Circuit Lake

Electronic Project and Circuit Collection

Frequency Marker Generator

06/07/2010 Category: AVR, Ham Radio, Microcontroller, Project

COLT45 is a small multi-purpose device intended for the Amateur Radio operator, short wave listener, or other experimenters. Functions include a versatile frequency marker generator, a Morse code keyer, and a Morse code beacon generator with room for more functionality.


The Colt45 uses an 8-pin Atmel AVR ATtiny45 microcontroller. To produce these precise low frequency square waves, an inexpensive and readily available 4 MHz crystal is used as the clock oscillator and several different program loops of varying (but specific) lengths are used to toggle an output pin at the frequency desired. This scheme has the advantage over the 4060 old design of being able to produce output frequencies on more desirable boundaries such as 100, 50, 25, and 10 kHz.

“Additional bonus functions of the Colt45 make use of the same hardware with the addition of a output keying transistor controlling the on-off keying of a transmitter. The same output used for the marker frequencies is used for headphones or power speaker to monitor code sending with an audio tone. Sending speed is adjusted by pushbutton control,” Don L. Jackson, project designer, explained.

The beacon keyer and a manual key input takes advantage of existing circuitry with only minor software additions. The station’s call sign is stored in the tiny45’s EEPROM memory. Beacons are sometimes used to study radio conditions and propagation with low power transmitters.

Colt45 Project
Project Detail and Documentation