# FM 'bug' transmitter help (Pleeeeeease? please?)

Discussion in 'Homework Help' started by OneForTan, Nov 30, 2013.

1. ### OneForTan Thread Starter New Member

Nov 30, 2013
3
0
Actually I'm Working on this project assigned in college.
I gathered all other equipment other than the wire mentioned here.
The only wire available in neighborhood market is 30 SWG, whereas this requires 25 SWG.
If I use 30 SWG, will it affect the circuit tremendously?
Second, does this circuit work on breadboard?
Third, How the antenna should be soldered to coil? in circuit the antenna is in no direct contact with coil.
How to calculate frequency of the transmission & length of antenna?

Do we have to adjust trimmer to vary capacitance after all connections?

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2. ### Ramussons Active Member

May 3, 2013
562
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If I use 30 SWG, will it affect the circuit tremendously? No

Second, does this circuit work on breadboard? Yes, but the "calculated" frequencies will be different due to stray capacitances.

Third, How the antenna should be soldered to coil? in circuit the antenna is in no direct contact with coil. One end of the capacitor C4 is extended as the antenna

How to calculate frequency of the transmission & length of antenna? You need to do a basic study on Oscillators, Tuned Circuits, Antenna, ....

Do we have to adjust trimmer to vary capacitance after all connections? Yes. The purpose of a Trimmer (or any preset) is to "adjust" the value to get the final result.

Ramesh

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3. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
These one-transistor circuits are hard to make them working but it is possible. I the voltage from the battery is changing, the tune will change too.

I built two transmitters- one with 3 and one with 7 transistors and both occupied me for a day.

4. ### DickCappels Moderator

Aug 21, 2008
2,775
673
And #25 wire should work too. You might have to change the length of the coil a little bit (squeeze or stretch it) to get the tuning range into the FM band, and that would be the case regardless of wire size.

Please don't do it on one of those plug-in plastic breadboards. Those are good for a few MHz, but at 100 MHz, you are likely to have some "extra" effects from the parasitic inductance and capacitance. If you don't solder, you can just twist your component leads together; that should be much better than one of those breadboards.

As for an antenna, you use no antenna, but if you do make an antenna, keep it to several cm or less because the antenna will load down the oscillator, and it large enough, could stop the oscillator. Also consider that with an antenna attached, the longer it is, the more susceptible the circuit is to having it frequency "pulled" by capacitance to other objects. Then there is the radio interference issue...no antenna works find for short distances, and a few cm is probably optimum.

5. ### OneForTan Thread Starter New Member

Nov 30, 2013
3
0
Well, after some shitty calculations frequency turns out to be 2.25 MHz.
If, I take that trimmer of 50p into consideration. should i consider 1n capacitor? I mean, for calculating frequency, with the 'L' of the coil, which 'C' should I consider?

6. ### OneForTan Thread Starter New Member

Nov 30, 2013
3
0
I constructed the circuit & of course, it failed. Can someone help me about calculating frequency of transmission? In the formula 1/(2π√LC), what is "C" in MY circuit attached above? Please help me!!!!

7. ### DickCappels Moderator

Aug 21, 2008
2,775
673
The 50 pf capacitance seems a little large, but with the right inductor, the circuit should oscillate in the FM band, and there is little chance of it oscillating as low as 2.5 MHz.

http://www.qsl.net/in3otd/indcalc.html

How to calculate the oscillator frequency of a Colpitts oscillator. Note that you will have to estimate or better yet, use the datasheet to find the capacitance contributed by the transistor.

http://en.wikipedia.org/wiki/Colpitts_oscillator

http://www.zen22142.zen.co.uk/Analysis/colfr.htm

For reference:

Note: It is recommended that 4.7k resistors be used on the audio inputs.

Last edited: Dec 6, 2013
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