Thursday, April 20, 2006

Telephone FM Transmitter

By leorick  |  4/20/2006 12:04:00 AM 51 comments

Descrition & Circuit

This project is taken from
With some modification, I create my own version of FM Telephone bug complete with stripboard layout.
Theortically, the frequency range is from 88MHz to 94MHz but you may experiment with the coil structure to tune it


Strip Board Layout

Parts List

Diode IN4384 4
Resistor 12k 1
1k 1
200R 1
Capacitor 470p 1
330p 1
25p 1
12p 1
Transistor A933(PNP) 1
Coil *See note 1
*Coil note
7 or 8 turns of 22AWG(diameter .6mm) around 9/64inch(3.6mm) drill bit. Antenna using the same wire 22AWG, 5inch long



With antenna!

Put it online!

Get ready to tune your radio!


fxtreme said...

Why can't you put this at

Sounds like a decent project

Bob said...

Hey this is really cool! And thanks for the link to Electronics Lab I am going to check this out for sure.. //bob

Just like the CIA used to make in the good ole days

how far does it transmit

Hi your blog is nice. I found this site about Reverse Cell Phone Lookup, take a look at it some time.

What is the purpose of this and what can be done with it - Newbie

Transmitting the telephone conversation to FM radio. (spy kit)

kquick said...

what is IN and OUT??

IN, to the telephone line (wall socket), OUT, to the telephone.
It doesn't matter actually. It just a pair label.

mohamed said...

Can any one tell me where can i find this wire cause i searched alot for itwith no success
Thanks alot.

This page should help.
Try to use a coil from old tv booster for example.
If you are not sure which wire to use, just measure the wire diameter using caliper or micrometer.
It should be around 6 mm in diamater.

vinita said...

how do we tune the coil?

johnygfunk said...

I understand the circuit fairly well but the phone line connections are a bit confusing. Phone lines have multiple wires and thus just two ins and two outs is a bit confusing. Anyone mind telling me what wires to connect from the wall to the circuit and from the circuit to the the bugged phone. It would be much appreciated.
fixed spelling errors

"how do we tune the coil?" From my experiment, the output carrier frequency depend on the coil form/structure such as the diameter of the core, how many turns, stretching and compressing the coil layers. This is what I mean by "tune the coil".

Shaunak said...

I have a suggestion,
why not add a SPDT swich with one throw connection the above circuit and another shorted to the trird leg so that YOUR conversations can remain private. ;)

Anonymous said...

You set the transmit frequency by spreading or compressing the coil. This is a trial & error method and may take several tries at it before achieving the desired frequency. Once you are on the desired frequency, you can apply several drips of hot wax on the coil to stabilize the windings. However you may still encounter some frequency drift from such a simple circuit.

Anonymous said...

Sorry to sound stupid but what is the difference between the circuit and the schematic? Obviously they are different but what is the purpose of both?

Sue said...

Hey, I am a way beginner but I would love to learn how to make this. Does anyone know or is able to give me more indepth instructions about how to make this? You can email me at anything would help! Thanks so much!

"Diff. between schematics and circuit",
I'm not sure about this.
Did you try to look at wikipedia (Schematics and Circuit_diagram) yet?
Any language expert here?

Sue said...

i meant the circuit and the stripboard, why are they different? Also how do i connect the phone line to the transmitter if i dont have that nifty little cream colored square that is shown in the picture?

I hope this may help. The circuit above is a simplified conventional pictorial representation of an electrical circuit. The strip board layout is the actual physical assembly layout of the transmitter it self. Click here see the side by side comparison image;

If you don't have a "little cream colored square", try to find a pair of RJ11 male and female like below;

RJ11 Male/Female image

Sue said...

oookay, i think im getting it! :) thanks so much! one more question: what wires go in the fm bug, where it says to phone and to wall, which out of the four wires in a phone line go where?

Anonymous said...

LeorickSimon said...
IN, to the telephone line (wall socket), OUT, to the telephone.
It doesn't matter actually. It just a pair label.
16 February, 2007 18:53

Connect the "OUT" end to the phone and
the center pair of your wall socket (red and green) to "IN" end.
(RJ-11 detail reference).

Martin said...

I recently built very cool stereo transmitter from this site

This telephone transmitter is really small. Is it possible to use phone line as antenna?

BERK said...

hi i m interested in any kind of spy surveillance products and projects.İt can be anything like spyphone,phone spying,software or hardware,room bug,telephone related spy and surveillance devices,remote covert listening,micro recorders,spy gps devices etc.if anyone have any good idea or any project for sale just contact me please.

Alex said...

that sounds cool, if you find out any good ones would you let me know too?

sammy said...

Hey, can i buy a complete kit or assembled one from you? I tried to do it myself, but for some reason it's not working. U can email me at

sammy said...

To add to my comment above, Do u know if it can work with VOIP phones( phones through internet)?
Your comment will be appreciated.

