Shortwave SW Transmitter Based IC BEL1895

Here the SW transmitter circuit based on IC BEL1895. This particular transmitter circuit works in shortwave HF band (6 MHz to 15 MHz), and can be applied for shortrange communication and for educational purposes.

Shortwave (SW) Transmitter Circuit diagram :

The circuit is composed of a mic amplifier circuit, a variable frequency oscillator, and modulation amplifier stages. Transistor T1 (BF195) is put to use as a simple RF oscillator. Resistors R6 and R7 determine base bias, while resistor R9 is utilized for stability. Feedback is provided by 150pF capacitor C11 to maintain oscillations. The primary of shortwave oscillator coil and variable condenser VC1 (365pF, 1/2J gang) form the frequency determining network.

By altering the coil inductance or the capacitance of gang condenser, the frequency of oscillation can be modified. The carrier RF signal from the oscillator is inductively coupled through the secondary of transformer X1 to the next RF amplifier-cum-modulation stage assembled around transistor T2 that is run in class ‘A’ mode. Audio signal from the audio amplifier assembled around IC BEL1895 is coupled to the emitter of transistor 2N2222 (T2) for RF modulation.

IC BEL1895 is a monolithic audio power amplifier intendeded for sensitive AM radio applications. It can deliver 1W power to 4 ohms at 9V power supply, with low distortion and noise characteristics. Since the amplifier’s voltage gain is of the order of 600, the signal from condenser mic can be straightly linked to its input without any amplification.

The transmitter’s stability is managed by the quality of the tuned circuit parts as well as the degree of regulation of the supply voltage. A 9V regulated power source is required. RF output to the aerial consists of harmonics, because transistor T2 doesn’t have tuned coil in its collector circuit. However, for short-range communication, it does not create any trouble. The harmonic content of the output may be lowered by means of a high-Q L-C filter or resonant L-C traps tuned to each of the prominent harmonics. The power output of this transmitter is about 100 milliwatts.

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2 Transistor Transmitter Schematic

A compact 2 transistor transmitter for use at VHF frequencies.

2 Transistor Transmitter Schematic Circuit Diagram

Notes:
Transistor T1 works as an audio preamplifier, gain is fixed at approximately R2/R1 or 100 times. The audio input is applied at the points LF in (on the diagram). P1 works as gain control. After amplification this audio signal now modulates the transmitter built around T2. Frequency is tunable using the trimmer CT and L1 is made using 3 turns of 1mm copper wire wound on a 5mm slug. The modulated signal passes via C6 to the antenna. A dipole can be made using 2 lengths of 65cm copper pipe. A DC power supply in the range 3 to 16 volts is required.

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300m FM Transmitter

This fm transmitter circuit is very simple and it has a acceptable transmission . The signal transited from this fm transmitter circuit can be received at almost 300 meters in open air .The circuit require a 3volts operating voltage and can be tuned anywhere in the FM band.The coil should be about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in diameter.

300m FM Transmitter Circuit diagram

After the coil in soldered into place spread the coils apart about 0.5 to 1mm so that they are not touching. If you don’t have a trim cap you can use a fixed value capacitor and you can vary the TX frequency by adjusting the spacing of the coils or placing a small piece of ferrite inside the coil , but the better way to change the transmission frequency is to use a variable capacitor .Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively.

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Miniature FM Transmitter

Miniature FM Transmitter Circuit Diagram 

Notes:
The default for the capacitors type is ceramic, preferably the npo 1% (low noise) type or equivalent. But basically nothing critical here. Use any capacitor you have laying around, but NO electrolytic or tantalum caps. Only if you intend to use this circuit outside the home you may want to select more temperature stable capacitors.

To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit wont work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. Im sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

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300m FM Transmitter

This fm transmitter circuit is very simple and it has a acceptable transmission . The signal transited from this fm transmitter circuit can be received at almost 300 meters in open air .The circuit require a 3volts operating voltage and can be tuned anywhere in the FM band.The coil should be about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in diameter.

300m FM Transmitter Circuit diagram

After the coil in soldered into place spread the coils apart about 0.5 to 1mm so that they are not touching. If you don’t have a trim cap you can use a fixed value capacitor and you can vary the TX frequency by adjusting the spacing of the coils or placing a small piece of ferrite inside the coil , but the better way to change the transmission frequency is to use a variable capacitor .Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively.

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4 Transistor Transmitter Schematic

This circuit provides an FM modulated signal with an output power of around 500mW. The input Mic preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset. The oscillator is a colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF capacitors and the inductor. ( Click here for Colpitt Oscillator Resonant Frequency Equation.) Frequency is around 100Mhz with values shown.

4 Transistor Transmitter Circuit Diagram

Audio modulation is fed into the tank circuit via the 5p capacitor, the 10k resistor and 1N4002 controlling the amount of modulation. The oscillator output is fed into the 3.9uH inductor which will have a high impedance at RF frequencies.

The output stage operates as a class D amplifier, no direct bias is applied but the RF signal developed across the 3.9uH inductor is sufficient to drive this stage. The emitter resistor and 1k base resistor prevent instability and thermal runaway in this stage.

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Shortwave SW Transmitter Based IC BEL1895

Here the SW transmitter circuit based on IC BEL1895. This particular transmitter circuit works in shortwave HF band (6 MHz to 15 MHz), and can be applied for shortrange communication and for educational purposes.

Shortwave (SW) Transmitter Circuit diagram :

The circuit is composed of a mic amplifier circuit, a variable frequency oscillator, and modulation amplifier stages. Transistor T1 (BF195) is put to use as a simple RF oscillator. Resistors R6 and R7 determine base bias, while resistor R9 is utilized for stability. Feedback is provided by 150pF capacitor C11 to maintain oscillations. The primary of shortwave oscillator coil and variable condenser VC1 (365pF, 1/2J gang) form the frequency determining network.

By altering the coil inductance or the capacitance of gang condenser, the frequency of oscillation can be modified. The carrier RF signal from the oscillator is inductively coupled through the secondary of transformer X1 to the next RF amplifier-cum-modulation stage assembled around transistor T2 that is run in class ‘A’ mode. Audio signal from the audio amplifier assembled around IC BEL1895 is coupled to the emitter of transistor 2N2222 (T2) for RF modulation.

IC BEL1895 is a monolithic audio power amplifier intendeded for sensitive AM radio applications. It can deliver 1W power to 4 ohms at 9V power supply, with low distortion and noise characteristics. Since the amplifier’s voltage gain is of the order of 600, the signal from condenser mic can be straightly linked to its input without any amplification.

The transmitter’s stability is managed by the quality of the tuned circuit parts as well as the degree of regulation of the supply voltage. A 9V regulated power source is required. RF output to the aerial consists of harmonics, because transistor T2 doesn’t have tuned coil in its collector circuit. However, for short-range communication, it does not create any trouble. The harmonic content of the output may be lowered by means of a high-Q L-C filter or resonant L-C traps tuned to each of the prominent harmonics. The power output of this transmitter is about 100 milliwatts.

Read More..