This project combines a simple LC oscillator with a transistor astable multivibrator to transmit an audible Morse code signal. When the pushbutton is pressed, the multivibrator starts oscillating and sends an audio-frequency signal to the base of the oscillator transistor.
The resulting signal can be received on an ordinary AM radio tuned to approximately 1200 kHz. When the correct frequency is found, the background noise disappears and the Morse code tone becomes audible.
Coil: approximately 60 turns, 4 cm diameter, with a centre tap
How does it work ?
The circuit consists of two main sections: an LC oscillator and an astable multivibrator.
The LC oscillator
The first section uses a PNP transistor, a coil and two capacitors to generate a high-frequency signal.
The coil and capacitors form a resonant LC circuit. Energy alternates between the magnetic field of the coil and the electric fields of the capacitors. This resonance determines the oscillator frequency, which is approximately 1200 kHz.
The transistor provides amplification and feedback to sustain the oscillation.
The astable multivibrator
The second section uses two NPN transistors connected as an astable multivibrator. Two capacitors and resistors determine its oscillation frequency, producing a repeating audio-frequency signal.
When the pushbutton is pressed, the multivibrator receives power and starts oscillating. Its output signal is coupled through a capacitor to the base of the PNP transistor in the LC oscillator.
This audio signal modulates the oscillator, making the transmitted signal vary at an audible rate. The AM radio detects this variation and reproduces it as a tone.
When the pushbutton is released, the multivibrator stops, and the audio tone disappears.
By pressing and releasing the button in short and long intervals, it is possible to transmit Morse code manually.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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