Build this low power dirty AM transmitter and learn about RF modulation.
In the 21st century we have all become accustomed to our connected high-tech lifestyle and we more or less take our internet based social and commercial connectivity for granted until the ‘network goes down.’ Much of the technology that drives our digital being are computer based, but another essential element is radio technology, without which we would find ourselves much less connected.
How does ‘radio’ work?
Long before manipulated radio waves were used to provide the backbone of today’s digital cellular and cloud based information network, those of us who grew up in the twentieth century knew radio primarily as a means to enjoy music, news and talk, via the AM and FM radios in our homes and cars. But what is radio?
Simply put, radio is the transmission and reception of electromagnetic waves that are encoded with data. The data could be spoken word, music, or digital data like text and email messages or larger digital files like software or other data files.
The data that can be transmitted via radio waves exists as low-energy signals and they must be attached to a high-energy signal called a carrier wave in order to be transmitted. A carrier wave is at a significantly higher frequency than the input signal and is typically sinusoidal.
The mixing of the signal with the carrier wave is a process called modulation. There are several ways a carrier can be modulated, the two most common modes of modulation for an audio signal is amplitude modulaion (AM) and frequency modulation (FM). (Another common method of modulation a carrier signal that is beyond the scope of this article is phase modulation (PM)).
Amplitude modulation involves varying the signal strength, or amplitude, of the carrier wave in direct proportion to the message signal. Frequency modulation involves varying the frequency of the carrier wave in dirrect proportion of the message signal.
A simple low power AM radio transmitter
It is possible to construct a very simple low power transmitter built around the ubiquitous 555 Time Integrated Circuit which is capable of demonstrating the principle of amplitude modulation by mixing an audio signal with a carrier signal that can be transmitted to and received by a AM radio receiver.
The 555 timer chip was designed in 1971 by Hans Camenzind and has remained one of the most popular and versatile integrated circuit chips ever produced. Simple when compared to today’s chips which may contain tens of billions of transistors, the 555 has 25 bipoloar transistors, 15 resistors and 2 diodes.
The 555 chip has three distinct modes of operation – monostable, bistable, and astable.
The monostable mode is also known as the one-shot mode and will output a single pulse of current for a specified length of time. The bistable mode is also known as the flip-flop mode and alternates between two stable states, on and off, controlled by the trigger and reset pin.
The astable mode is also known as the oscillator mode and can be used to generate a carrier wave that could be modulated in a model RF transmitter circuit. We will employ this feature of the 555 chip for our transmitter circuit described below.
Traditionally the carrier wave in a radio transmitter is a continuous sinusoidal wave. However, the 555’s output is digital, either on or off, so the resulting wave is a square wave. An alterenating current sine wave is smooth and continuous and is optimal for carrying an information signal, but the square wave will work for our purposes.
I will now present and describe a simple AM radio transmitter circuit that you may wish to construct to observe and demonstrate radio modulation. This circuit design can be found on hundreds of websites oftentimes with small variations component value changes or minor modificaitons to the basic circuit. The circuit I present here is based on the core circuit, nominally modified to reflect what I have found to work well on my bench. I encourage readers who take the time to build my circuit to experiment and share modifications letting us know how you may have enhanced performance.
We will start by looking at the transmitter circuit by its three sub-circuits. The first is the oscillator circuit that produces the carrier wave, the second is the audio input sub-circuit, and thirdly we add the output amplifier and antenna.
1.) The Oscillator
The schematic diagram below is built upon the basic 555 astable circuit and this is the heart of the transmitter. The frequency of the carrier wave output on pin 3 is controlled by how rapidly capacitor C1 charges and discharges. The values of resistors R1, R3 and R4 will determine how quickly C1 charges.
When the charge of C1 reaches 2/3 of the control voltage Vcc, the output at pin 3 goes high. When the charge of C1 discharges to 1/3 of Vcc, the output at pin 3 goes low. Changing the resistance of the RC circuit by adjusting R4 will inversely change the output signal frequency. Increasing R4 will decrease the frequency and decreasing R4 will increase the frequency.
2. Audio Input
When a low energy audio signal is injected at the control pin, pin 5, the 555 will mix the input signal with the signal of the carrier wave and produce an amplitude modulated output signal at pin 3, output.
It is important to note that because the carrier wave generated by the 555 oscillator circuit is a digital pulse wave, where the output goes from completely on to completely off, the modulation of the carrier isn’t pure amplitude modulation, but pulse amplitude modulaton.
The pulse modulated RF signal will not be able to capture all of the fine detail of the input signal and you will notice that as a result, that the sound coming from the receiver speaker will sound a bit choppy and not as pure as you are accustomed to.
3. RF Amplification and Signal Output
The modulated RF signal is output at pin 3 of the 555. Attaching a simple piece of wire to pin 3 will serve as a crude antenna causing the modulated signal to be transmitted, or radiated into space where it can be picked up by properly tuned radio receivers.
