The Simplest FM Transmitter... Without Coil/inductor

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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

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

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:

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82 Discussions

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cyber_cematicyber_cemati

Reply 2 months ago

Hi.
The circuit working. Thank you.
So, how do boost signal a simple way?
I'm gonna do a baby monitor.
I would appreciate if you could help.

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abdelrazzac10cyber_cemati

Reply 2 months ago

Hi,
Well, there is no simple way to boost the signal. I tried with some amplifiers but wasn't successful. You need high frequency amplifiers (that can amplify 100 MHz signals) to do so.

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abdelrazzac10cyber_cemati

Reply 2 months ago

Good work, but it won't work on breadboard cemati. Note that the metal bars at the bottom of the breadboard form a capacitance value that disturbs the original capacitance of the capacitor. Because we are working on very high frequencies (100 MHz), a tiny change in the capacitance will affect the signal and makes it undetectable on radios (whose range is 88 to 108 MHz). Unfortunately, this circuit (like all FM transmitter circuits) cannot be tested on breadboard; you have to move directly to the prototype board realization.

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15WestJ

1 year ago

can I have a parts list because it is not clear what I need to build it.

2 replies
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abdelrazzac1015WestJ

Reply 2 months ago

SN7413 NAND chip, 20 pf capacitor, 20 pf trimmer capacitor

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abdelrazzac1015WestJ

Reply 1 year ago

The SN7413 NAND chip, 20 pF varicap (20 PF trimmer capacitor), a fixed 22 pF, a mic, and a 70 cm wire as an antenna

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KapoioO

4 months ago

I have some question
1 how i will connect a audio jack
2 the trimmer capacitor is to adjust the transmission ?
And 3 what other chips can i use ?

2 replies
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abdelrazzac10KapoioO

Reply 2 months ago

You can replace the mic with an audio jack. The trimmer capacitor is able to adjust transmission channel in kilohertz values, so the adjustment will be around 100 MHz only. Note that the peak of the signal is close to 100 MHz, and adjusting the channel will decrease its strength. I tested the circuit with other NAND chips, but none gave high frequency square wave (close to 33 MHz). This means that the transmitted signal cannot be detected using regular radios.

Good luck...

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abdelrazzac10abdelrazzac10

Reply 2 months ago

In the file, the gates stated below are said to be suitable for this project, but I couldn't find them. I tried a regular NAND gate but it didn't work

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tedb39

1 year ago

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?

2 replies
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abdelrazzac10tedb39

Reply 1 year ago

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.

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ElijahWabdelrazzac10

Reply 1 year ago

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.

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abdelrazzac10laith mohamed

Reply 2 years ago

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.

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.

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CarlosV84

2 years ago

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

1 reply
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abdelrazzac10CarlosV84

Reply 2 years ago

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.

Stereo-mono.jpg