Introduction: The Simplest FM Transmitter... Without Coil/inductor

Picture of The Simplest FM Transmitter... Without Coil/inductor

I made this project three years ago, and it works perfectly. I have found the circuit of this project in a book I bought eighteen years ago when I was in high school.

Most of the available FM transmitter circuits have, in my opinion, a very important drawback: the use of inductors. Some, including myself, find it hard to make the perfect coil and tune it for the intended frequency. I found the solution in the circuit of this project.

The circuit uses the SN7413 NAND chip as oscillator to generate a carrier frequency of about 100 Mhz which lays inside the FM radio frequency range (88 Mhz - 108 Mhz). This transmitter has a range of about 50 m.

Step 1: The Circuit

Picture of The Circuit

The circuit is extremely easy, and is based on the SN7413 chip. The range of the transmitter is about 50 m.

The transmission frequency of the circuit is around 100 Mhz, and can be adjusted by adjusting the 20 PF trimmer capacitor. Using a fixed 22 PF capacitor, you will receive the signal at channel 93.2 Mhz.

The aerial/antenna is a 70 cm long wire. I used a high gain microphone as the one shown in the figures above. If you would like to use a condenser microphone, it is better to use an audio amplifier circuit after the microphone. Example circuits are shown above.

I was planning to use a radio frequency (RF) amplifier to extend the range of this simple circuit, but never proceed.

Step 2: How Does It Work? + Video

Picture of How Does It Work? + Video

The theory behind this circuit is very easy. First we generate a square wave (carrier signal) using the SN7413 NAND gate chip (shown in the first figure above), then we change the frequency of this carrier signal according to the sound wave (this is called modulation). The modulated signal is then transmitted via the antenna. The radio receiver receives the modulated signal and de-modulates it to recover the sound wave.

How the carrier signal is generated?

the SN7413 is a dual four input NAND gate chip. Dual means that it holds two NAND gates inside the chip, and we are going to use only one of these chips. When the inputs of a NAND gate are connected to its output (called feedback), the it is transformed to an inverter (or NOT gate). The output of a NOT gate is always the inverse of its input. If a digital 1 (5 v TTL) is the input of a NOT gate, its output is digital 0 (0 v TTL) and vice versa. This means that by feedbacking the NAND gate, it will start generating a square wave. The frequency of this square wave is between 30 and 35 Mhz. Note that the feedback is assured by connecting pins 1, 2, and 4 of the chip to its pin number 6.

The frequency range of FM radio is 88 Mhz-108 Mhz, so how are you using a 30-35 Mhz signal to transmit FM signals?

Note that the signal generated by the NAND oscillator is a square wave. It is known that the square waves embed an infinite number of sine waves. These sine waves are called harmonics, and have the following properties: the first harmonic is called the fundamental signal, and has frequency and amplitude equal to the square wave signal. the second harmonics has its amplitude equal to half that of the square wave, and its frequency double that of the square wave. The third harmonics has its amplitude equal to one third of the square wave, and its frequency three times that of the square wave. This series goes to infinity following the same procedure.

Explanation above means that our carrier signal (square wave with 30-35 Mhz frequency) has its third harmonics with frequency about 100 Mhz which lays inside the FM radio range. What we are actually doing is changing the frequencies of all the harmonics according to the audio signal (modulating all the frequencies), but only the modulated third harmonics can be detected using FM radios.

For those who can understand Arabic, I added a PDF copy of the reference below in the attachment.

Recently I found a similar circuit using the 7413 chip but with both a coil and a capacitor. The circuit is shown above, and its link is:

http://www.next.gr/circuits/FM-bug-Spy-transmitter...

Note that I didn't test this circuit.

The reader "bmaverick" has found our circuit with good explanation in english and s/he, thankfully, provided us with the link (page 75, Warning: an e-book, very big file):

https://www.iz3mez.it/wp-content/library/ebook/Cir...

