Phruwt Issue 04 008

eZine lover (@eZine)

Published in 

Phruwt

 · 22 Aug 2019

  

			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