The Simplest FM Transmitter... Without Coil/inductor

30,048

388

74

Published

Introduction: 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

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:

Share

Recommendations

  • Clocks Contest

    Clocks Contest
  • Make it Move Contest

    Make it Move Contest
  • Planter Challenge

    Planter Challenge
user

We have a be nice policy.
Please be positive and constructive.

Tips

Questions

74 Comments

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

1 reply

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

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

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.

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.

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.

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

1 reply

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

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?

1 reply

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.

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.

1 reply

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!!!

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.

1 reply

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.