Anonymous said...

What is the difference between SWG and AWG wire.

Anonymous said...

can you modify it even further and take out the transmitter part? i dont have the coil so im going to have to go with a tape recorder or a set of head phones.

If you are interested in wired version, you might wanna take a look here, Inductive Telephone Tap. You could plug it into a microphone input.

Snigger said...

Hi, I can't find 1N4384 diode ; Can I use 1N4007 or something else instead?

Theoretically, yes!
Telephone circuit voltage range from 40V to 150V (20Hz to 40Hz) while ringing. Other than that, 48 volt DC.
1N4384; VRRM=400, VR(RMS)=280, IO=1A
1N4007; VRRM=1000, VR(RMS)=700, IO=1A
VRRM, the maximum amount of voltage the diode can withstand in reverse-bias mode, in repeated pulses.
VR or VDC, the maximum amount of voltage the diode can withstand in reverse-bias mode on a continual basis.
IO or IF, the maximum average amount of current the diode is able to conduct in forward bias mode.
The limit is higher than 1N4384. Why don't you give'm a try?
Read more here;

Snigger said...

Hi again ,
I assembled circuit with 1N4007 and it transmits properly but ...

1. it works just when I connect IN1 & OUT1 to phone line !And not when IN 1 & IN2 are connected to line!

2. When I connect it to line it transmits conversation properly, but when I put back phone on the hook or press Flash button on the phone the connection doesn't finish and it continues while the circuit is connected to the line!.for example if I call and answer machine or a machine responded number like clock announce and put phone on the hook and put it back to my ear ,I can listen to the rest of conversation!

I re checked connections between elements and all are correct!

What's on?

leorick said...

Snigger, for your case, if IN1 & OUT1 go to the wall/phone line then I guess IN2 & OUT2 must go to the telephone unit. From the circuit diagram, this will cause no current flow at all. The circuit will not work since there is no current flow through the transmitter circuit. As you can see from the circuit diagram IN1 & OUT1 go directly to the diode bridge array. IN2 & OUT2 on the other hand, both share the same node (short circuit).

But if you said that it works somehow, I strongly suggest that you should take a better look at the diodes polarity. I hope this might help

Snigger said...

Hi again,
Yeah I know and that is why I'm confused!
It works in that way. and 330pF capacitor is on fly!
I checked connections and diode direction many times, Nothing!
This diagram shows how I connected elements. all are correct !
Can You Check it Once ?
Maybe my brain is dead!!!

Snigger said...

Excuse me I forgot image,
Here it is:

leorick said...

"it works just when I connect IN1 & OUT1 to phone line !And not when IN 1 & IN2 are connected to line!" While testing it with IN1 & IN2 connected to the wall/line which is not working with you circuit, did you connect it with the phone? Phone must be connected to node OUT1 & OUT2 and pickup the handset (off-hook) in order for it to works. BTW, my theory was wrong in previous comment. After thinking it over and over again, connecting IN1 and OUT1 to the phone line meaning to put all phone line load into the transmitter circuit. I never try this before. A picture of of your circuit board may help in our troubleshooting.

Snigger said...

No I'm not connected phone to OUT1 & OUT2 ,
What kind of Tap could it be ?!
What use we have for a transmitter while we can hear via phone itself!!

Ok here they are:


Thanks For help

leorick said...

This circuit was designed to transmit others conversation, something like spy kit :). That is why OUT1 & OUT2 must be connected to the phone and it will be turned on whenever the phone handset is being picked up (off-hook). At least now you found another way to use it (connect to IN1 & OUT1). BTW your circuit grid compression ratio is better than me :) (8 by 11).

Snigger said...

Hi, And thanks again for reply.
So this circuit must be connected to phones in serial?
So I prefer inductive taps, if i have access to wire like that!!

And Thanks again for responding.

Snigger said...

I meant in series when I said serial.

Hi, it is hard to find 1N4384 diode ; Can I use 1N4007 or something else instead?I would liketo crate many of them for resell in my online spystores.

setu said...

this project is really very interesting.
can you plz tell me that from where i can get working of the circuit for my project report.

kernales said...

hi leoric,

are you selling this transmitter?
im located in sg. is it possible?

leorick said...

kernales, sorry to say that I'm not selling it. I built only one unit. the one that you see above.

good project

This looks like a great student project. Did you simulate this circuit as well ? Congratulations and thanks for posting this.

Thank you for the useful information it was looking for listening devices

pawan said...

can you plzz specify how the transmitting frequency can be calculated and what is the exact inductance required for the circuit to transmit in the RF frequency range

leorick said...

7 or 8 turns of 22AWG(diameter .6mm) around 9/64inch(3.6mm) drill bit will you around 100nH.
Check this link

Meawhile 25pF with 100nH resonator will give you around 100MHz

The original design can be found here at;

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