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Thursday, July 31, 2008

Active Sub Whoofer

Variable Regulated Powersupply(1.2 to 30V @ 1.5 Amps)

Parts List
BR1 = Bridge Rectifier, 100V - 3A
C1 = 2200 ìF, 63V
IC1 = LM317, adjustable regulator
C2 = 0.1 ìF
V = Meter, 30V,
Ri = 85 ohm
C3 = 1 ìF, 40V
TR1 = Transformer, 25V, 2A Plug = 3-wire plug & cord
R1 = 1K8, 5%
S1 = On-Off toggle switch
R2 = 220 ohm, 5%
D1 = 1N4001
R3 = 27K, 5%
Fuse = 110V, 500mA, slow-blow
P1 = 5K, potentiometer FuseHolder, wire, solder, case, knob for P1
P2 = 10K, 10-turn trim-pot Red & Black Banana Jacks

Couple Notes:

This is a simple, but low-ripple powersupply, and an excellent project if you're starting out in electronics. It will suit your needs for most of your bench testing and prototypeapplications. The output is adjustable from 1.2 volts to about 30 volts. Maximum current is about 1.5 amps which is also sufficient for most of your tinkering. It is relatively easy to build and can be pretty cheap if you have some or all the required parts. A printed circuit board is not included and I'm not planning on adding one since the whole thing can easily be build on perforated or vero board. Or buy one of Radio Shack/Tandy's experimenters boards (#276-150). Suit yourself. The meter and thetransformer are the money suckers, but if you can scrounge them up from somewhere it will reduce the cost significantly. BR1 is a full-wave bridge rectifier. The two '~' denotes 'AC' and are connected to the 25vac output coming from the transformer. IC1 is a 3-pin, TO-220 model. Be sure to put a cooling rib on IC1, at it's max 1.5 A current it quickly becomes very hot...
Most of the parts can be obtained from your local Radio Shack or Tandy store. The physical size of the power supply case depends largely on the size of the meter &: transformer. But almost anything will do, even wood. Go wild.

Circuit Description:
The 110V-AC coming from the powercord is fed to the transformer TR1 via the on-off switch and the 500mA fuse. The 30vac output (approximately) from the transformer is presented to the BR1, the bridge-rectifier, and here rectified from AC (Alternating Current) to DC (Direct Current). If you don't want to spend the money for a Bridge Rectifier, you can easily use four general purpose 1N4004 diodes. The pulsating DC output is filtered via the 2200ìF capacitor (to make itmore manageable for the regulator) and fed to 'IN'-put of the adjustable LM317 regulator (IC1). The output of this regulator is your adjustable voltage of 1.2 to 30 volts varied via the 'Adj' pin and the 5K potmeter P1. The large value ofC1 makes for a good, low ripple output voltage.
Why exactly 1.2V and not 0-volt? Very basic, the job of the regulator is two-fold; first, it compares the output voltage toan internal reference and controls the output voltage so that it remains constant, and second, it provides a method for adjusting the output voltage to the level you want by using a potentiometers. Internally the regulator uses a zener diode to provide a fixed reference voltage of 1.2 volt across the external resistor R2. (This resistor is usually around 240 ohms, but220 ohms will work fine without any problems). Because of this the voltage at the output can never decrease below 1.2volts, but as the potentiometer (P1) increases in resistance the voltage across it, due to current from the regulator plus current from R2, its voltage increases. This increases the output voltage.
D1 is a general purpose 1N4001 diode, used as a feedback blocker. It steers any current that might be coming from the device under power around the regulator to prevent the regulator from being damaged. Such reverse currents usually occur when devices are powered down.The 'ON' Led will be lit via the 18K resistor R1. The current through the led will be between 12 - 20mA @ 2V depending on the type and color Led you are using. C2 is a 0.1ìF (100nF) decoupler capacitor to filter out the transient noise which can be induced into the supply by stray magnetic fields. Under normal conditions this capacitor is only required if the regulator is far away from the filter cap, but I added it anyway. C3 improves transient response. This means that while the
regulator may perform perfectly at DC and at low frequencies, (regulating the voltage regardless of the load current), at higher frequencies it may be less effective. Adding this 1 ìF capacitor should improve the response at those frequencies.
R3 and the trimmer pot (P2) allows you to 'zero' your meter to a set voltage. The meter is a 30Volt type with an internal resistance of 85 ohms. I you have or obtained a meter with a different Ri (internal resistance) you will have to adjust R3 to keep the current of meter to 1mA. Just another note in regards this meter, use the reading as a guideline. The reading may or may not be off by about 0.75volts at full scale, meaning if your meter indicates 30 volts it may be in reality almost 31volts or 29 volts. If you need a more precise voltage, then use your multimeter.

