Monday, February 28, 2011

555 Design Contest Entry - Direct Conversion Radio Receiver for the 80M Amateur Band

My design is a direct-conversion radio receiver for the 80 meter amateur radio band. It covers the lower end of the band, 3.500 - 3.535 Mhz, the most-used portion of the band for CW ("Morse code") operation.

"Direct conversion" refers to a receiver architecture where the radio signals are directly converted to audio frequencies (as opposed to the more complex "superheterodyne" design).  Direct-conversion receivers are by far the most popular design for homebrew amateur radio equipment, and even for some simple commercially-available products.

I set as my goal the design of a radio receiver that used no active devices other than the mighty 555 timer.  The circuit I produced contains only three 555 chips, plus four diodes for the mixer.

The radio's performance is really quite good.  It would be perfectly usable as part of  a low-power ("QRP") setup.  The 555 turns out to be very noisy when used as this sort of amplifier, but on 80 meters the high level of naturally-occurring noise makes this less significant. 

The primary limitation of the unit is selectivity (the ability to receive the desired station, while excluding other nearby stations).  You can hear this in the demonstration at the end of the first video -- several stations are often heard at once.  You also hear a lot of noise, but that's normal for the 80 meter band, not a deficiency in the design. The selectivity issue isn't a fundamental problem with the design; I just couldn't be bothered to design a better audio filter.

A great deal more detail is contained in the two videos:

Video #1

Video #2

The second (very brief) video demonstrates a significant design improvement.  I didn't have time to rework the radio hardware to incorporate it, so I submit it for your consideration as part of the overall design project.

The project from various angles:

Block diagram and schematics:

From video #2:


  1. This is an amazing project! Mixer based receivers are already a challenge, but to use 555s as the only active component is ever better. I wounder if you could have also done a PLL using the latch inside the 555. I will have to try this circuit out in my free time.


  2. I bet that when Signetics first brought out their NE555 IC circa 1970 they never envisaged that it would end up 40 years later being used in this way, congratulations OM on this very innovative application.
    Good Luck es 73 de G3OTH

  3. Embed those youtube videos on the page! The links to them are too small - I almost missed them while skimming the page! -AJ4VD

  4. VERY cool! I bet you could replace the 4 diodes in your mixer with the input clamp diodes in two 555s, and make it a TOTALLY 555 implementation. Good luck!

    73 Dave NM0S

  5. Hi there,

    Would there be any benefit in using the CMOS version of this chip, the 7555?

    Stephen Walters G7VFY