Project: Mini Spy FM transmitter

Thread Starter

chzulfiqar4

Joined Sep 7, 2007
4
Parts List

R1#10K ohm
R2#47K ohm
R3#10K ohm
R4#100K ohm

VC6# (Trimmer) 6pF to 8pF
C1#4.7uF
C2#0.02uF
C3#0.02uF
C4#6pF

Q1# (Transister) C828 or C945

Mic 2pin

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This transmitter can be made very compact,my designed PCB is about 1.5" x 1"
It can be hide any where..even in a match box.
You can tune it on any FM radio..if voice is not clear vary the VC6 slightly
 

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bloguetronica

Joined Apr 27, 2007
1,372
What is the frequency and range? I am interested, but in my country is forbidden to have a range more than 30m on commercial frequency range. However, if it is in the amateur band (CB, I think), then no problem.
 

Mazaag

Joined Oct 23, 2004
255
Could you please explain to me the different stages of this circuit ? Like, what is required by a transmitter, and where it is in the circuit... This will help me and others understand the circuit a bit better..


Thanks
 

SgtWookie

Joined Jul 17, 2007
22,210
What is the frequency and range? I am interested, but in my country is forbidden to have a range more than 30m on commercial frequency range. However, if it is in the amateur band (CB, I think), then no problem.
Without doing any extensive analysis on the circuit, my SWAGS (Scientific Wild-Arsed Guesses) are:

1) Operates in the commercial FM band and fringes, approx. 86 MHz - 108 MHz
2) The range of this transmitter would exceed your allowed distance by a significant amount.
3) Wouldn't be useable on a CB receiver, since they are AM and SSB, not FM.
 

bloguetronica

Joined Apr 27, 2007
1,372
Without doing any extensive analysis on the circuit, my SWAGS (Scientific Wild-Arsed Guesses) are:

1) Operates in the commercial FM band and fringes, approx. 86 MHz - 108 MHz
2) The range of this transmitter would exceed your allowed distance by a significant amount.
3) Wouldn't be useable on a CB receiver, since they are AM and SSB, not FM.
If it is FM and in the commercial range, then I can't use it (unless I shield one room with aluminum paper). The transmitter didn't looked that powerful.

Thanks for answering my doubt!
 

SM5JAB

Joined Oct 1, 2007
5
Could you please explain to me the different stages of this circuit ? Like, what is required by a transmitter, and where it is in the circuit... This will help me and others understand the circuit a bit better..

Thanks
The actual transmitter is the oscillator around the transistor. The LC-parallell circuit determines the resonance frequency and the cap between collector and emitter gives enough feedback for this to resonate (become an oscillator). Changing the voltage on the base causes a frequency deviation (thus FM-modulation). So the transmitter is as far as the two base resistors. You could cut vertically through the schematic and have a pure transmitter.

I imagine another transistor for modulation would also be a good thing unless a mic is used that gives some voltage out - electret with internal amp for example.
 
first: You can tune it on any FM radio
then: Wouldn't be useable on a CB receiver, since they are AM and SSB, not FM.
now:) i want an answer before i get to the store ;)
thx
 

Audioguru

Joined Dec 20, 2007
11,249
The Micromitter uses a fairly new line of FM stereo transmitter ICs. Some can be tuned with data from a PIC.
The Micromitter has a quartz crystal in a phase-locked-loop frequency synthesizer circuit for crystal accuracy of the desired frequency which is selected by a DIP switch.
It has pre-emphasis (treble boost like radio stations have) to match the de-emphasis in every FM radio.
It is stereo.

One circuit has a pretty high RF output. The other has an output attenuator so it doesn't cause interference when it transmits an MP3 player in a car to the car's FM radio.
http://electronics-diy.com/schematics/BH1417_fm_transmitter.htm
http://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php
 

gee_emm

Joined Jan 16, 2008
34
I built that circuit awhile back when looking for things to make. And had one hell of a time tuning it below 108. I ended up changing the var. cap. to an adjustable coil by adding more turns, and using a small screw inside of it. Worked out pretty good. :)
 

Eyas

Joined Dec 23, 2007
10
Good day everyone

I and two of my friends have made an FM transmitter as a project of a university course. I attached the PDF file we used to build the circuit. However, we found that the values given in that file will make the circuit works in frequency above 108MHz. I can give you the project report where you can find what we have done and what values we changed to get better performance and to select the proper frequency "like the LC tank circuit" and small explaination of how the circuit works. We also trace the signal using an oscilliscope. Well, if you want the report please mail me.

GOOD LUCK
 

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Weevic

Joined Feb 2, 2008
1
This is my 1st post on this forum so bear with me. A good rf transistor for 88-108 in a bug would be a ferranti ztx300. this micromitter is good! nice to see some decent small phase locked looped bug kits.
 

Audioguru

Joined Dec 20, 2007
11,249
VC6# (Trimmer) 6pF to 8pF

^ does that stand for Variable capacitor? and what does i mean by (trimmer)?
Yes, VC stands for "variable capacitor".

A trimmer capacitor is small and is adjusted with a plastic screwdriver. A metal screwdriver will add stray capacitance and change the frequency of the tuned circuit.

6pF to 8pF is hardly any change. A trimmer capacitor will be rated for 5pF to 30pF or more.
 

DickCappels

Joined Aug 21, 2008
6,317
If you want to reduce the range, just make the emitter resistor larger - say 470 Ohms, and/or reduce the battery voltage -I have used 1.5 volt penlight cell with success, and/or omit the antenna -the inductor will radiate just fine.

BTW, its a Colpitts oscillator. You can use FM on the Citizens Band -I did that many decades ago, and if its narrow band then, you can receive it on a CB receiver by using slope detection (tuning to the side of the center frequency). If you want to transmit on CB, then you should really be going AM, DSB, or SSB, IMHO.
 
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