A video of this circuit along with the output shown on the oscilloscope:

Comments

ElijahW (author)2017-09-17

So, I have this antenna that is four feet long, but it broke, so I hot-glued the antenna from the remote of an old RC truck onto the end of it. Would I be able to hook up that antenna to the circuit somehow and still get it to work?

abdelrazzac10 (author)ElijahW2017-09-21

You cannot hot-glue the antenna, you have to solder it. Antennae are transducers that transform the electric signal into electromagnetic signal ( or RF signal) so it can travel in the space. They do NOT "boost" the signal! boosters are amplifiers. Second, an antenna can be as simple as a wire, but its length matters. The length of an antenna is defined by the wavelength it is going to be used with. Longer antenna will attenuate the signal, while shorter antenna will not give the RF signal time to be strong; both result in weaker performance. Kindly check the following links for the length of an antenna (you may have a quick search in the internet as well)

http://www.csgnetwork.com/antennagenericfreqlencalc.html

http://www.modelboatmayhem.co.uk/forum/index.php?topic=13093.0

ElijahW (author)abdelrazzac102017-09-23

Well, I would obviously have to solter the wires together, but I would have to make it stick to the circuit board somehow. Anyway, so if I had a longer antenna, it would make the signal weaker? Describe to me how it would affect the signal as if I don't know any big words. Pretend I'm an Idiot.

abdelrazzac10 made it! (author)ElijahW2017-09-25

Actually, the idea is very easy and straight forward and related to the natural frequency idea. Have you ever oscillate a rope? You should have seen that for the rope to oscillate in a very clear manner you have to lower and rise your hand with a given speed (Fig.1, a). This speed is called the natural frequency of the system formed of the rope and the node on the wall. Now if you get a shorter rope, you will find that you have to increase the frequency of your hand in order to keep the rope moving nicely (shorter ropes have higher natural frequency). If you move your hand with frequencies higher or lower than the natural frequency, you will note that the rope will move insanely and will stop after a while. This means that any frequency other than the natural frequency will generate a damped motion that will stop the rope from oscillating. On the other hand, for a given hand frequency, we have only one rope length that ensures natural frequency.

The same thing applies for antennae. For a given communication frequency (for example 100 MHz as in our FM transmitter), only one antenna length ensures the natural frequency of the antenna, thus an undamped oscillation at this antenna. With longer antenna length, the antenna material will generate a resistance for the signal resulting in weaker signal. For shorter antenna, the signal requires more length for one period to pass through the antenna, resulting in weaker transmission.

Each antenna shape has a formula that should be used in order to find its length for a specific frequency. Yes, the antenna is just a piece of wire, but there are a lot of restrictions that should be taken into account while connecting an antenna.

ElijahW (author)abdelrazzac102017-09-23

So the longer the antena, the weaker the signal?

ElijahW (author)ElijahW2017-09-17

And, even if I could, would it boost the signal in any way? Thank you in advance for your help, your time, and your response.

tedb39 (author)2017-06-12

hi

I would like to find out if I can use this circuit to create static signal and transmit it to my neighbor device. You see they play their radio very loud at all times of day and mostly at night. So I figured if I transmit static to their radio I will be able to get a good night sleep. Is there a way to get such circuit?

abdelrazzac10 (author)tedb392017-06-13

Well, I don;t think so. First, the signal generated by this circuit is too weak, and is not able to suppress station signals. Second, you have to know the frequency currently received by your neighbor's radio in order to suppress it efficiently. Third, what if your neighbor is listening to an AM station? You may use a spark transmitter to prevent their device from receiving radio signals but this is illegal to the best of my knowledge.

ElijahW (author)abdelrazzac102017-09-17

Or you could always use an EMP (ElectroMagnetic Pulse) machine, and completely destroy any electrical system the car has. But then it could travel through the entire world destroying every way of us utilizing electricity ever again. But then again, that is not only illegal (maybe), but I enjoy using electricity, so plz don't. HOWEVER!! You could always use the Trojan Horse example: make a "music" Cd that has tiny coils of copper running through half of the Cd. Then you could build a super tiny system that turns the heat of the Cd-reading laser into electricity, powering the electromagnet with just enough electricity to kill the car's audio system, but not nearly enough electricity to travel outside of the car, let alone to the rest of the world.

Senthil sivaraj (author)2017-01-05

Nice

laith mohamed (author)2016-11-26

Can i use this ic.....?

Well, I am not sure. Feedback-ing this chip will -most probably- produce an oscillation, but I don't know if the frequency of this oscillation will be 35 MHz. It is better to try. Please tell us if you succeed.

Detectingdragon777 (author)2016-11-15

Where did you get the ic chip?

It was not easy to find the chip, but it still available in the market. In the document I have, it says that the following ICs can be used as well:

FLH361, HD7413, UPB213, TL7413, M63213, and MC7413. However, I can't guarantee this because I didn't test these ICs.

