FM Bug
This FM transmitter circuit will allow you to eavesdrop on individuals. The transmitter can be put in any room, and a regular FM radio can be used to listen to the conversation from a lon…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
FM Bug is a electronics project. This FM transmitter circuit will allow you to eavesdrop on individuals. The transmitter can be put in any room, and a regular FM radio can be used to listen to the conversation from a lon…
- Source pages
- 446-448
- Named parts
- 2
- Build goal
- Working, tested prototype
Tools you need
- Digital multimeter
- Wire stripper and side cutters
- Soldering iron with a fine tip
- Current-limited bench supply
Use eye protection, good lighting, and a clean insulated surface throughout the build.
Safety precautions
- Disconnect every power source before changing a connection.
- Check component polarity, pinout, and supply voltage twice.
- Use a current limit for the first power-up.
- Keep liquids, loose metal, and uninsulated wires away from the bench.
Gather, identify, and understand every part.
Use the standardized inventory, then open What's this? to learn each part's role, advantages, limitations, handling, and specifications.
What's this?Image, role, pros, cons, handling & specifications
2N3904
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for 2N3904; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
T1 - transistor stage identified in the circuit
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for T1 - transistor stage identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
Connect one verified path at a time.
Explore the named components and standardized signal flow, then use Source Check to verify exact physical pins, values, and topology before applying power.
Trace this circuit.
FM Bug: interactive parts, standardized terminals, responsive anchored wires, student explanations, and the original circuit reference in one shared system.
Trace before wiring
Follow power, ground, inputs, processing, and outputs in that order. Never guess a pin from package shape alone.
Connect with power off
Make short, labelled connections and share a common ground only where the schematic requires it.
Inspect every joint
Check continuity, polarity, adjacent shorts, and loose connections before the first power-up.
Open all source diagrams and build views 1 visuals
Confirm the hardware-only control path.
This project does not include firmware in the source. The circuit itself provides the required behaviour.
How to connect
- Match every controller label to the circuit view and source pin map.
- Join grounds before signal wires when separate low-voltage supplies are used.
- Keep motors, relays, pumps, and other loads on a suitable driver and external supply.
Common mistakes
Reversed VCC/GND, board-label versus GPIO-number confusion, missing common ground, and charge-only USB cables.
Troubleshoot
Disconnect loads, continuity-test one path at a time, then test with a current limit.
Software preparation
No IDE, board package, library, or firmware upload is required for this project.
If you add a programmable controller as an extension, document its pin map separately.
How to upload code
The original design is implemented entirely in hardware, so proceed after verifying the circuit and supply.
Assemble, deploy, test, and troubleshoot.
Use the complete source notes in build order, then pass the final checks before calling the project finished.
Build in functional stages
- Power and regulation
- Controller or processing stage
- Inputs and sensors
- Outputs and loads
- Enclosure and strain relief
Power up safely
- Inspect unpowered continuity first
- Apply the lowest safe current limit
- Measure supply rails before signals
- Add one load at a time
- Record expected and actual results
Work from simple to complex
- Confirm power, ground, polarity, and orientation
- Compare each pin with the source
- Test inputs separately from outputs
- Replace only one variable at a time
- Power off before every correction
Follow the documented instructions.
These notes come from this project's source and remain in their original order.
Project overview
Project build noteThis FM transmitter circuit will allow you to eavesdrop on individuals. The transmitter can be put in any room, and a regular FM radio can be used to listen to the conversation from a long way away. The circuit is built around a single transistor 2N3904 (T1), a custom-made coil (L1), three capacitors (C1 through C3), a trimmer (VC1), two resistors (R1 and R2), and, of course, a condenser microphone (MIC1). The circuit sends signals between 88 and 105 MHz. The range of transmission is 100 meters.
Figure: FM Bug circuit
Project build noteThe way the circuit works is simple. It uses analogue modulation, in which the message signal changes the carrier signal. The microphone picks up the sounds around it to make an electrical signal that matches the sound. This is the signal for the message that needs to be sent on the FM band. The signal for the message is sent to the base of transistor T1. The carrier frequency is made by the tank circuit, which is made up of trimmer VC1 and coil L1. Using the trimmer, you can change this frequency. The audio signal from the condenser microphone is added to the carrier signal made by the tank circuit by the transistor T1. This signal with changes is sent through the antenna (ANT.). Using VC1 to adjust the carrier frequency in the FM band and an oscilloscope to check it, when you tune the frequency of the FM radio set to match the frequency of the carrier, you will hear the conversation that MIC1 picked up. Make the coil L1 with a 25SWG wire length of about 25cm.
Wrap the wire eight times around a cylinder with a diameter of 6 mm, then take it out. Put the circuit together on a general-purpose printed circuit board (PCB) and put it in a suitable cabinet. Fix the S1 switch on the front of the cabinet after the soldering is done right. Make sure the oscillator is tuned right. By adding a dipole antenna to the FM bug transmitter, the range will be increased. A 3V battery powers the circuit.
You built FM Bug.
You followed the full workflow from understanding the mission to testing the finished project. That is a real engineering achievement - well done.
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