The Privacy Box

by obitus  (obitus@marmoset.net)

The purpose of this box is to add a measure of privacy to your phone calls.  It does this by blocking your phone when someone else in the house picks up another phone on the same extension.

Theory/Background

This box is based on the Fuschia Box that was included in Hacker's Information Report, Issue #2.

I was not able to get that box working so I set out to make my own, simpler version.

Basically this is the theory behind the device: your phone line has electricity running through it.  When you are talking to someone, the voltage is around 20 or so volts.  When someone picks up another phone in the house, the voltage is cut in half.

The box runs on two 15V Zener diodes.  The diodes only allow the electricity to flow through it if it is above the preset voltage of the diode.  So when there are two phones in the house off the hook, the voltage on the line is only like 10 volts.

That isn't enough to flow through the diodes, which causes your phone to be blocked.  You have to use two Zeners because, depending on how you have the box hooked up, the electricity flows through differently.  With only one Zener, the box would only work 50 percent of the time because the Zener only tests the voltage if the electricity is flowing through it from a certain direction.  From the other direction, the electricity can flow through freely.

Construction

The first thing you want to do is run over to your local RadioShack and pick up a few things.

Here's what you need:

• 1 - Modular phone jack.
• 2 - 15V Zener diodes.  They come in a two-pack.  (1N4744 - Part No. 276-564)
• 1 - Small switch  Such as an SPST micro-mini toggle switch.  The type really doesn't matter - you just want it small enough to fit in the phone jack.
• You will also need a couple of feet of phone cord.

Assembly

1.)  Open everything up and spread it out on a clean workbench.  You will want a screwdriver, something to strip wires with, and these directions close at hand.

2.)  Locate your modular phone jack and open it up.  Inside should be eight screws with eight wires running to them.  The two that we are working with are the RED and the GREEN.

3.)  Unscrew the other screws.  You may want to keep the BLACK and the YELLOW wires.  Cut the rest as close to the socket as you can.

4.)  You should have a RED wire and a GREEN wire running from the socket to two separate screws and six empty holes.

5.)  Move the GREEN wire and screw it into an empty hole.

6.)  Next, solder two short wires to the poles on your switch.

7.)  Then solder the two anode ends of the two Zener diodes.  (The anode end of the Zener is the end not marked with a black stripe - look at the back of the package that they came in.)

8.)  Take your phone cord and cut off one of the plugs.  Peel back the insulation and expose the GREEN and RED wires.  Strip the ends of these wires.

9.)  You will want to screw the RED wire from your piece of phone cord to the screw that is holding the RED wire from the socket.

10.)  Next you will want to screw the GREEN wire from your phone cord to the screw that isn't holding anything at the moment.  One wire from the switch and the cathode from one of the Zener diodes will also be screwed to that screw.

11.)  The other wire from the switch and the cathode of the other Zener will be screwed to the screw that is holding the GREEN wire from the socket.

12.)  Lastly, drill a hole in the cover of the modular jack and push the switch through.  The cover should just snap on.

That was easy, wasn't it?

Use

To use this sucker, just hook it between the wall and the phone.

You will have to figure out which way is "privacy mode" and which way is "bypass mode" if you used the toggle switch.

To do this, call up a friend and tell them to chill for a second.  Flip the switch back and forth.  You should be able to talk to your friend with the switch in either position.

Next, run and take another phone off the hook in the house.  Run back to the phone with the box connected to it.  Flip the switch back and forth.

In one position of the switch, you should be able to talk to your friend.  This is "bypass mode."  A flip of the switch should yield a dead phone.  This is your "privacy mode."

Conclusion

This is a pretty easy box to build.

There is a limited amount of soldering involved, so even the novice phreak should be able to build one.

As I said before, the concept of this box is based on the Fuschia Box article by Axon in Hackers Information Report, Issue #2.  I just simplified the design a bit.

I have found that these modular phone jacks are useful for building boxes in.  They are fairly small and portable.  They can be used to add features to almost any phone.  If you screwed some wires with gator clips attached to them to the same screws that the piece of phone cord is screwed to, you could make a Beige Box that would block your phone if the line you were trying to phreak was in use.