A radio receiver with a candle flame detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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Figure 1 shows a diagram of a detector receiver with an unusual detector, which is the flame of a burning candle. The candle has two metal conductors located in its flame, one above the other. To do this, unbend a couple of paper clips, and fasten them next to the candles.
Rice. 1. The simplest circuit of a detector receiver
The principle of operation of the detector is based on the fact that inside the flame there is an area called plasma, consisting of ionized gas, which is a good conductor of electricity. It is important that the electrodes are located one above the other, the bottom one in the hottest place of the flame. The detector works on temperature difference. The hottest electrode will emit electrons and become the cathode, the second electrode located higher, in the less hot section of the flame, will act as the anode.
The flame is not only suitable for detecting RF signals, it can also be used as a highly sensitive microphone and broadband sound emitter. Even ancient people noticed that if someone is talking in a quiet room, lit by candles, then the flame always flickers. This is because the flame is susceptible to invisible sound waves.
Rice. 2. A more advanced detector receiver circuit with a detector in the form of a Bunsen burner.
Figure 2 shows a more advanced detector receiver circuit. The use of a Bunsen burner is much more convenient, since it melts and its flame remains at a constant height, unlike a candle. In addition, the burner flame is hotter and easier to regulate. It is necessary to adjust the burner so that the lower electrode glows with a red glow.
Circuit components:
L1 - Antenna coil - 75 turns (180 μH), taps from 15 and 30 turns;
C1 - Variable Capacitor (KPI) - 500 pF;
D1 - Diode - VD1 - Candle detector, two wires in flame;
R1 - Load resistor - 200 kOhm, used only with crystal phones;
C2 - Filter capacitor - 1000 pF, used only with electromagnetic phones;
If the sound frequency is applied to two electrodes installed in the burner flame, the flame will behave as a high-quality sound emitter. The horn voltage must be adjusted to match the resistance of the flame. The flame and its plasma are very similar to vacuum tubes, they also have a high impedance. A step-up transformer can be used to match the impedances. To increase the output power, it is necessary to increase the area of the electrodes.
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