This simple FM (frequency modulation) transmitter is powered only by a 1.5V battery and uses only 1 transistor.The frequency of this transmitter is controlled by the L-C resonance circuit and operates from 80 to 110 MHz.
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| simple FM transmitter schematic |
The inductor L1 is made of 8 turns of #22AWG magnetic wire wound with the diameter of 4-5cm or diameter of a pencil. The antenna is a 6 inches copper wire connected at the middle of L1 inductor. Other parts
are not critical and can be replaced by its closest value. Resistor are 1/4 watt type and capacitors are ceramic except the 10uF electrolytic capacitor. 5-60pF capacitor is a trimmer type or variable type.
Note: Modifying this simple fm transmitter circuit such as increasing its power capacity is illegal.Review the fcc limits regarding fm transmitter if you are planning to build a more powerful one.
Notes by Acme Fixer:(Thanks!!)
1. Add 0.01uF capacitor across 1.5V supply near the transistor.
2. You can remove the antenna.
3. Mic is an electret condenser microphone type.
4. See the rest of his recommendation based on his experience in building the circuit. See comment section below.
Notes by Acme Fixer:(Thanks!!)
1. Add 0.01uF capacitor across 1.5V supply near the transistor.
2. You can remove the antenna.
3. Mic is an electret condenser microphone type.
4. See the rest of his recommendation based on his experience in building the circuit. See comment section below.
I started with 8 turns wound on a screwdriver slightly smaller than a pencil (6.5mm) but the frequency was well below the bottom of the FM band (88 MHz). I removed turns until at 5.5 turns the frequency would get up to the bottom of the FM band. I spread the coils apart a bit with a toothpick to increase the frequency. Getting this coil right was the hardest part. The microphone is not very sensitive, I had to talk close to it (I think it's because it uses only 1.5V for the battery). Other wireless transmitters add a second transistor to boost the microphone signal, and I think it would help a lot. Other than that, it's working. Thanks.
ReplyDeleteA few other things I found helpful. I found the antenna makes the circuit sensitive to handling, so I left it out. The circuit with just the coil can be picked up more than 50 yards/meters without the antenna. The circuit is more stable after I put a 0.01 uF ceramic capacitor across the 1.5V supply, close to the transistor. It isn't said, but the Mic must be an electret condenser microphone. I also tried an earphone as a microphone but I did not need the 4.7k so I temporarily disconnected it. Thanks.
ReplyDeleteTo Acme Fixer
ReplyDeleteSharing your experience and knowledge in constructing the circuit is a great help to everyone. Lots of thanks to you!!
I would like to know what kind of receiver I will need to listen to it's transmission. I mean, if I am close enough it can be received with an ordinary radio or I'll have to build something else?
ReplyDeleteordinary fm radio receiver is ok. since the transmitter is very simple, it can be heard only in a very short distance. You need to be close enough so it can be received by the receiver. You can use also the fm radio from phone.
ReplyDeletewhere is the negative in the 10uF capacitor .how i can determine where is the polarity?
ReplyDeletePositive of 10uF is connected in 2K resistor and negative is in the base of the transistor. Other capacitors are ceramic type and no polarities.
ReplyDelete