All files are for educational and/or historic purposes only. [back to library]

ÿþHow to build a black box?

 I found this tutorial quite a long time ago and I don't know if it still works. Nevertheless , it might be interresting to read, you might even learn something from it! I would have gladly given credit to it's author but won't since I have no idea who he is. So here is his tutorial that remained unchanged ( I hope!):


is a device that is hooked up to your phone that fixes your phone so that when you get a call, the caller doesn't get charged for the call. This is good for calls up to 1/2 hour, after 1/2 hour the phone Co. gets suspicious, and then you can guess what happens.

What this little beauty does is keep the line voltage from dropping to 10v when you answer your fone. The line is instead kept at 36v and it will make the fone think that it is still ringing while your talking. The reason for the 1/2 hour time limit is that the

phone CO. thinks that something is wrong after 1/2 an hour of ringing.

All parts are available at Radio Shack. Using the least possible parts and arangement, the cost is $0.98 !!!! And that is parts for two of them! Talk about a deal! If you want to splurge then you can get a small PC board, and a switch. There are two

schematics for this box, one is for most normal fones. The second one is

for fones that don't work with the first. It was made for use with a Bell

Trimline touch tone fone.

** Schematic 1 for most fones **

** LED ON: BOX ON **

FROM >--------------------GREEN-> TO

LINE >--! 1.8k LED !---RED--> FONE


! !



Parts: 1 1.8k 1/2 watt resistor

1 1.5v LED

1 SPST switch

You may just have two wires which you connect together for the switch.

** Schematic 2 for all fones **


FROM >---------------GREEN-> TO

LINE >------- ---RED--> FONE

! LED !


! !



ýÿParts: 1 2 watt resistor

1 1.5v LED

1 DPST switch

Here is the PC board layout that I recommend using. It is neat and is very easy to hook up.

Schematic #1 Schematic #2

************** ****************

* * * ------- *

* --<LED>--- * * ! ! *

* ! ! * * ! <SWITCH> *

* RESISTOR ! * * ! ! ! *

* ! ! * * ! ! / *

* -------- ! * * ! ! *

* ! ! * * ! <LED>! / *

* --SWITCH-- * * ! ! *

* ! ! * * ! ! / *

L * ! ! * F L * ! ! ! * F


N>-----GREEN---->N N>-----GREEN------>N

E * h * E E * * E

************** ****************

Once you have hooked up all the parts, you must figure out what set of wires go to the line and which go to the fone. This is because of the fact that LED's must be put in, in a certain direction. Depending on which way you put the LED is what controls what wires are for the line & fone.

How to find out:

Hook up the box in one direction using one set offor line and the other for fone.

*NOTE* For Model I switch should be OFF.

*NOTE* For Model ][ switch should be set to side connecting the led.

Once you have hooked it up, then pick up the fone and see if the LED is on. If it is, the LED will be lit. If is doesn't light then switch the wires and try again. Once you know which are which then label them. *NOTE* - If neither directions worked then your

ýÿlable the switch in its current position as BOX ON.

How to use it:

The purpose of this box is for poeple who call you not to pay so it would make

sence that it can only be used to receive! calls. When the box is *ON* then you may only recieve calls. Your fone will ring like normal and the LED on the box will flash. If you answer the fone now, then the LED will light and the caller will not be charged. Hang up the fone after you are done talking like normal. You will not be able to get a dial-tone or call when the box is on, so turn the box *OFF* for normal calls. I don't recommend that you leave it on all the time, as you don't want it to answer when Ma Bell calls!

Hope it brought something to those who read it.