My 555 contest entry.

*Gasp*, it's 03/01/2011, and i've just finished my entry for the 555 contest!

http://www.555contest.com/

There is a lot of "555 transmitters" on the web, most of them are very simple but have several drawbacks.

-The range is very low, i've havent succeded to transmit beyond one or two meters in the same room.
-Square signals are full of harmonics, meaning that you're tranmitting in several frequencies. Your roommate would be irritated if you make his radio buzzing! .

I wanted to know if it's possible to clean the signal and extend the range by adding a tuned circuit.

Coil's and variable capacitor's values are not critical. The LC circuit just have to be tuned to the sending frequency. I've wounded enameled copper wire around a cardboard tube (use hot glue to hold everything in place), and the variable capacitor come from a modern radio.

The 555 is working in astable mod. This simple design doesn't allow it to reach the MW AM band (around 1MHz). But, it can send pulses at the right frequency to make the tuned circuit oscillate.

Oscillations in the tuned circuit before resonance:

5µs/Div 5V/Div

We can see that the square signal from the 555 timer induce damped oscillations in the tuned circuit.


When the 555's frequency reach F=F'/N where F' is the tuned circuit's resonant frequency and N a natural number, the tuned circuit reach resonance. The oscillations are so important that i can't monitor them on my oscilloscope.

The audio input on the pin 5, modulate the 555's frequency (yes, this is FM). But, as the frequency's changing, the 555 go out of tune, and the oscillations in the tuned circuit decrease, and we've got amplitude modulation!

This circuit work very well on my basement, harmonics are present near the device, but fade out rapidly beyond one meter.


This is my first entry in english, so please, tell me if i'm totaly unintelligible! :)