Here is the schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug.
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| PC Board Layout |
Part Total Qty. Description Substitutions
C1 1 0.001uf Disc Capacitor
C2 1 5.6pf Disc Capacitor
C3,C4 2 10uf Electrolytic Capacitor
C5 1 3-18pf Adjustable Cap
R1 1 270 Ohm 1/8W Resistor 270 Ohm 1/4W Resistor
R2,R5,R6 3 4.7k 1/8W Resistor 4.7K 1/4W Resistor
R3 1 10k 1/8W Resistor 10K 1/4W Resistor
R4 1 100k 1/8W Resistor 100K 1/4W Resistor
Q1, Q2 2 2N2222A NPN Transistor 2N3904, NTE123A
L1, L2 2 5 Turn Air Core Coil
MIC 1 Electret Microphone
MISC 1 9V Battery Snap, PC Board, Wire For Antenna
Notes
L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it.
C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
Q1 and Q2 can also be 2N3904 or something similar.
You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
You may want to bypass the battery with a .01uf capacitor.
An antenna may not be required for operation.
A thanks goes out to Mike_Springer@iname.com for cleaning up the original image once displayed on the page. It looks a lot better now!
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Owen FM Transmitter Thursday, December 09, 2010 9:33:35 AM
Is there any matched receiver circuit? I would like to build a walkie talkie type thing.
Th3_uN1Qu3 FM Transmitter Tuesday, November 23, 2010 1:25:00 PM
The second inductor is for decoupling the antenna from the tuned circuit so that moving the antenna does not affect the frequency (a lot). No matter what you do to it this type of circuit will never be stable. Best way to transmit in FM now is to get a car FM modulator with audio input or a Belkin Tunecast and hack that up. With an amplifier using a single RF transistor it is easy to boost the output of those things to several watts. Be on the lookout for the car modulators with LED display and no fancy features, the ones with complex LCD displays tend to have whistling noise that you can't get rid of.
maggie FM Transmitter Tuesday, November 09, 2010 7:25:01 AM
can some one tell me the use of the second inductor connected to the antenna. if i remove the second inductor and connect antenna directly will it work? thanks
omkar FM Transmitter Friday, October 15, 2010 9:20:22 AM
i have found many ckts for a radio transmitter, will anyone tell me how many ckts do we have for a single device coz i m not able to understand which one to choose. i have already tried to make a radio transmitter from two different types of ckts but none of them worked.........plz someone help me with this
Basit khan FM Transmitter Wednesday, October 13, 2010 10:16:22 AM
can u run this on 4.9 Volts. will it work if not can u suggest a circuit that works on this much input
ciaran FM Transmitter Friday, September 03, 2010 8:00:10 PM
currently working on this one with my group for our subject Communications..while the other group in my class was assigned to make an AM receiver...
RAHUL FM Transmitter Wednesday, August 18, 2010 8:09:30 AM
HEY WORKING REALLY GOOD...!! BUT HAS VERY SHORT RANGE... FOR THOSE WHO ARE HAVING PROBLEM ... REGARDING INDUCTOR: 1.BUY ENAMEL COATED MAGNETIC WIRE TO BUILD INDUCTOR. 2. CUT WIRE INTO TWO PIECES : 8cm EACH. REMOVE ENAMEL COATING FROM BOTH ENDS OF WIRES.& CHECK THE CONTINUITY 3. TAKE A PEN REFILL (LEXI,CELLO ,TECHNO TIP.ETC ) WHICH HAS OUTER DIAMETER OF 3.2mm approx. 4. WIND WIRE ON REFILL (5 TURNS). 5. DONT BEND WIRE WHILE WINDING, REGARDING CAPACITOR: 1. IF YOU DONT HAVE VAR. CAPACITOR THEN CONNECT ANY FIXED CAPACITOR (disk) BETWEEN RANGE OF 3-18pf. REGARDING ANTENNA: 1.YOU CAN CONNECT SIMPLE WIRE ALSO. REGARDING RECEIVER. 1, IT WORKS VERY..WELL WITH CELL PHONE RADIO RECEIVER DUE TO HIGH SELECTIVITY.( VERY CLEAR.) TUNE IT IN AUTOMATIC MODE. 2.WORKS VERY VERY POOR WITH TRAD. RADIO RECEIVERS. REGARDING TRANSISTOR: ...I USED 2N2222A . DRAWBACK: 1.SHORT RANGE. BEST SOLUTION: 1.BUY LESS TOLERENCE COMPONENTS. FOR MORE COVERAGE.. 1. PUT POWER AMPLIFIER BEFORE ANTENNA. (FOR POWER AMPL. .....SEARCH ON GOOGLE ).
anonymous FM Transmitter Wednesday, June 16, 2010 11:14:01 PM
For those asking to further the range. You can easily amplify the signal, but after grabing a certain distance the fcc will get on your tail.
fm TRASCE FM Transmitter Thursday, June 10, 2010 2:47:38 AM
DOES NOT WORK!!!! i have put 18 AWG coils! What to do!
anonymous FM Transmitter Thursday, June 03, 2010 11:38:36 PM
Im a thirteen year old and i want to build 1
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