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Spy Circuits – FM Bug Transmitters

Here we’ll discuss how to make an electronic spy bug circuit using two methods, one that consists of wire link from the transmitter to the receiver, and the other which is completely wireless and can be used to eavesdrop a particular conversation over a range of about 30 meters, over an ordinary FM radio.

All the spy circuits presented below are significantly powerful, hard to trace in their hidden positions, and equipped for grasping even the weakest of whispers in the vicinity. Moreover the designs are capable of transmitting the picked information upto radial distances exceeding 2 kms.

The above extraordinary capabilities have forced the legal authorities to enforce stringent laws against the use of these transmitters without permission, so before you make and use one of these make sure you have all the legal formalities completed.

Spy Bug Using Wire Connection:

The proposed simple electronic spy bug circuit is basically a high gain amplifier using the IC 741 as the heart of the circuit and also a couple of high gain output transistors.
The IC 741 if configured as a non inverting amplifier which performs the function of a pre-amplifier stage.
The gain of this IC 741 preamplifier stage may be varied as desired, using the pot across its input and output pin outs.
The gain setting is used to set the sensitivity of the amplifier and is set to maximum so that even low volume speech conversation may be picked through it.
The mic at the input transforms sound vibrations into minute electrical pulses, which is further amplified by the IC 741 to suitable levels before applying it to the output amplifier stage consisting of a standard push-pull stage. This push pull stage is made using a couple of high gain transistors 187/ 188.
Here, the signal received from the 741 output is suitably amplified so that it finally becomes audible over the speaker.

Parts List for the above opamp based spy bug circuit

R1 = 10K,

R2 = 10k,

R3, R4 = 27K,

R5 = 1.5 M,

C1 = 104,

C2 = 220uF/25V,

T1 = 188,

T2 = 187,

MIC = electret mic,

IC1 = 741,Power = 9 volt battery

Headphone = 64 Ohms, or a small speaker of 8 Ohms, 2 inches

Wireless spy bug circuit description:

The purpose of this circuit is also the same as above, that is for catching weak sound signals or for hearing some hidden conversation, however the caught voice signals here is transmitted into the air instead of forwarding it through wire links.

The sent signals can be received over any standard FM radio, tuned accurately to the respective frequency.

The above shown wireless spy bug circuit is basically a small RF transmitter built around a single transistor.

The circuit functions quite like a Colpitts oscillator incorporating a tank circuit for the generation of the required oscillations.

The frequency mainly depends on the positioning and the values of the inductor, C1, C2 and C3. The coil turn distance and diameter may be manipulated a little for optimizing best response over the FM receiver.

A small antenna in the form of a 3 inches wire may be attached at the shown point for making the “bug” highly responsive and generate distortion free signals.

For the 741 circuit, the speaker is only positioned and used as the receiver and may be placed in some other room, where the eavesdropping may be intended to be carried out.

The linking of the speaker from the amplifier circuit may be done through wire connections, preferably by using thin wires and escorting the entire length up to the speaker in some hidden way, probably by laying it under the carpet or across the corners of the room.

For the wireless spy bug circuit everything becomes pretty simple and you just have to hide the transmitter circuit in some suitable place, like under the table, couch, sofa etc.

Circuit Diagram

Parts List for the above wireless spy bug circuit.
R1 = 3k3,

R2 = 100K,
R3 = 470 Ohms

C1 = 10 pF,C2 = 27 pF
C3 = 27pF,
C4 = 102 disc
C5 = 10uF/10V,
Mic = condenser mic
T1 = BC547
L1 = 3 to 4 turns of 22SWG super enamel copper wire, 5 to 7 mm diameter, air corePlease refer the scanned image of the prototype for getting an idea regarding the coil dimensions.

Now let's discuss a few FM spy or bug transmitter circuits which can be built using different configurations and features.

One Transistor Design

You might have already come across a host of these extremely basic one transistor FM transmitter circuits, however these may incorporate certain drawbacks as mentioned below:

No substantial transmitting range.

No enhanced sensitivity range

Use 1.5V for operating which render limited capabilities.

Among the first in the line, which is probably the simplest is shown in the following circuit diagram.

Surprisingly it does not employ a MIC, rather the antenna coil itself performs a dual function of detecting sound vibrations and also transmitting it into the atmosphere.

The design is void of a frequency determining stage and thus does not come under tuned transmitter circuits (we’ll discuss about these later on in the article).

