This project shows how to build a simple AM radio transmitter based on 555 timer IC. The circuit parts are: the 555 timer IC, a NPN transistor three caps, three resistors and a potentiometer. The circuit is able to generate an amplitude modulation signal at 600Khz and you are able to receive it using a plain AM receiver. The range is about 30-40 feet.
You are going to build an AM radio transmitter AND you will be shown how it works. When you finish your radio, it will look something like mine in the picture above.
These are the components you will need
- 555 timer chip
- NPN transistor
- two #103 capacitors (0.01 microfarads or 10,000 picofarads)
- #102 capacitor (0.001 microfarads or 1,000 picofarads)
- some short wires
- two 1 Kilohm resistors
- 10 Kilohm resistor
- 1/8 inch (3.5 millimeter) female audio jack (yours may have more or less than three wires, but it must have at least two)
- 5 Kilohm potentiometer
- 1/8 inch (3.5 millimeter) male audio cable
- AM radio receiver
- Antenna. Yours doesn't have to be made out of a pop can, but the pop can works
Test your radio!
To test the AM radio transmitter, simply set the antenna next to your AM radio receiver (Alarm clock) tuned to approximately 600 KHz. Then play with the potentiometer until you can hear your music on your radio. The frequency generated by this devise will be anywhere from 100-480 Kilohertz if you used all the correct component values.
If you hear weird sounds when you turn the potentiometer (and do not hear the audio signal) That means that your radio is working, but your audio signal needs to be configured. try turning the volumed of your audio signal up.
~Is there power applied to your transmitter?
~Is the audio signal on?
~try turning the potentiometer.
~try turning up the audio signal
So how does this work?
The audio signal is controlling when the radio signal is being transmitted using its amplitude. this is called amplitude modulation (hence A.M. Radio) (See picture above).
However, as kr.baker pointed out in the comments, "The audio is "modulating" the RESET pin on the 7555. This means that the signal is turning the carrier completely on or off, as opposed to linear amplitude modulation. Consequently, the audio will be distorted." He raises a very good point that I hadn't given much or any thought to. You aren't modulating signals like they normally would be modulated. The audio quality will be greatly distorted because of this lack of having "linear amplitude modulation" - kr.baker
YOU AREN'T TRANSMITTING AM RADIO FREQUENCIES
You are transmitting at a low frequency that can be heard at higher AM frequencies. Lets say I transmit music at a base frequency of 300 Kilohertz (KHz) This music can be heard at the frequencies of 300 KHz, 600 KHz, 900 KHz, 1200 KHz,... (etc.) This is called harmonics. When the radio receiver's picks up a 300 KHz signal on a 2400 KHz band, the signal is heard only faintly. If you were to pick the very same 300 KHz signal on the 600KHz band, it would be exponentially stronger. This is why harmonics are only useful to a degree.
Modify your radio
I want you to modify your radio and post a comment below that tells us what you did and how it worked! it's that simple!
Although, I do have some suggestions:
Try changing R2 to a 3.3Kohm resistor.
Try cutting C3 out of your circuit.
try connecting the radio antenna to ground through a 1Kohm resistor
The radio transmitter we made can only transmit at frequencies from 110KHz to 480KHz. The AM radio band is from 520KHz to 1610KHz. Harmonics are essential to be able to hear audio signals transmitted from our radio transmitter.