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Phruwt Issue 04 008
Improved Silver Box
After looking at the box plans that were available on
silver boxes, and deciding that they weren't all that useful,
I've written this short phile on a method to make a generic
silver box, which should work on most phones. Although its
only been tested on desk type phones, the principals can be
applied to any touch-tone phone.
Intro.
A silver box is a device that can produce the Dual Tone
Multifrequency (DTMF) tones that are associated with an
extended keypad. That is gives you four extra buttons
(labeled A, B, C and D), which are not normally assessable.
If you want to know what these frequencies are look in the
alt.2600 FAQ.
Basic Principle
The DTMF tone is generated by an IC which sends dual
tones down the phone line. One of the tones (the low
frequency one) corresponds to the row on the keypad, and the
other (the high frequency one) corresponds to the column on
the keypad. So when a key is pressed on the keypad the tone
you hear is made up of two separate tones. On a normal
keypad there are four rows (labeled R1 to R4) and 3 columns
(labeled C1 to C3). If the button "1" is pressed this is
indicated by output from R1 and C1. These outputs are fed
into the DTMF generator, which produces the correct tones
according to row and column inputs. On all the DTMF
generators I've encountered there is also an input for C4
(for example with the DTMF generator TCM5087 this is pin 7),
so all one has to do to generate the correct tones
corresponding to A, B, C or D is send a signal into R1-C4 to
generate A, R2-C4 to generate B and so on.
Putting this into Practice
You'll Need:
A Phone (make it cheap, say from Goodwill, about
$5. Desk ones work best)
4 DPDT switches
Some wire
Solder
Volt/Ohm meter is also useful
First thing to do is take apart the phone, and you
should find several sections in there. The ones of interest
are 1) the keypad and
2) the DTMF generator. The DTMF generator will be the chip
nearest the keypad, and will have a 3.579 MHz crystal wired
to it.
After you've identified the parts you need to look at
the keypad. There will be at least 8 pins with the keypad,
one carrying the signal in, the 7 out (4 rows and 3 columns).
Sometimes there are 9 pins, with the ninth pin used for C4,
but isn't wired up. You'll want to identify which of the
pins is carrying the signal into the keypad. One method to
do this which I use, and is quick, is to plug the phone in
and with a voltmeter connect one clip to earth (say that wire
on the plastic sheet which goes around the keypad) and then
test each pin connecting the keypad. One will give you a
signal of about 3.2V (this may vary). This pin is the live
pin (it was pin 5 on the ones I've used). Know you want to
find outwhat the other pins are for. Put the meter into
resistance mode and connect one clip to the live pin. Then
hold down key "1" and test the other pins for no resistance.
This will give you 2 "hits", R1 and C1. Go through the other
keys until you have the pins for R1-4 and C1-3. An
alternative method is to pull the keypad apart and follow the
wiring around and figure it out that way (not very elegant
though).
Now we want to follow the pinouts to the DTMF generator
and find out which pin they go into. This should present no
problem as there are no components between the pins of the
keypad and the chip. You'll notice that the chip inputs seem
to go in order, i.e. R1-14, R2-13, R3-12 and R4-11. This may
help you find out where C4 goes (look at C1, C2, and C3). If
that doesn't appear to be working try looking for a pin on
the chip that goes nowhere. This will be C4. Alternatively
if your phone has a 9-pin output from the keypad, follow the
unconnected pin, and this will lead to C4.
You should now have enough information to put together a
silver box. To complete the box take the 4 switches and
connect them to the live pin (all 8, its easiest to run one
wire from the connection between the keypad and the printed
circuit board and one pin of one switch, then connect each
pin in series with the one next to it ie 1 to 2, 2 to 3....7
to 8. Then for switch one (A) run one output to R1 and the
other to C4 (again its easiest to solder the wire to R1 at
the connection between the keypad and the printed circuit
board). A quick crude diagram would look like this:
R1 R2 R3 R4
| | C4 | |
| | | | |
| _____________ | ______|______ | ___________ | _
\ \ \ \ \ \ \ \
\ \ A \ \ B \ \ C \ \ D
\ \ \ \ \ \ \ \
| | | | | | | |
| | | | | | | |
| | | | | | | |
____________________________________________________
|
|
|
Live Pin
Then just take the keypad plate in the phone and cut 4
holes for each of the switches to go through, and reassemble
the phone, which should now be a fully working silver box.
If anyone has question/queries/comments/threats/sisters
phonenumber I can be reached on the PHRuWt mailing list. For
a more information check out Understanding Telephone
Electronics by Stephen J. Bigelow for a nice introduction on
how a phone works.
Standard disclaimers apply.
Nixx