CarlosV84 (author)2016-08-23

so it looks like the transmitted signal is a mono signal. is ist possible to transmit stereo? and how?

abdelrazzac10 (author)CarlosV842016-08-23

This circuit is a transmitter, it transmits whatever signal existing at the input. To the best of my knowledge, a stereo signal is generated by using two microphones placed apart to record the same voice signal. The signals from the two mics are then merged to form a stereo signal. Stereo signals are transmitted using one carrier frequency; for the transmitter here, this carrier is near 100 MHz. I think yes, this transmitter is able to transmit a stereo signal. Note that at the receiver, you should have a stereo decoder in order to recover the L and R signals.

ggarcía-2 (author)2016-08-03

Its there a way to scramble audio transmitted and then de-scrambled on a FM receiver? In order to get a secret communication and avoid other to hear the audio?

abdelrazzac10 (author)ggarcía-22016-08-03

We can do it by transforming audio signals to digital signals and then encrypting them. This requires the use of an ADC (Analog to Digital Converter) - a parallel to serial converter then sending data bit by bit. At the receiver, data is received bit by bit, transformed to parallel via a serial to parallel converter, then transformed to audio signals using a DAC (Digital to Analog Converter). Such system is not trivial though.

One can use transmission over multiple frequencies. You make four transmitters for example, each one transmitting on a given frequency. Then you use a switch to travel between these two frequencies. At the receiver, you use another switch synchronized with the first one, receive signals from different frequencies, and deliver them to the audio amplifier. This is very hard because the synchronization between the transmitter and the receiver switches should be perfect.

Another way to have safe communication is to add well defined noise to the signal (a noise with well known frequency) and send the signal. Anyone receiving this signal will hear noise only. When you receive the signal, you filter it with Low-Pass Filter with cut-off frequency less than the frequency of the noise you add. This will recover you the original signal. You can add multiple noise signals as well, and pass the received signal to multiple filters.

bdobrica (author)2016-07-04

Connecting a simple coil (6-7 turns around a pencil with a copper wire) in parallel with the tuning capacitor creates a filter that removes the other harmonics of the oscillator. Helps you limit the interference with other devices. Also, you can use the coil to fine tune the transmitter (by closing or widening the gaps between turns). I think the sound quality should improve if you use a variable potentiometer to bias the audio input closer to the center of the logic thresholds.

abdelrazzac10 (author)bdobrica2016-07-09

Thank you very much for this idea. I always try to avoid using coils, because you have to choose the capacitor and coil values to respect the formula f = 1/(2*3.15*sqrt(L*C)) with f is your transmitting frequency. It is not tirvial to make the intended coil; you can search the internet for FM coil making (wire diameter, coil diameter, coil length, etc.. everything affects the coil value! ), choose one coil, make it, and choose the convenient capacitor. Then you have to fine tune the coil in order to "catch" the right channel which is a great annoy!!!

SimonG84 (author)2016-06-28

Hi there, how can I make the antenna?

abdelrazzac10 (author)SimonG842016-06-29

Well, the antenna can be as easy as a 70 cm long piece of wire. For more complex and more powerful antenna you can check the following pages:

http://www.willus.com/author/antenna.shtml

https://www.instructables.com/answers/how-can-i-mak...

http://fmradio-transmitter.blogspot.com/2011/06/an...

http://electronics-diy.com/electronic_schematic.ph...

swander (author)2016-06-15

Remember, in the US, FM offset is odd numbers so some electronic tuners wont be able to find an even frequency such as 93.2. Im sure a variable could be used to tune it to a vacant frequency.

abdelrazzac10 (author)swander2016-06-16

Thank you for the information. Actually, the 20 PF trimmer capacitor is used for fine tuning. The frequency is between 30-35 Mhz so the reception will be between 90-105 Mhz.