Because this is an exteremely low power transmitter, I have added a single NPN bipolar junction transmitter (2N3904) at pin 3 to perform as an amplifier to boost the output signal strength. The base of the transistor is connected to output pin no. 3 of the 555 and the antenna wire will be connected to the transistor emitter. Power is supplied to the transistor via the collector.
An ideal length for the transmitting element for a radio antenna is 1/4 of the signal wavelength. This length gives us an efficient antenna length where the signal will resonate with minimal loss of signal. What would that length be for this transmitter?
Radio wavelength is calculated as the speed of light, (the speed at which RF waves also travel,) expressed in meters per second divided by frequency expressed in Hertz. The speed of light is 299,792,458 meters / second and the frequency range of the standard AM broadcast band in North America is 550 – 1700 KHz. Doing the math we learn that the broadcast band wavelength ranges from 176.3 meters to 545.1 meters.
Dividing these figures by four, we calculate the optimal antenna length for our transmitter would need to be between 44 and 136 meters or 144.36 to 446.19 feet! Constructing such an antenna for a simple circuit would be impractical and costly.
In this build we are using a piece of wire that is approximately one meter or a bit over 39 inches long. Because such a short antenna length is non-resonant and because the transmitter signal output is very week, you will find draping your antenna wire over the radio receiver establishing a loose coupling to the receiver will give you best results.
If you decide to build this circuit you may wish to experiment with different antenna designs to see how the may affect the range of your output signal. You may try a longer wire and another website suggests using a(n empty) beer can as an antenna.
Ham radio operators often make use of coils when antenna space is limited. Adding a tightly wound wire coil will add inductance allowing a physically shortened loaded antenna become more resonant at a longer wavelength. The concept of designing inductance loaded antennas is a concept goes beyond the scope of this article, but there is no shortage of excellent websites available that cover this topic if you are curious and know how to use the Google.
Using the transmitter
Once you have built the transmitter circuit, using it is straightforward and should be fairly intuitive. In addition to connecting the antenna wire to the transistor emitter and draping it over the receiver, you will of course need to supply a control voltage and an audio signal.
I have found that a 9 volt battery works well with this simple circuit as it is compact and will provide plenty of current to drive the circuit. The 555 will work with a Vcc ranging from 5 to about 15 volts giving you plenty of options.
For my audio source, I initially used the audio output from my laptop but have found that using my Activo CT10 MP3 player works much better as the MP3 player puts out a stronger high res audio signal.
Once you have made the above connections, start the audio source music playing and tune the radio receiver to about 600 kHz. Slowly fine tune the receiver up and down until you can hear the audio signal. It should be heard somewhere near 600 kHz, either slightly above or below.
Once you find the signal, try tweaking it by adjusting R4 on the transmitter using either an RF tuning tool or a mini-screwdriver.
Experiment by moving the antenna wire back form the receiver and raising it and lowering it. How does position change the receiver’s ability to pick up the signal?
What are some other ways you can improve the transmitters performance?
I mentioned above that this is a dirty transmitter. This term means that the output signal is not well-filtered. Transmitted radio waves by nature produce harmonic signals on even multiples of their frequency. In fact, when you are listening to the radio output around 600 kHz, you are actually tuned to the first harmonic as the circuit’s output frequency is somewhere between 200 and 300 kHz (see oscilloscope photo above).
Try tuning the radio receiver slowly up the dial. You ought to be able to also hear the transmitted signal just below 900 kHz and again around 1200 kHz and perhaps just under 1500 kHz. These are all harmonic signals and this type of unfiltered signal would not be acceptable for any FCC licensed radio transmitting station – wheter commercial or amateur. Harmonics can be mitigated through the use of electronic filters.
It should also be noted that it is of course illegal for anyone not holding a valid FCC license to transmit radio signals. A commercial license is required to broadcast radio signals on the US AM band and it is also illegal for anyone to interfere with commercially licensed stations.
If you successfully build this circuit, you will see that it is not capable of transmitting a signal to receivers located more than a foot or two away at best. You should not modify the circuit presented here to further amplify the signal to gain further range.
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If you would like to build this circuit, I have prepared a complete kit that includes a solderless breadboard, all of the necessary electronic components including the audio input jack, the battery snap, pre-cut jumper and antenna wires. I have also written highly detailed and illustrated step-by-step instructions that should guarantee a successful build every time.
The kit requires no soldering with a limited number of step-by-step instructions to make the project easy and fun withe guaranteed success.
For a limited time, the cost of the kit including postage-paid shipping via USPS to domestic addresses is currently $22.00. The complete kit will be shipped in a padded envelope and instructions and documentation will be sent in PDF format via email.
Solderless 555 AM Transmitter Kit
Domestic US Customers Only
Thanks for reading and please share your thought and experiences. You may drop me a line at firstname.lastname@example.org.