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Wednesday, October 28, 2009

Long Range FM Transmitter Circuit

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This circuit is a circuit diagram fm transmitter. This circuit is somewhat different from the previous fm transmitter circuit. Transmitter circuit described here has the additional RF power amplifier stage, after the oscillator stage, to increase the power output of 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element yagi antenna, this transmitter can provide a good enough signal strength to a distance of about 2 kilometers. The circuit built around transistor T1 (BF494) is the basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts.
Transistor T2 (2N3866) forms a VHF-class power amplifier. This increases the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power produced at the collector of transistor T2. For better results, assemble the circuit on a good quality glass epoxy board and house the transmitter in the case of aluminum. Shield the oscillator stage using aluminum sheets. Transistor T2 must be mounted on the heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

Coil winding details are given below:
L1 – 4 changes of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 changes of 24 SWG wire near top end of L1.
(Note: There is no core (ie air core) is used to coil on top)
L3 – 7 changed from 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 changed from 24 SWG wire-wound on ferrite beads (choking)
Potentiometer VR1 is used to change the fundamental frequency whereas potentiometer VR2 is used as power control.

Simple DC to AC Inverter

This DC to AC inverter circuit work based on unstable multi vibrator does. In this circuit, IC CD4047 is chosen as a heart of unstable multi vibrator, because this IC type gives a complementary output that has opposite phase to another ( pin 10 and 11 as seen in Figure 1), and has 50 % duty cycle that satisfy to generate a pulse for inverter.

In order to increase the current out of multi
vibrator so enough to generate a higher AC power too, then we must use MOSFET IRFZ44. IRFZ44 gives out high current to drive a step-up transformer, so AC power is available at the high voltage side of transformer.
This circuit is called as simple DC to AC inverter because of the output haven't a sinusoidal signal

yet, so there are many harmonic signal at the output. To suppress this signal we must use a filter such as a capacitor C. Because of this simplicity this circuit is suitable only for lighting demand. To build a sinusoidal DC to AC inverter, we can use a PWM signal for driving a step-up transformer, such as at page of DC to AC inverter using AT89C2051.

Friday, October 23, 2009

Parrot Sounding AC door Bell

Here is a mains-operated doorbell that produces parrot-likesweet voice without requiring any musical IC. The circuit is cheap and easy to construct. The AC mains is fed to the circuit without using any step-down transformer.

The complete circuit is shown in Fig. 1. The main components of the circuit are a resistor-capacitor network, transistor BC337 and audio output transformer X1. The oscillation frequency depends on the combination of resistors R4 and R5 and capacitors C3, C4 and C5. When switch S1 is closed, the audio signal generated due to oscillations is amplified by transistor BC337 and parrot-like sound is reproduced from loudspeaker LS1 connected across the secondary of transformer X1. Here we have used an 8-ohm, 0.5W loudspeaker. The audio output transformer (X1) is normally used in transistor radio. The function of the audio output transformer is to transform the high impedance of the output amplifier to match the much lower impedance of the speaker. This is necessary to get an efficient transfer of the audio signal to the speaker. If a wrong audio transformer is used, the result can be low output and loss of tone quality.

The audio frequency tone across the speaker terminal is about 3 kHz. The dimensions of the audio transformer used in the experimental setup are shown in Fig. 2. The circuit is powered directly from 220V AC mains. The operating DC voltage obtained at the cathode of diode D1 is about 6V. However, if you press switch S1 continuously for a few seconds, the maximum voltage developed at this point may go up to 20 volts, which must be avoided to prolong the life of the circuit. R1 limits surge current in the circuit. The parallel combination of resistor R1 and capacitor C1 limits the circuit current to a safe level for circuit operation. R2 across C1 provides DC path for the current as well as a discharge path when the circuit is switched off. This is to prevent a possible shock to the operator by charged capacitor C1.

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