FM transmitter installed in a matchbox, Tapping Device
Keywords: FM microphone, wireless microphone
Almost every electronic enthusiast has a strong sense of radio, regardless of a aircraft or communicating with the outside world, expressing the expectations of their launch.
Here, a pocket transmitter introduced here is very suitable for beginners, simple circuits, low cost, no output power, no more than 5-8MW, the launch range can be around 300 meters, with a normal FM radio Receive, showing its sensitivity and clarity, the richest part of the circuit design is to have such launching capabilities only with 3V power and half-wave antennas. In addition, because the parts required for the circuit are very small, it can be placed in a matchbox (larger than the domestic-all match box), which can be placed in the baby room, gate or corridor channel, monitor the actual situation, in addition It can also be used as a night security device.
The current loss of the circuit is less than 5 mA, and two dry batteries can be used to work between 80 and 100 hours.
The circuit is very stable under normal operation, the frequency drift is very small, the test: After 8 hours, no need to recover again. The only thing affecting the output frequency is the condition of the battery. When the battery is aging, the frequency is slightly changed.
With this production, learn about FM transmission, you can understand its superior place, especially it produces no noise, even if it is transmitted, it is also easy to achieve well.
Circuit work principle
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From the figure (1) circuit, it is visible to two levels, first-stage audio amplifiers, and one-stage RF oscillator.
There is a FET in the actual microphone in the eternal microphone. If you like, the visual is level, and FET amplifies the capacitance of the microphone precursor membrane, which is the sensitive reason for the eradicacy.
The audio amplification is carried out by its emitter crystal thrust Q1, and the gain is from 20 to 50, and the zoomed signal is sent to the base of the oscillation level.
The oscillation level Q2 is operating at a frequency of about 88 MHz, which is adjusted by the oscillation coil (a total of 5 laps) and 47PF capacitor, which is also determined in the transistor, 18pf back-activator, and a few biasing elements, such as 470 Ω emitter resistance. And 22K base resistance.
When the power is turned on, the 1NF base capacitor is gradually charged by 22K resistance, and 18 pf is charged with 470 Ω resistance of oscillating coil, but is more fast, and the 47PF capacitor is also charged (although only two ends of the two ends), the coil generates a magnetic field. .
When the base voltage gradually increases, the transistor is turned on and effectively connects the internal resistance and connected to both sides of 18pf. When the 1nf capacitor is charged to the working voltage of the pole, several messy week waves have occurred, so we assume that the discussion is approaching the working voltage.
The base voltage continues to rise, the 18NF capacitor attempts to prevent the movement of the emitter from being moved, and when the energy depleted in the capacitor and no block movement, the substance voltage is lowered, the transistor is turned off, and the current of the transistor is also stopped. Magnetic field declaration.
Magnetic field declaration, producing a voltage in the opposite direction, the column voltage is reversed from the original 2.9V to exceed. 3V, and in the opposite direction 47PF capacitor charge, this voltage also affects the 18PF capacitor charging, and the voltage on the 470 Ω emitter resistor fuel to the crystal pipe into a deeper turn-off.
When the 18PF capacitor is charged, the radiation voltage has fallen and falls to a crystalist to start turning on, current flows into the coil, and confronts the declaced magnetic field.
The voltage on the coil is reversed, forming the collision voltage drop, this change is transmitted to the emitter by 18PF, resulting in a deeper conduction, shorting the 18PF capacitor, and the cycle start repeat.
Therefore, Q2 is here to form an oscillation, generate an AC signal of 88MHz. After the zoom, the audio signal is collected into the 0.1uf capacitor, the base of Q2, changing the oscillation frequency, generating the required FM signal.
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