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A compact transmitter introduced here is ideal for beginners. The circuit design is simple and easy to build, with a low cost and an output power of no more than 5-8mW. It can transmit up to 300 meters indoors, making it perfect for use with a regular FM radio. The receiver shows excellent sensitivity and clarity, proving that even a basic setup can deliver great results. One of the most challenging aspects of this design is achieving transmission with just a 3V power supply and a half-wave antenna. Additionally, since the circuit uses very few components, it can easily fit inside a matchbox (slightly larger than a standard one), making it almost undetectable.
This device is not only useful as a bug but also has various other applications. You can place it in a baby’s room, near a gate, or in a hallway to monitor what’s happening. It can also serve as a night-time security tool, offering a discreet and effective way to keep an eye on your surroundings.
The circuit consumes less than 5mA of current, allowing it to run continuously for 80 to 100 hours on two standard dry batteries. It is highly stable under normal conditions, with minimal frequency drift. During testing, after running for 8 hours, there was no need to recalibrate the receiver. The only factor affecting the output frequency is the battery condition. As the batteries age, the frequency may shift slightly, but this is usually negligible.
Building this transmitter is a great way to learn about FM transmission. It demonstrates how even with low power, you can achieve high-quality signal transmission without noise. The range is impressive, especially considering the low output power.
Looking at the circuit diagram (Figure 1), it consists of two main stages: an audio amplifier and an RF oscillator. There's actually a FET built into the electret microphone, which can be considered as a separate stage. The FET amplifies the capacitance changes caused by the diaphragm movement, making the microphone extremely sensitive.
The audio amplifier stage uses transistor Q1, which provides a gain of around 20 to 50. The amplified signal is then sent to the base of the oscillator stage. The oscillator stage, using transistor Q2, operates at approximately 88 MHz. This frequency is controlled by a 5-turn coil and a 47pF capacitor. The exact frequency is also influenced by the transistor itself, along with a 18pF feedback capacitor, a 470Ω emitter resistor, and a 22KΩ base resistor.
By building and experimenting with this circuit, you'll gain a better understanding of how FM transmitters work, and appreciate their efficiency and performance even with minimal components.
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