Introduction: DIY FM Radio Jammer Circuit Using LC Oscillator

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About: Masters in electronics from MIT

A very simple FM blocker circuit works within 10 meters of range. Let's build one with theoretical approach.

Supplies

Components required:

1) 2N3904 NPN transistor

2) 1mH inductor coil (air core)

3) Variable capacitor(30pf)

4) 1M, 100K, 10K, 1K and 220R resistors

5) 4.7pf, 10pf, 1nf and 22nf capacitor.

6) 9v battery

7) Mic for audio

Step 1: Story

Most of the jamming devices work on the principle of disrupting the transmitted signal by generating high power noise signal in the same frequency region. This will hamper the receiver to choose between 2 similar signals. In this way both the signal interferes and possibly gives a disrupted signal. Now the received signal has two components, first is main information signal and other is purely noise signal generated in that region. To be more specific about the frequency range, we need to tune the circuit properly.

FM region is the radio frequency region works in very high frequency range of 88.0Mhz to 108.0Mhz. An oscillator with single input can work very fine, the frequency can be tuned through a tank circuit. This idea is the same as used in previous IR jammer circuit but there we are dealing with infrared frequency instead of RF. This article is brought to you by PCBway, one of the leading PCB manufacturers from China. Get hands of the best PCB service just in $5 for the 5pcs. What we are basically designing is a radio transmitter with variable frequency range.

** The ministry said that it has been stated that the use of cellular signal jammer, GPS blocker or other signal jamming device is generally illegal, except specifically permitted by the government of India. That’s why I can only share a low power jammer having a range of at most 10 meters.

Step 2: Selection of Components:

While working with Radio range of VHF band 30-300Mhz, Colpitts oscillator and common emitted oscillator configurations come into mind. To keep it simple and shorter range we are using common emitter configuration on which a tank circuit is placed at collector terminal. I decided to use 2N3904 small signal NPN transistor because of its stable transition frequency on 100Mhz region. I personally recommended to use BF494 if it is available in your region.

Step 3: Circuit Diagram:

This is the most delicate part of this crude radio transmitter, which decides the frequency response of the device. Here we are using 1 milli-henry inductor with a variable capacitor. Tuning the capacitor will help to set the frequency.

The frequency of LC tank circuit is given by: 1/2π√LC

Lower the capacitance will cause in generation of high frequency. But this LC tank circuit produce damped oscillation. Which comes to zero after some oscillation to keep it triggered a transistor with feedback circuit is used. 4.7pf capacitor and emitter resistance is used to keep the transistor stable.

For antenna either we direct couple the terminal with collector point or inductor coupling can be used. The simple solution is to solder a wire of ½ meter on the collector end of the transistor.

Attachments

Step 4: PCB Layouts:

I always prefer to design a high frequency oscillator on a PCB, Because it will reduce the overall noise and give a better look for understanding and designing. While making the design keep the form factor as small as possible. For antenna use a spiral wire of 1/2 meter(any diameter).

Here is my designs if you want to use the same then download them from here. Sign-up to PCBWAY using this link and get free new user coupons to order the PCBs. PCBWAY is dealing in a lot of services like SMT assembly, PCBA, STENCIL, 3D printing and CNC. Try these services from here and see why I prefer PCBWAY for my prototypes.

Step 5: Making of Tank Circuit:

I am using a 19SWG(1mm) copper enameled wire, wrap 5 turn of wire on a 5mm diameter screwdriver.

Keep the winding tight and don’t leave space between wire. The overall inductance is depended on diameter, wire length and space between windings. Now rub both the ends of wire and solder them properly.

Step 6: Tuning the Tank Circuit:

I have a 100Mhz digital oscilloscope where I can barely see the waveform of above 100Mhz but we can adjust the frequency and hence we can adjust the capacitor according to the channel which is playing on radio. It will disrupt the signal with noise and now we can’t hear any information on radio.

Step 7: Working:

As I am using a Mic on the input, it will automatically produce a noise signal. If you don’t want to use mic then connect an AUX cable there with 100nf capacitor. This circuit has a range of 10 meters with ½ meter antenna so this is a very low power system. If you want to increase the power a little bit so that it can work within 20-25 meters, then design both the sections separately. Separate the audio amplifier section from the oscillator section. Use this verified schematics to design that circuit.

I tried a lot of circuit but the PCB files and circuit given in the above section is quite simple and easy to make then this circuit. TRY PCBWAY pcb prototype service and convert your projects into products.