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
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.
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.