project to record a telephone conversation

[Thunderchild]

I need to buy something cheap or build a project that will allow me to record (onto cassette or computer) the telephone conversation, I'm not after a spy gadjet but an interface circuit and I don't really want to use an FM braodcaster project I have the diagram for one of those already

 

[aquamon]

I need to buy something cheap or build a project that will allow me to record (onto cassette or computer) the telephone conversation, I'm not after a spy gadjet but an interface circuit and I don't really want to use an FM braodcaster project I have the diagram for one of those already

There is free software for this and it records automatically and quietly to your hard drive (from modem) but is illegal in some parts...

 

[Thunderchild]

well my dad needs to be able to use it so the easy way is an output of standard audio level that can be fed to a cassette deck. Yes some listening devices are illegal and i think it is if they are inted for secret use (ie the user whos phone is being bugged is unaware), well ok the person the other end of the line is not to know but this is for our own use as we need to record our conversations over the phone

 

[colin55]

You didn't say if you wanted it to be automatic or not. Here is an automatic circuit:
**broken link removed**

Circuit details on Talking Electronics website.
Here is the simplest circuit:
**broken link removed**

 

[Thunderchild]

so essentially to get the audio from the line it is blocking caps (maybe one enough ?) and a resistor to help prevent loading of the line

 

[colin55]

You need two caps as the tape recorder is earthed when connected to a charger and the phone line can also be earthed.

 

[MrAl]

Hi there,


Yes, two caps in series with two resistors is a good idea. Also, a set of clipping diodes after the resistors to limit the input to the audio amp.
This way you get cheap AC coupling plus limiting. Total of 6 parts: two caps, two resistors, two diodes.
Some places really require a line transformer in there too though, to provide direct isolation, which costs about another 2 dollars USD.
The transformer makes it a bit safer to use.

 

[colin55]

First of all, the caps are not in series.
Secondly they are not clipping diodes.
and thirdly, what is "direct isolation?"
You cannot put a 600:600 isolation transformer into the "Phone Tap" circuit.
And finally; if any voltage strikes the phone line, lots of other things will be "blown off the wall" before the tape recorder gets damaged.

 

[Hero999]

Tandy (RadioShack in the US) used to sell a pick up coil which could be fixed to the back of the receiver with a suction cup. I assume the idea was to connected it to a dynamic mic input. I don't know how well it worked because I didn't buy it.

 

[Externet]

Connect any audio transformer low impedance winding in series to only one of the telco wires before your telephone.

The other winding of the transformer can feed a recorder or inject audio into the line. A couple of counter paralleled diodes to that winding will protect against excessive amplitude of signals.

Do not use capacitors.

 

[AlainB]

Hi,

Here is an old thread about that.

https://www.electro-tech-online.com/threads/phone-line-to-sound-card-adaptor.88269/

I have been using my adaptor since I build it and it is still working very well.

Alain

 

[colin55]

Old phones had a microphone transformer in the base. The coil picked up the stray magnetic flux from the transformer. You could also place the inductor near the earpiece and pick up the conversation. But new phones don't have the microphone transformer or an open magnetic earpiece.

 

[MrAl]

First of all, the caps are not in series.
Secondly they are not clipping diodes.
and thirdly, what is "direct isolation?"
You cannot put a 600:600 isolation transformer into the "Phone Tap" circuit.
And finally; if any voltage strikes the phone line, lots of other things will be "blown off the wall" before the tape recorder gets damaged.

Hello,

Maybe the caps in your version are not in series, but they are in mine.
Maybe the diodes in your version are not for clipping, but they are in mine.
Direct isolation would refer to the kind of isolation you get by using a transformer, as opposed to that which you get by using series caps that only isolate the DC.

Also in my version the clipping diodes are used AFTER the resistors so that they have a significant impedance BEFORE them to work with.

Yes that's true that if a voltage high enough comes in contact with the line that much damage will be done. If a meteor hits the house that will also damage the circuit and diodes wont help there either.