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Temperature Controlled Fan
Posted on Tuesday, April 19, 2011   •   Category: Sensors

With this simple circuit you will be able to control the speed of a DC fan according to temperature measured by a temp sensor. Itís an ideal add-on for your PC cooling fans to eliminate produced noise. Requested by some correspondents, this simple design allows an accurate speed control of 12V dc fan motors, proportional to temperature. A n.t.c. Thermistor (R1) is used as temperature sensor, driving two directly coupled complementary transistors wired in a dc feedback circuit. An optional circuitry was added to remotely monitor fan operation and to allow some sort of rough speed indication by means of the increasing brightness of a LED.

Four Channel Power Amplifier
Posted on Monday, April 18, 2011   •   Category: Amplifiers

I am a big fan of surround sound. Up until the moment I finished this amp, I was using two two-channel amplifiers to power four speakers. This was very annoying, because it meant I had two volume controls. This invariably meant that the balance between front and rear was off. Added to this, was that the amplifier for the rear speakers was not very good. It produced too much noise, and allowed clicks and pops from the power grid through, to be amplified and heard. I decided I was going to build a four channel power amplifier, and later a pre-amplifier to feed it. You might wonder why I didn't choose to make a five channel one instead. This is because I don't see the use of the center channel. I will probably write an article detailing why, but for now, suffice it to say that four channels is enough for 2 dimensional sound, which is what 5.1 is as well.

1KM Power FM Transmitter
Posted on Sunday, April 17, 2011   •   Category: FM Transmitters

This small power FM transmitter can transmit more than 1 km in good conditions. The modulation can be made so much with a microphone or audio source. Circuit of power fm transmitter is built around 2n2218 transistor. Transmitter coil is 5 turns of enameled 22 AWG wire, with diameter of 1 cm without nucleus. Look at the capacitors that it should be ceramic. The antenna should possess from 15 to 40 cm. For transmission it ties a receiver of FM (radio) in the proximity to half volume in a free frequency (that there is not any radio operating), with a wood or plastic key, rotate the screw of CV to capture the frequency of the transmitter.

Digital ON-OFF Tactile Button Relay Switch
Posted on Saturday, April 16, 2011   •   Category: Miscellaneous

Small circuit, with which we can use to get rid of the standard switch and operate any type of load with a push of a tactile button. Voltage between 5 and 18 V DC ( depends on the operating voltage of the relay coil ) in terminal CN1. Output Current is 10A ( depends on relay contacts ) in terminal CN3 activated by external positive pulse or short-circuiting the terminal CN2. Circuit is protected against double pulses and rebounds.

1.5V Battery Operated Rebroadcast FM Transmitter
Posted on Friday, April 15, 2011   •   Category: FM Transmitters

This implementation is adapted to rebroadcast the output of a CD player, television receiver, or radio receiver. I use it so that I can move about the house and listen to my favorite programs without disturbing others. Within and the house, I find that I can get 10 to 20 meters away from the transmitter with the small pocket FM receiver I carry in my shirt pocket. Your mileage may vary. The transmitter as built and pictured below (the transmitter is in the blob of hot melt glue on the end of the battery holder) does not have an on-off switch. I put a 1.5 AA cell that was run down too far to run my CD player in this transmitter and it ran for over a month before I replaced it. The one in the transmitter at this moment has been running it continuously for over three months. Current draw is only about a milliamp with a new battery (assuming you don't have a super-high beta transistor in which case the theoretical limit is about 2.5 ma). An on-off swich is not necessary, though it may satisfy an emotional need.

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