BettyP3 (author)2016-06-14

30-35mhz is used by some state & county Sherriff & fire department communications. The FCC will be arriving at your door with a big fine or jail sentence by pumping a 30-35mhz square wave into the air.

abdelrazzac10 (author)BettyP32016-06-15

Thank you for the information. Better for all to check this issue at their countries before making this circuit. Using a high pass filter (with 80 Mhz cut-off frequency for example) could help; good to try.

ivanR22 (author)2016-06-14

why not just use a two input NAND gate?

abdelrazzac10 (author)ivanR222016-06-14

Good question. I tried using the regular two input NAND gates with no success. Honestly, I don't remember whether the feedback-ed NAND gate didn't oscillate or the oscillation frequency was far less than the required frequency (approx. 35 Mhz). I tested using NOT gates as well, but they oscillate in very low frequency. Note that FM broadcasting is within the range 88 Mhz - 108 Mhz, and any signal outside this range cannot be detected using regular radios.

abdelrazzac10 (author)ivanR222016-06-14

Good question. I tried using the regular two input NAND gates with no success. Honestly, I don't remember whether the feedback-ed NAND gate didn't oscillate or the oscillation frequency was far less than the required frequency (approx. 35 Mhz). I tested using NOT gates as well, but they oscillate in very low frequency. Note that FM broadcasting is within the range 88 Mhz - 108 Mhz, and any signal outside this range cannot be detected using regular radios.

abdelrazzac10 (author)ivanR222016-06-14

Good question. I tried using the regular two input NAND gates with no success. Honestly, I don't remember whether the feedback-ed NAND gate didn't oscillate or the oscillation frequency was far less than the required frequency (approx. 35 Mhz). I tested using NOT gates as well, but they oscillate in very low frequency. Note that FM broadcasting is within the range 88 Mhz - 108 Mhz, and any signal outside this range cannot be detected using regular radios.

zukeman (author)2016-06-13

I think I'm going to swap the mic for an audio jack and make a little transmitter for my old stereo

abdelrazzac10 (author)zukeman2016-06-13

Thant's a great idea. But you have to check the reliability of the transmitter, it's affected by close channels

scoluccio (author)2016-06-10

you said something about a referance book in a comment. can i have the name of the book? unless it is also in Arabic.

abdelrazzac10 (author)scoluccio2016-06-10

Unfortunately the book is in Arabic

t.rohner (author)abdelrazzac102016-06-13

I don't know where these "mini spy books" originally come from. I have three of them in German. (sold by Franzis back then)

The 74LS13 isn't a regular Nand circuit, it's a Schmitt Trigger by the way. (we used to have the "Pocket Guide" from TI to look them up...)

abdelrazzac10 (author)t.rohner2016-06-13

You are right, it's a schmidt trigger. Thanks

bmaverick (author)scoluccio2016-06-12

Found it in English. Took some digging.

https://www.iz3mez.it/wp-content/library/ebook/Cir...

See page 75.

Also, based on the same NAND IC, there is a FM receiver that will detect if one of these devices are in the area. It will help towards locating it. Guess the spy tech of the 1960s was easy and small to make.

scoluccio (author)bmaverick2016-06-13

thank you

rafununu (author)bmaverick2016-06-13

There were no ICs in the 60s ! I mean available for public users.

abdelrazzac10 (author)rafununu2016-06-13

In the e-book I added the following is written:

Couple Notes:

This circuit is old (70's) but since the 74LS13 is from the regular 74xx series and easy to obtain, I decided to keep the circuit in place until components are no longer available.

abdelrazzac10 (author)bmaverick2016-06-13

Thank you very much for the link, it's very helpful.

bmaverick (author)scoluccio2016-06-12

It's also in German per this Youtube vid of the same circuit.

https://www.youtube.com/watch?v=O7F33IZkJrU

bmaverick (author)bmaverick2016-06-12

Further info ...
"

Here simplest FM transmitter. The circuit oscillates already with only 3
components. The transmitter needs 10mA at 4.5 volts, since a TTL IC is
should not be created more than 5 volts. The circuit is from the book
mini spies, it is the TTL Mini Spy."

barryrk (author)2016-06-13

Could this work to make a wireless tv speaker? My hearing isn't what it used to be. Thank you

abdelrazzac10 (author)barryrk2016-06-13

Well, the transmitted signal was not immune against noise and close channels. I would recommend you to use an FM car transmitter. Kindly check:

https://www.instructables.com/id/Re-Transmitter-1/

and this one

https://www.instructables.com/id/Hacking-FM-Transmitter/

ak08820 (author)2016-06-12

What is the application?

Kevanf1 (author)ak088202016-06-13

Eavesdropping or spying on conversations in a different room or location. Place your morals in the box outside the door ;)

★DigiDavidex (author)ak088202016-06-12

Pirate Radio :D

abdelrazzac10 (author)ak088202016-06-12

FM transmission in a close range

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