Wireless Doorbell Transmitter Circuit

Posted by Micro System

on Thursday, August 5, 2010

This is a circuit for transmitter that can be used in doorbell circuit. This circuit is a simple form. The circuit is based on three transistor. This is the figure of the circuit;


The 303MHz oscillator consists of a self-oscillating circuit made up of the coil on the PC board and a 9p (9 puff) capacitor (actually 4p and 5p in parallel). The circuit starts-up by the transistor producing noise. This rising-and-falling signal on the collector is passed to the parallel tuned circuit (the tank circuit) and the base sees a very smooth sine wave at a frequency of 303MHz. This sine wave is then amplified by the transistor and this is how the 303MHz frequency is generated. Now we come to the purpose of the 15microhenry choke on the tank circuit. When the circuit oscillates, it takes a larger and small amount of current. This current passes through the choke and the turns produce a back-emf or back voltage that fights against the flow (change) in current. The end effect is a voltage created at the point where the choke is connected to the track-work on the board. This effectively allows the track work to produce a waveform and since the frequency of this wave is very high, a percentage of the energy is radiated into the air as electromagnetic energy. The choke allows the track-work to effectively rise and fall while providing a very low resistance path for the flow of current during certain parts of the cycle.

Stereo FM Transmitter Circuit

Posted by Micro System

on

This is a circuit for transmitter circuit. This circuit can be used for stereo FM radio. This is the figure of the circuit;


The circuit is a result of many hours of testing and tweaking. The goal was simple; to test many existing BA1404 transmitter designs, compare their performance, identify weaknesses and come up with a new BA1404 transmitter design that improves sound quality, has very good frequency stability, maximizes transmitter's range, and is fairly simple for everyone to build. We are happy to announce that this goal and expectations have been met and even exceeded. The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range.

FM Transmitter Circuit Using LM358

Posted by Micro System

on Monday, August 2, 2010

This is a low power FM Transmitter using an op-amp as the audio preamp and a single transistor as the RF amplifier. This circuit is based on LM358. This is the figure o the circuit;


L1 is 0.112uH (this tunes to the middle of the FM band, 98 MHz, with VC1 at its centre value of 33pF). L1 is 5 turns of 22 swg enameled copper wire close-wound on a 5mm (3/16") diameter former. Alternatively, you can have a fixed 33pF cap instead of VC1 and have L1 as an adjustable molded coil (eg UF64U from Maplin). VC1 will give you a tuning range of 85 - 125 MHz, and a possible choice is the Philips type polypropylene film trimmer (Maplin code WL72P). Two sets of oscillator bias resistors are given, the ones in the brackets give about 20% more RF power. Q1 is configured as a Clapp oscillator. Frequency modulation results from the audio voltage changing the transistor's base-emitter capacitance.

Part;
R1 4K7 R4 150K R7 3K9 (2K7)
R2 4K7 R5 220R R8 120R (82R)
R3 4K7 R6 4K7

All resistors except R8 are at least 0.25W rated. R8 is at least 0.5W rated
(the 0.6W metal film M-series from Maplin can be used for R1-R8).

C1 1n C4 22uF C7 10n C10 1n
C2 4u7 C5 1n C8 1n
C3 1n C6 10n C9 33pF
VC1 5-60pF IC1 LM358 Q1 ZTX108

Wireless Doorbell Transmitter Circuit

Posted by Micro System

on Saturday, July 31, 2010

This is a circuit for transmitter circuit that can be used for wireless doorbell circuit. The transmitter circuit is made up of two building blocks - the 303MHz RF oscillator and the 32 KHz crystal controlled oscillator. This is the figure of the circuit;


The 303MHz oscillator consists of a self-oscillating circuit made up of the coil on the PC board and a 9p (9 puff) capacitor (actually 4p and 5p in parallel). The circuit starts-up by the transistor producing noise. This rising-and-falling signal on the collector is passed to the parallel tuned circuit (the tank circuit) and the base sees a very smooth sine wave at a frequency of 303MHz. This sine wave is then amplified by the transistor and this is how the 303MHz frequency is generated.

When the crystal is added, the frequency increases (because the effect of the 2n2 and crystal in series creates a lower capacitance than 2n2) and as it rises, the amplitude of the feedback signal increases until it reaches a maximum at the resonant frequency of the crystal. The crystal exhibits the lowest impedance (the highest capacitance) at the resonant frequency. This is how the circuit stabilizes at the frequency of the crystal. When the device is turned on, the 150k on the base of the second transistor turns the transistor on. The third transistor has 0.65v on the collector and the base also receives very close to 0.65v, via the 220k resistor. The third transistor is not fully turned on and it produces a small amount of noise. This noise is passed to the second transistor and appears on the collector. The collector passes this noise to the base of the third transistor and the noise very rapidly increases to a maximum. It comes to a point where the waveform above is generated and the reason why the spikes are so narrow is easy to explain.

PC16550D Universal Asynchronous Receiver/Transmitter Circuit with FIFOs

Posted by Micro System

on Friday, July 23, 2010

This a circuit for universal transmitter and receiver circuit. This circuit is based on PC16550D IC from National Semiconductor. This circuit have basic circuit that can be control using microcontroler. This is the figure of the circuit;


The PC16550D is an improved version of the original 16450 Universal Asynchronous Receiver/Transmitter (UART). Functionally identical to the 16450 on powerup (CHARACTER mode)* the PC16550D can be put into an alternate mode (FIFO mode) to relieve the CPU of excessive software overhead. In this mode internal FIFOs are activated allowing 16 bytes (plus 3 bits of error data per byte in the RCVR FIFO) to be stored in both receive and transmit modes. All the logic is on chip to minimize system overhead and maximize system efficiency. Two pin functions have been changed to allow signaling of DMA transfers.

The UART performs serial-to-parallel conversion on data characters received from a peripheral device or a MODEM, and parallel-to-serial conversion on data characters received from the CPU. The CPU can read the complete status of the UART at any time during the functional operation. Status information reported includes the type and condition of the transfer operations being performed by the UART, as well as any error conditions (parity, overrun, framing, or break interrupt).

[Circuit schematic source: National Semiconductor Notes]