GW4ALG's QRP Radio Pages

[ GW4ALG went QRT in February 2007 ]

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A Morse Sidetone Oscillator


1.   Features Summary
2.   Introduction
3.   Overview
4.   Circuit Diagram
5.   Pin-out Diagrams
6.   Feedback from other constructors


Features Summary

This sidetone oscillator has been developed for use with a CW transmitter that does not have a built-in sidetone oscillator for monitoring the transmitted morse code.  It has the following features:
- 9 v battery operation;
- Morse practice oscillator;
- Uses 555 timer IC;
- RF actuated option;
- Simultaneous keying of both the oscillator and a transmitter.


This morse practice oscillator and sidetone oscillator is the result of several email exchanges with John, M0CVB after we both became disappointed with some of the other 555 designs with which we had experimented. 

This design has a low component count, but produces a good level of audio output into a low impedance speaker.   Using the suggested component values, an audio frequency of about 600 Hz can be expected.  This frequency can be changed by altering the value of the 5.6 k resistor and/or the 20 nF capacitor.   Increasing the stated values reduces the frequency; reducing these values increases the frequency.



The oscillator uses a 555 timer IC which produces a square wave.  Square waves are not the nicest sounding of tones, but this design provides room filling sound with a low component count and low power consumption.

The circuit can be battery powered using, for example, a PP3 9 volt battery.

The keying input (from the morse key) keys a positive voltage to ground via a diode.  By using a second diode, the keying input can simultaneously key an associated transmitter if that transmitter also keys a positive voltage to ground.

To allow for situations where it is not convenient to key the transmitter via the oscillator circuit, provision has been made to trigger the oscillator when a strong RF signal is present.  In this mode, a sense wire is used to detect RF in the shack so that the oscillator is actuated without connecting a morse key.


Circuit Diagram

Pin 4 of the 555 chip is used to key the oscillator.   This pin needs to go positive to enable the oscillator.   Pin 4 can go positive under two conditions:
1) when the the base of the PNP transistor goes 'low' through operation of the morse key; and/or,
2) when an RF signal is detected from the sense wire.

A 10 k pull-down resistor is used between Pin 4 and ground to prevent the chip being enabled when it shouldn't be! 

Depending how this oscillator is to be used, it may be helpful to add a 500 ohm variable resistor in series with the speaker to adjust the volume.

Pin-out Diagrams


Feedback from other constructors

None yet!