Circuit Operation

The following single transistor FM bug circuit may be understood as follows:

When switched ON, the capacitor 22n inhibits the transistor from switching until it gets charged. A soon as this happens the transistor switches ON via the 47k resistor forcing the pulse through the inductor which feeds back a negative pulse to the base of the transistor discharging the 22n capacitor.

This switches OFF the transistor until 22n yet again charges fully. The procedures take place rapidly generating a frequency across the coil which is transmitted as carrier waves through the connected antenna.

In the course if the coil is subjected to an external vibrational pulse, it’s forced to mount the above explained carrier waves in the air and could be received and retrieved over a standard FM radio positioned and tuned at the same frequency nearby.

The circuit may be expected to work at around 90MHz frequency band.

Using Tuned Circuit

The second example below shows another single transistor FM spy circuit that incorporates a tuned circuit or a frequency determining stage in it.

In the original prototype the coil was created by etching a spiral track layout on the PCB itself, however for optimal gain and performance such etched antenna coil must be avoided and the traditional wire wound type of coil must be employed.

Incorporating Q Factor

Below's another circuit you would like to know about. The circuit basically makes use of the “Q factor” of the tank network achieved from the coil and the capacitor for generating a relatively high voltage. This stepped up potential attributes the circuit with a rather longer range of transmission.

For an improved performance make sure the coil and the capacitor are positioned as close as possible. Insert the coil leads as deep down the PCB as possible in order to make it tightly hugging the PCB. C2 value could be tweaked for achieving even better response from the circuit.

Preferably a 10pF could be tried. The coil is made of 5 turns of 1mm thick super enamelled copper wire, with 7mm diameter.


Better Saturation Capability

The next FM transmitter design is a bit different than the above types. Fundamentally the design could be classified as a common emitter type, unlike the others which are rather common base types with their design.

The circuit employs an inductor at its base which adds a better saturation capability to the device which in turn allows the transistor to respond in a much healthier way.

Adjustable Coil Slug

The next design in the list is much superior to its previous counterparts since it uses a slug based variable inductor.

This enables the transmitter to be tuned by adjusting the slug core using a screwdriver. In this configuration we can see the coil being attached to the collector of the transistor which allows a massive 200 meters range to the design, with a current that may be not more than 5mA.

The MIC stage is isolated from the base with the help of a 1u capacitor and the gain of the mic could be well tweaked by adjusting the series 22k resistor.

This circuit could be rated as the best as far range is concerned however it may lack stability which could be improved, we’ll learn how in the following explanation.

Improved Stability

The stability of the above circuit could be improved by tapping the antenna from one top turn of the coil as shown in the following figure.

This actually enhances the response of the circuits due to a couple of reasons. The antenna gets aloof from the collector of the transistor allowing it to function freely without unnecessary loading, and the slipping of the antenna to the top further allows the relevant side of the coil to get a higher stepped up voltage induced across itself and also the coil generating a higher concentration of transmission power on the antenna.

Although this enhancement may not actual increase the range of the device, it makes sure that the circuit does not get rattled when hand held, or when the grip is encircled close over the circuit inside its enclosure.

Transmitting Music

If you want your bug circuit to transmit music instead of spying or eavesdropping, you would probably find the following design interesting.

The proposed FM transmitter will allow combining a stereo input simultaneously from the source so that the info contained inside both the channels get into the air for an optimal reception.

The design configuration is quite identical to the one that’s discussed above so does not need much of an explanation.

Analyzing a Two Transistor Spy Circuit

Adding a transistor stage to the above discussed single transistor FM transmitters could enable the designs with extreme sensitivity.

An electret MiC itself has a built in FET which makes it very efficient and makes it a stand alone vibration amplifier device. Adding another transistor stage with it enhances the sensitivity of the device to overwhelming limits.

As may be witnessed in the following diagram, the involvement of an extra transistor stage adds up to the gain of the MIC making the entire unit highly sensitive such that it now picks even the sound as low as a pin dropping on the floor .

The extra transistor prevents excessive loading of the MIC thereby ensuring better efficiency to the sensitivity.

Five things that that make the circuit extremely good with it reception are:

  1. The use of a fix capacitor in the tank circuit along with a adjustable trimmer.
  2. A low value coupling capacitor with the MIC sufficient to handle the capacitive reactance of the MIC which may be around 4k at 3kHz.
  3. A 1u coupler is included between the oscillator and the audio amplifier in order to make up for the low impedance rendered by the 47k base resistor.
  4. The coil used is wound practically using super enameled copper wire which ensures higher efficiency than PCB etched type of coil.
  5. The entire circuit could be compactly constructed over a small sized PCB for acquiring better stability and a drift free frequency response.

