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TV Transmitter Circuit
Posted on Wednesday, January 4, 2012   •   Category: TV Transmitters


This is a small TV transmitter circuit which transmits in VHF band, negative sound modulation and PAL video modulation. It is suitable in countries where the B and G system is used.


Capacitance Meter
Posted on Tuesday, January 3, 2012   •   Category: Test and Measurement


RF laboratory often requires capacitance meter for small capacitors in pF range. Such a device can easily be built by yourself. Here, a measurement converter for PC serial port is presented. The frequency of an oscillator is reduced by the target and measured on a PC. The appropriate conversion then allows the direct display of the capacity. The input uses a short and low capacitance probe tip. The opposite pole is clamped to ground cable with a crocodile. The NE555 precision timer receives its operating voltage directly from the serial interface and produces a measurement object without C is a square wave with a frequency of 3.5 kHz. The signal is processed via the CTS input of the interface.


Function Generator XR2206
Posted on Saturday, December 31, 2011   •   Category: Test and Measurement


For measurement purposes in the electronics laboratory is needed again and again signals of different frequency and waveforms. A common function generator provides sine, for example, triangular and square waves. The frequency must be adjustable and at least cover the low frequency range. The low-cost IC XR2206 provides a very simple function generator with only a few external components. XR2206 data sheet provides complete basic circuit for a simple function generator. It requires an operating voltage of 12 V and delivers sine and square wave signals. Instead of the sine wave output is obtained after opening of S1 a triangular output wave. XR2206 IC contains an internal VCO (Voltage Controlled Oscillator, Voltage Controlled Oscillator) with triangular and rectangular output. The capacitor C and the power to determine the frequency at pin 7. With a pot of 2 megohms and a fixed resistor of 1 kOhm variation gives a ratio of 1 to 2000 and may include a range of 10 Hz to 20 kHz sweep.


Phone Spy Transmitter
Posted on Friday, December 30, 2011   •   Category: FM Transmitters


Here is a very simple telephone broadcaster transmitter which can be used to eavesdrop on a telephone conversation. The circuit can also be used as a wireless telephone amplifier. One important feature of this phone transmitter is that the circuit derives its power directly from the active telephone lines, and thus avoids use of any external battery or other power supplies.


1Watt AM CW Transmitter for 10 Meterband
Posted on Wednesday, December 28, 2011   •   Category: FM Transmitters


In this project, you will make a simple 3-stage low-power broadcast-type circuit, using a crystal oscillator integrated circuit and an a collector modulated AM oscillator with amplifier. You can connect the circuit to the an electred microphone or amplified dynamic microphone. Using an electred microphone is shown (in gray) in the diagram below. (no amplified dynamic microphone has a to low output voltage to work. at least 100mv is needed). You could also add a LF preamp stage of one transistor to allow connecting a dynamic microphone directly. You'll see that you can receive the signal through the air with almost any AM radio receiver. Although the circuits used in radio stations for AM receiving are far more complicated, this nevertheless gives a basic idea of the concept behind a principle transmitter. Plus it is a lot of fun when you actually have it working! Remember that transmitting on the 10 meter band you'll need a valid radioamateur license!! A wide range of different circuits have been used for AM, but one of the simplest circuits uses collector modulation applied via (for example) a transformer, while it is perfectly possible to create good designs using solid-state electronics as I applied here (T1 BC557). The transmitter is build as a Colpitts Oscillator with a BSX20 transistor. HF-output of the oscillator is approx. 50 mW, depending on the supply voltage of 6 to 15 Volts. This is amplified by the BD135 and brings the power up to approx. 1 watt @ 12volts. The transmit frequency is stabilized with the 28Mhz crystal. A slight detuning of approx 1kc is possible when using a 120pF trimmer capacitor for C8. The oscillator signal is taken from the collector of T2 and guided to the input of T3 which output is lead via an L-filter and low-pass PII filter circuit cleaning up the signal pretty good and ensuring spectral purity. The oscillator is keyed by T1 and the morse key (S). By keying the morse-key T1 is not been used for modulation and is biased, hence lets T2 freely oscillate.


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