Musical Fidelity would let you believe that this is a Class A design. However, in common with most commercial amplifiers, it is a class AB amplifier - it simply has a rather high standing current in the output stage, which results in the first 8 watts or so being class A.
Posted on Wednesday, April 23, 2008 • Category: FM Transmitters
A simple FM transmitter links your home-entertainment system to a portable radio that can be carried around the house and into the back yard. For example, you can play music on the CD changer in your living room, and listen to it on a portable radio by the back-yard barbeque.
IC1 is a voltage-controlled oscillator with integrated varactor. Its nominal frequency of oscillation is set by inductor L1, and a 390nH value places that frequency at 100MHz. Potentiometer R1 then lets you select a channel by tuning over the FM band of 88MHz to 108MHz. Output power is about -21dBm into 50 (most countries accept emissions below 10dBm in the FM band).
The home system's left and right audio signals are summed by R3 and R4, and attenuated by the (optional) potentiometer R2. R2's wiper signal serves as a volume control by modulating the RF frequency. Signals above 60mV introduce distortion, so the pot attenuates down from that level.
In the absence of a standard FM radio antenna, 75cm (30 inches) of wire will suffice as a transmitting antenna. For best reception, it should be mounted parallel with the receiving antenna. The IC operates on a single supply voltage in the range 3V to 5V, but you should regulate the applied voltage to minimize frequency drift and noise.
Posted on Tuesday, April 22, 2008 • Category: Robots
The robot is controlled by a NXT-G program that listens to Bluetooth messages that tell it what to do. Normally, the commands are movement commands. These commands control the speed of the robot, whether it will go straight or turn left or right, or whether it will stop. It can also receive a command to do a radar-style scan with the ultrasonic sendor. When it gets such a command, it stops moving, and performs a sequence of ultrasonic measurements while rotating the sensor. It sends these measurements, along with data about what angle the sensor was looking at, back to the PC. When the scan ends, the robot starts responding to movement commands again.
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