This project demonstrates how a high-frequency (HF) signal can be transmitted through the air and detected by a simple transistor receiver.
When the pushbutton is pressed, the transmitter starts operating and emits a high-frequency signal. The receiver picks up the signal, causing the LED to light up.
Coil: approximately 60 turns, 4 cm diameter, with a centre tap
How does it work ?
The circuit consists of two separate sections: an HF transmitter and a receiver.
The HF transmitter
The transmitter uses a PNP transistor, a coil and capacitors to form an LC oscillator.
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.
A pushbutton controls the transmitter's power supply. When the button is pressed, the oscillator starts operating and produces an HF signal. This signal is coupled to the surrounding space, where it can be detected by the receiver.
The receiver
The receiver is built around a single NPN transistor. Its antenna is connected to the transistor's base, allowing it to pick up the transmitted HF signal.
When the transmitter is activated, the signal received by the antenna influences the transistor's base current. The transistor then conducts, allowing current to flow through the LED and causing it to light up.
When the pushbutton is released, the transmitter stops oscillating. The received signal disappears, the transistor stops conducting sufficiently to illuminate the LED, and the LED turns off.
The circuit can also operate without an earth connection. However, the signal transfer is generally less effective, and the receiver may need to be placed closer to the transmitter. An earth connection can improve the performance of this simple setup.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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