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

57 thoughts on “Spy Circuits – FM Bug Transmitters”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hello Mr Swag, I bought a BC547 transistor but C547B W73 was written on it.. Is is correct sir or the components seller just sold to me the wrong transistor,, thanks.

  3. Regarding the ckt for transmitting music:
    Can I use it as a mono input by removing the capacitor/resistor from one of the input side?
    Does it need to change values for other components? Thanks. 🙂

      • Referring to Ckt. 3.3 Incorporating Q Factor:
        Can I use it to transmit music instead of mic? and if possible what changes should be made? Thanks

        • yes definitely, all the shown designs can be used for transmitting music or a high frequency content simply by removing the MIC and connecting the relevant points with a music source…

          • Referring to the ckt with the fewest component..what AWG is the coil? Can I use 23 AWG? cause I got 1 here haha..Thanks a lot

            • AWG is the US version for SWG which is the universal version for presenting wire thickness. you can 23 AWG, instead it may be better to check the thickness of the wire…ideally a 0.6 to 0.8 mm thickness works best as per my experience 🙂

  4. what is trimmer
    can we use five pin air trimmer that we use in fm radio
    and sir you also tell me how to use five pin trimmer please

    • praveen, you can try the following circuit, it has a PCB design and all the details regarding the connections.


  5. Sir, i want to ask if i can make changes to the electrolitic capacitors in the last circuit, …by the increasing(not reducing) range(nf) of capacitors

    • Tun, Please do not repeat your question more than once…your questions appears at least 25 to 30 times which becomes very disturbing while moderating.

      there's only one electrolytic capacitor in the last diagram and its value is 1uF…which can be increased

  6. hi SWAGATAM , CONSTRUCT FOR ME A CIRCUIT OF SIMPLE FIELD STRENGTH METER which can measure the freguency radiated by transmit antenna. thanks

  7. Hi,
    I want to design a simple tranmitter and receiver but for a very limited range(around 2-3 feets). And at the same time their operating frequencies should be configurable. Can I do that with the one transistor transmitter and receiver circuits that are shown. How can I configure the operating frequencies.

  8. At the receiver end buzz sound comes means sound is not clear when I use it with 9v adapter. I want to use this circuit with adapter do I have to do change in adapter circuit or in Tx circuit.

  9. Sir I want to use 9v adapter for the 9v fm transmitter circuit. but when I connect it it makes buzzz sound how to clear the noise.

    • what makes buzz sound?

      try 3V first and confirm the working and after that try 9V….or increase the 330 to 1K and test again

  10. Sir you're second bug has an ot of 87.62mhz , i use 50p trimmer , so what coil should i use? What is the dimeter and how many turns ?

    • yes it will catch but from a closer proximity not from greater distances….a normal radio will catch the signals from greater distances also.

  11. sir,
    last circuit me coil ke sath 39p cap fixed.var cap ke jaga trimer use kar sakte hey. trimer ka 3 pin hey.mene trimer kavi use nehi kiye.pin con.kesa hoga plz thora batanese acha hota. mujhe green trimer ka valu nehi pata.apki suggsation ke liye wait karege.

    • yes you can try a 39pf trimmer in place of the 39pf cap in the last circuit.

      use the center and any one of the outer pins for the trimmer.

      you will have to confirm the value from the retailer, he will know better about it.

  12. Hello sir.
    As iam a verry begginer I wanted to ask you couple questions
    In the verry first diagram what are 6t and 15t, also what is the thikness of the antenna and thanks for your great help

    • hello Mohamed, those indicate the number of turns in the coil.

      antenna thickness is not critical, any ordinary flexible wire can be used as the antenna.

  13. Hello sir…..
    Again i have question.
    {{for upper most curcuit}}

    Can i use old mobile's small ANTINA and "copper 22 Swg "wire to coil ?
    IS IT WORK?????
    AND can i use my NOKIA 310 for tunning. On which frequency ??

    • Hello Saurabh,

      for antenna you can simply use a 6 inch long flexible wire, a mobile antenna is not required.

      22SWG will do for the coil.

      for tuning use an FM radio as it would have a better receiving power than a mobile phone.

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