Posted on Tuesday, April 5, 2011 • Category:
FM Transmitters
Here's a PLL FM Transmitter using LMX1601, ATtiny2313 or AT90S2313 microcontrollers. The common characteristic of all of the previous low power FM transmitters I've built over the decades, is that their operating frequency is determined by an LC resonant circuit. Some of them had excellent stability, some of them didn't, but I had always wanted to make one that is crystal controlled. Various schemes had been considered from time-to-time, including the direct approach of modulating the load capacitance of a a crystal oscillator, a whimsical phase modulation scheme involving a phase shifter, some balanced modulators, and limiting amplifiers, and at times, the down-to-earth and sober approach of modulating a VCO within a phase locked loop (PLL). While browsing Digikey's online catalog, I found the LMX1601 frequency synthesizer chip and thought: "Just maybe, the PLL approach is finally within my grasp."
The LMX1601, which apparently was designed for use in cell phones, includes everything need to make two phase locked loops except for the VCOs. More importantly, one of the PLLs, specifically the "AUX" PLL, is specified to work in the FM broadcast band. The LMX1600 and the LMX1602 were also considered, but the LMX1601 was selected because it has a "500 MHz option", meaning that it can work down to about 50 MHz.
Posted on Tuesday, April 5, 2011 • Category:
USB Interface Adapters
Here's the new 6 channel USB relay card for switching different appliances, lamps or motors by a computer program via the USB interface. There are certain switching sequences can be programmed via computer. The card was dispensed on SMD technology and is thus the ideal craft project for beginners and novices. Those who do not always wanted with his computer what Steruern. This can be done with the help of a web application even over the Internet. About the SPI connector could even imagine the relay card to cascade with other relay boards.
The relays max. 250V AC and have a switching current of 8A (depending on the used relay). Furthermore also 4 inputs, which are separated optocouplers, and monitors can be queried. AVR simulates a USB RS 232 interface can thus this relay, the card without problems using a terminal program or other software to be easily addressed. The USB driver in the AVR firmware was developed by Objective Development ( http://www.obdev.at/products/avrusb/projects-de.html ) and under the GPL2. Through this USB driver could have very special devices and SMD technology abandon. Her come out is a very compact board with dimensions of 100mm x 85mm. The setting of the simulated serial port is: 9600 baud, 8Datenbits, no parity and 1 stop bit further handshaking. To power supply reaches a normal 9 - 12V power supply.
Posted on Monday, April 4, 2011 • Category:
Miscellaneous
MOSFET transistors are excellent choice for driving high current devices such as motors or high power RGB LEDs. They offer very low switching resistance and very small heat dissipation compared to bipolar transistors. This guide is designed to explain how to drive N-Channel MOSFETs with a microcontroller such as PIC or ATMEGA. Transistors heat up when driving large loads because they have a voltage dropped over them (Vce), and Heat (Watts) = Voltage * Current. This leads to thermal runaway within the transistor, eventually driving the device to destruction if not handled carefully.
FET's are like digital switches, capable of turning on and off between the Drain and Source via a voltage potential at the Gate. When a FET is on, it usually has a resistance of less than 0.01 ohm, and when off, its like an open circuit. Because of the low resistance during the FET's on state, it can allow large amounts of current to pass through it without heating up.
FET's turn on by voltage potential, not an electric current, and in return they have a very high input impedance. With this in mind, you only need a voltage to turn them on, perfect for digital electronics.
Posted on Monday, April 4, 2011 • Category:
Miscellaneous
Here's a simple microphone amplifier based around TDA7050 IC. There are many schematics, the choice fell on the amplifier chip TDA7050, the only downside was it that it was not appointed to the microphone. By adding a resistor R1 in the scheme by 4,7 kOhm - amplifier can no longer work with conventional condenser microphones.
If you remove the resistor R3 - the alarm will be put on a small speaker with an impedance of winding 32 Om'a.
Voltage amplifier is powered by 3 - 5 volts, but as the amplifier must be nourished from 12 in the scheme of linear stabilizer was added at 5 V.
This scheme was checked with headphones (current consumption was 50 mA) and connect to the line input of your TV.
Posted on Sunday, April 3, 2011 • Category:
Amplifiers
The amp described on this page, is a very simple poweramp based on the National Semiconductor chip LM3875. According to National it's a chip meant for TVs, compact stereos etc. But many people claim that these chips are great high-end amps...
So I decided to try building one. The "design" work was quickly done, as I just used the guidelines and sample circuit of the datasheet. I designed a small PCB for the amp (I'm lazy), and I made it double sided to make it easier to keep all the ground lines separate, as recommended by National.
The prototype board can be seen below with a 100VA toroid I used for testing.
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