A single NE555 timer IC, paired with a handful of resistors, capacitors, and one potentiometer, produces a 455 kHz beat frequency oscillator capable of unlocking CW and SSB suppressed-carrier reception on cheap ham receivers that lack a built-in BFO.
The circuit injects a carrier into the radio's intermediate frequency chain, and the potentiometer makes it tunable across roughly plus or minus 20 percent of 455 kHz. The 555 runs on anything from 4.5 to 16 volts, meaning it drops into almost any existing radio without a dedicated power supply. EDN published both this design and a more detailed alternative build worth comparing directly.
The original article is short, but the EDN links it points to contain the actual schematics and component values. If you want to understand why the 555's wide supply voltage tolerance makes it the right tool here, rather than a purpose-built oscillator chip, read both sources back to back.
[READ ORIGINAL →]