifier with the junk I have
lying around here?"
Mechanical
Now, before I can declare my independence from Bell South, I need to turn
the breadboard into a finished product. I'll have to rig some kind of
cradle for the phone that excludes ambient light and just illuminates the
photocell with the phone display backlight. Thinking about it, I also
realize that I'll want to make sure the system is deactivated when the phone
isn't in the cradle, or it will probably be going off all the time I'm
trying to talk. Probably will want to have an "arming" switch to keep things
quiet when the phone isn't in the cradle (any kind of microswitch to notice
the phone in the cradle is probably getting too ticky for me). From
electrical engineering, I need to move on to mechanical engineering if I am
to produce a fully functional gadget.
Lesson
So, everything is finished, the circuit board all soldered up, everything in
the box, completely done! Unfortunately, it doesn't work. Even though I was
able to make it work by pointing the phone at it at night, clearly the
conditions are different in the actual box as constructed. Measuring the
output from the op-amp with the phone in the shell shows the voltage barely
flicker when the backlight is on, and it never hits anything like the level
needed to trigger the rest of the circuit. The lesson is that I should
obviously have constructed the shell and tested the photocell with the phone
under actual conditions before committing to the whole design.
Things To Do
The most obvious flaw with my current design is the location of the "armed"
toggle switch in the circuit. I put it on the relay to prevent it from
triggering when the circuit is disarmed (which is good to do when the phone
isn't in the box, otherwise it is going off all the time). The problem with
this is that the 555 timer goes off anyway, so if I flip the switch to armed
soon after putting the phone in, it sounds the doorbell anyway. I should
have put it in series with the photoresistor. That way, when it is
disarmed, it would look like an infinite resistance was in place (i.e. total
darkness), and I wouldn't need to worry about anything else (that would also
probably make sure the minimum power is being sucked from the batteries when
it is disarmed). [Actually, I went ahead and did this - it was too simple
for too much benefit, and it does work much better now].
References
By the way, in case you are wondering, the circuit diagrams
were generated with xcircuit
and the postscript file it produces was rendered into a jpeg image
with gimp. All running on
a Fedora Core 3 system (Gimp comes with fedora, and xcircuit only took
about 10 minutes to download, build, and install). The photos and
sound file were downloaded off the phone using
bitpim with a cable for a LG-VX6000
from Radio Shack and
USB
drivers from futuredial.com.