Spy Stuff !

Hello  Jason and Lewis !

There are many characters inside us, all waiting to get out !. Your characters might be Superman, Spider man, a mad scientist, Doctor Who, or even a James Bond.  Historically spying has been a dangerous activity because before the introduction of electronics spies had to rely on their eyes and ears to gather information ... or 'intelligence' data.  If you wanted to hear what people were saying you had to be very close to them and there was always the chance that you would get caught and end up at the bottom of the moat !. For example if you wanted to hear what was being said inside a hut, you had to be standing under the 'eaves' of the roof, to do so.  This was known as 'eavesdropping' and is still a crime today !.  You had the same problem if you wanted to see what the opposition was doing as you had to be close enough to see, with the unaided eye. If the opposition saw you first, you had a shower of arrows fired at you.  Things became a little more easy when electronics were introduced. 

Telephones.

Telephone 'tapping'.

The first practical voice telephone was invented by Alexander Bell about 1876. Although the telephone was invented in America, Alexander Bell was actually a Scotsman !.   Elisha Gray also filed for a patent for a telephone on the same day as Bell's lawyers and both designs were similar and that raised the question of who might have been copying from who !.  In the early days connections between subscribers were made by lady telephone operators using electrical connection cords on a large 'jack' panel.  Telephone operators were not supposed to listen in to the subscribers private telephone conversations, but many did !. They called this illegal listening 'eavesdropping' even though no roofs were involved. The interesting thing to note is that if the operator could eavesdrop, then anyone else with access to the telephone line could also listen in by connecting a headphone across the telephone line. The illegal connection of a electromagnetic receiver to the telephone line became known as 'tapping the line' or a 'telephone tap'.

Automatic Telephone exchanges.

Legend has it that an undertaker caller Almon B. Strowger suspected that he was losing business because anyone phoning the operator asking for an under-taker was immediately redirected to another undertaker who just happen to be the husband of the operator !.  Strowger was very unhappy about this,  so he sat down and designed an automatic telephone 'exchange' that did not require an operator !.  The Strowger electro-mechanical telephone exchange is still in use in some parts of the world. Strowger did not invent the idea of automatic switching; it was first invented in 1879 by Connolly & McTigthe but Strowger was the first to put it to effective use in 1888.

Leap-frog telephone technology.

When Strowger designed his automatic telephone exchange, it had to work with existing telephones and telephone lines.  When the next generation of telephones were invented, they too had to be able to work with manual, as well as automatic telephone exchanges, as well as all of the old telephones and telephone cables !. When the next generation of automatic telephone exchanges were introduced they too had to also work with all of the previous systems. It was not until after the Transistor was invented in 1947 could a completely new telephone system be considered .... Oh yes ... it also had to work with all previous equipment !. In short all this lead to a standard set of line conditions, voltages etc..

In the exchange there is a 48 volt battery and two 700 ohm relays, and these are connected in series with the telephone cable to the subscribers premises and telephone. The remote telephone also contains an AC electric bell used to indicate an incoming call. We will be covering this circuit in detail because it is important to understand how it works, in order to also understand how many surveillance devices work.

Simplified diagram.

Above are the important elements involved in a telephone connection. On the left is the telephone exchange building and that contains two 700 ohm relays with one side of each connected to a large 48 volts battery. 

In the centre Is the line cable which connects the telephone exchange to the building the remote telephone is in. The resistance of the line depends upon how far away the remote telephone is from the exchange. The further it is, the higher the line resistance.

On the right  is the building and the telephone.

Line voltage 'on the hook'.

The telephone contains a 'hook switch' and more modern telephones have something similar. If the telephone is 'one the hook' (not being used), then no current is flowing in telephone, line or exchange relays. So if we measured the telephone line voltage just outside of the house it would probably be the same voltage as the exchange battery, ie 48 volts. 

Line voltage 'off the hook'.

If the handset is picked up, the hook switch closes completing the circuit. Current will flow from the exchange battery through the top relay, down the telephone line, through the telephone, back down the other telephone cable conductor, through the bottom relay back to the other terminal of the battery. This current does the following things .....

1.      It energises both relays in the exchange connecting the telephone to the number selection equipment in the exchange.

2.    It provides power to make the telephone microphone and earpiece work.

The  voltage actually applied to the telephone by this current depends on how far the telephone is away from the exchange and the type of telephone.  If we measured the line voltage where the cable enters the house it would typically measure 10 volts when the telephone is taken 'off the hook' and 48 volts when the receiver is placed back 'on the hook'.  This is very useful because it allows us to automate our surveillance equipment and in some cases actually power the surveillance equipment from the telephone line (parasitic). We can now consider some practical telephone surveillance devices.

The Parasitic serial Transmitter.

This is probably the simplest type of telephone 'bug'.

To be continued ......