FM Bug Transmitter Mint Box

[ThePyroElectro]

FM Bug Transmitter Mint Box Project Write-up

This project creates a very tiny FM transmitter with only 17 electrical parts and then shows you how to stick it into a slightly modified mint box to create the ultimate FM Bug. The transmission range is up to 100 feet indoors and even more outdoors.

Questions & Comments?

[blueintegral]

You mentioned that this circuit won't work on a breadboard. Why is that? Is it because the connections between components get so long that you need to worry about inductance?

[ThePyroElectro]

Hi,

There will end up being too much capacitance in the system because breadboards are basically large plates aka capacitors and when you reach higher frequencies (especially right after 500 MHz) this has a drastic effect on how the circuit works.

Since the LC oscillator is sensitive enough on the PCB, this is case enough that it won't work reliably on a breadboard, especially since we're using such small capacitance values to get to the FM frequencies.

The length between components is not as critical as you might think at 100 MHz, but it is something that you must eventually take into consideration in more complex circuits.

[Bazooka]

The FM transmitter worked like a charm. I wanted to transmit the music from my Mp3 player, so I used an audio jack instead of the microphone. It works just like I wanted.... ..Thanx.

[ThePyroElectro]

Sweet! Do you have any pictures?

[ashraf2011]

hi Chris,,i`v done all the circuit but i have a problem with the oscillator part,,the 6.8 p capacitor is not available in here,,so
i would use a 4 pico farad capacitor across the collector and emitter of the transistor and 27 pico farad parallel to the coil,
i make a coil with 9 turns, 0.5 mm wire diameter, 9 mm long,,approximately 0.1 uh so the l=0.1 uh, c=27+4?? like yours 6.8+6.8 ?? is the two 6.8 capacitors forms the oscillation frequency with the coil?? so that my 27 and 4 pico forms oscillation with the coil??

with my design and based on calculation ,, the frequency=1/[2pi * sq-rt( L*(27+4))] =90.4 MHZ

but the sound i received in the radio is not clear just a vibrations when i talk,the mic is new ,i used 3 volt coin battery, can i use 9 volt battery?? and the antenna i used is not 140 cm about 50 cm and it is copper insulated but without a plastic cover like the wire in the solenoid.

what would the problem be in??

the antenna? the Battery ?? the oscillation ??

[ashraf2011]

now i modified the coil inductance in the circuit ,i make a coil with 5 turns,2.3 mm long and 10 mm diameter to get .09uh
and capacitors for resonance 27 p and 4 p capacitor (connected between emitter and collector)
i successfully received the FM on the radio with at 95 MHZ but the problem is the distance is very short between transmitter and receiver
not exceed 20 cm.

the question is how to enlarge the distance???
ANY SUGGESTIONS PLEASE?

[ThePyroElectro]

now i modified the coil inductance in the circuit ,i make a coil with 5 turns,2.3 mm long and 10 mm diameter to get .09uh
and capacitors for resonance 27 p and 4 p capacitor (connected between emitter and collector)
i successfully received the FM on the radio with at 95 MHZ but the problem is the distance is very short between transmitter and receiver
not exceed 20 cm.

the question is how to enlarge the distance???
ANY SUGGESTIONS PLEASE?

Unfortunately the simplicity of this transmitter circuit means its not very reliable. Even though you've used accurate capacitor and inductor components, the way in which you solder them all together will also effect performance.

3 things that might help from the problems you've described:

[1]
Use an actual Quarter Wave antenna for 95 MHz (2 feet, 5.5 inch or 0.75 meter) and test how it works when you point the straight line quarter wave antenna in different directions.

[2]
Modify your RF coil like an accordion to change inductance and see if you notice any change better or worse.

[3]
Change your capacitors to be a little larger value if you have some laying around.

I didn't get the FM transmitter working on my first try, I went through 3 different PCB's before I was able to achieve the results seen in my project article. So don't beat yourself up if its taking time to get the results you want...RF electronics is a difficult field to navigate.

[ashraf2011]

hi Chris,,i have modified the antenna to be 40 cm long and now i can receive the signal 5 meters long,, the longer antenna the worst signal i receive .i also tried an audio input from mp3 played and i received it on radio, but the sound wasn't pure there was a distortion in the signal. here is a pictures of mt circuit.

i will try to make a pcb for that project ..i hope the performance be better isn't it?

[ThePyroElectro]

hi Chris,,i have modified the antenna to be 40 cm long and now i can receive the signal 5 meters long,, the longer antenna the worst signal i receive .i also tried an audio input from mp3 played and i received it on radio, but the sound wasn't pure there was a distortion in the signal. here is a pictures of mt circuit.

i will try to make a pcb for that project ..i hope the performance be better isn't it?

Thanks for showing us your pictures. And yes, using PCB will definitely make the transmitter work better!