[Picture] Making of Micro-FM Transmitter for Telephone

This is a device that uses ordinary telephone lines to send voice signals wirelessly at close range, and can be accessed through the telephone line. When the microphone is removed and the line is connected, the phone itself can be used to send voice without the need for a transmitting antenna or power supply (using a telephone line as an antenna and using a telephone signal as a power source). The transmitted signal is in the FM frequency band, and the FM receiver can be used to answer the phone line. The transmission frequency can be adjusted.
This line is a radio frequency (RF) oscillator operating at 93MHz. The circuit is powered by a full-wave rectifier bridge. C1, C8, L3, T1 make up the FM oscillator. Each transmitter (TX) requires an oscillator to generate the RF carrier. Groups L1, C6 and T2 were successfully derated. The audio signal from the telephone line is coupled to the base of T1 through R3 and C2 to modulate the oscillator. This is achieved by changing the junction capacitance of the transistor. Junction capacitance is a function of the voltage at the base of the transistor.
Such circuits usually require calibration. The resonance frequency of the L1, C6 amplifier circuit needs to be adjusted to match the resonance frequency of C1, C8, L3. In fact, this circuit can work without any calibration.
When C1 is 27p, the transmission band of this line is 86 ~ 95MHz. The capacitance band using 22p is 90 to 95MHz (related to the coil pitch). If you want to use a frequency band above 100MHz, C1 should be replaced with a 10-15p capacitor.
The standard voltage of the telephone line is generally 48V (DC). If it is on an extension line, the line voltage may be as low as 39V.
Note: When adjusting the frequency, the component cannot be held in the hand because the capacitance of the human body is sufficient to change the resonance frequency of the line.
If a 150cm long antenna is added to the collector of T2, a larger transmission range can be obtained.
This component is serially connected to one of the two wires connected to the phone, and both wires can be used. A wire connected to terminal 2 or 5 when in the UK. Cut the telephone line and connect it to the other end with an alligator clip. Pick up the phone, turn on the hook switch, adjust the frequency to about 93MHz, and take a portable FM receiver along the phone line to receive it.

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Floating Ball Liquid Level Sensor
The floating ball liquid level transmitter is composed of magnetic floating ball, measuring conduit, signal unit, electronic unit, junction box and mounting parts. Generally, the specific gravity of the magnetic floating ball is less than 0.5, which can float above the liquid level and move up and down along the measuring conduit. The conduit is equipped with measuring elements, which can convert the measured liquid level signal into a resistance signal proportional to the change of liquid level under the action of external magnetism, And convert the electronic unit into 4 ~ 20mA or other standard signal output.
The liquid level sensor is a modular circuit, which has the advantages of acid resistance, moisture resistance, shock resistance and corrosion resistance. The circuit contains a constant current feedback circuit and an internal protection circuit, which can make the maximum output current no more than 28ma, so it can reliably protect the power supply and prevent the secondary instrument from being damaged.
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