Because of the few components you can use a small case but use whatever you have available.I used a power cord from a computer and cut the computer end off. All computer power cords are three-prong. The ground wire, which is connected to the middle pin of the power plug is connected to the chassis. The color of the ground-wire is either green or green/yellow. It is there for your protection if the 110vac accidentally comes in contact with the supply housing (case). BE CAREFUL always to disconnect the powerplug when you working inside the chassis. If you choose to use an in-line, or clip-type fuseholder be sure to isolate it with heat shrink or something to minimize accidental touching.
I use perf-board (or Vero board) as a circuit board. This stuff is widely available and comes relatively cheap. It is either made of some sort of fiber material or Phenolic or Bakelite pcb. They all work great. Some Phenolic boards come with copper tracks already on them which will make soldering the project together easier. I mounted the LM317(T) regulator on a heatsink. If you use a metal/aluminum case you can mount it right to the metal case, insulated with the mica insulator and the nylon washer around the mounting screw. Note that the metal tab of theLM317 is connected internally to the 'Output' pin. So it has to be insulated when mounting directly to the case. Use heat sink compound (comes in transparent or white color) on the metal tab and mica insulator to maximize proper heat transfer between LM317 and case/ or heatsink.
Drill the holes for the banana jacks, on/off switch, and LED and make the cut-out for the meter. It is best to mount everything in such a way that you are able to trouble-shoot your circuit board with ease if needed. One more note about the on-off switch S1, this switch has 110VAC power to it. After soldering, insulate the bare spots with a bit of silicon gel. Works great and prevents electrical shock through accidental touching.
If all is well, and you are finished assembling and soldering everything, check all connections. Check capacitors C1 & C3 for proper polarity (especially for C1, polarity reversal may cause explosion). Hookup a multimeter to the power supply output jacks. Set the meter for DC volts. Switch on S1 (led will light, no smoke or sparks?) and watch the meter movement. Adjust the potentiometer until it reads on your multimeter 15Volts. Adjust trimpot P2 until the meter also reads 15volts. When done, note any discrepancies between your multimeter and the power supply meter at full scale (max output). Maybe there is none, maybe there is a little, but you will be aware of it. Good luck and have fun building!

Final Note:
You can add two silicon diodes (in series) to the output of the LM317 to drop the final 1.2V, giving the full 0-30V range. I built a similar supply using the LM317, to supply a wafer coating spinner motor. The 1.2V kept the motor spinning at over100rpm, which was unacceptable to the researcher, who needed to ramp the motor speed from 0-8krpm.

Active Antenna for AM/FM/SW

This simple little circuit can be used for AM, FM, and Shortwave(SW). On the shortwave band this active antenna is comparable to a 20 to 30foot wire antenna. It is further more designed to be used on receivers that use untuned wire antennas, such as inexpensive units and car radios.

L1 can be selected for the application. A 470μH coil works on lower frequencies and lie in AM, for shortwave try a 20μH coil. This unit can be powered by a 9 volt alkaline battery. If a power supply is used, bypass the power supply with a 0.04μF capacitor to prevent noise pickup. The antenna used on this circuit is a standard 18-inch telescoping type, but a thick piece of copper, bus-bar, or piano wire will also work fine.
The heart of this circuit is Q1, a JFET-N-Channel, UHF/VHF amplifier in a TO-92 case.It can be replaced with an NTE451.Output is taken from jack J1 and run to the input on the receiver.

FM Radio Antenna Amplifier

Active FM-Amplifier:

With only a small handful of parts you can built this trusty FM Amplifier. It works with only 1 UHF/VHF type transistor, MFE201. This amplifier will pull in all distant FM stations clearly. The circuit is configured as a common-emitter tuned RF pre-amplifier wired around VHF/UHF transistor MFE201. There are a couple other models that probably would work too, like the NTE107, 2SC2570, etc. but I have not tried it.

Adjust capacitor trimmers C1 and C2 for maximum gain. Input coil L1 consists of 4 turns of 20SWG enameled copper wire over a 5mm diameter former. It is tapped at the first turn from ground lead side. Coil L2 is similar to L1, but has onlythree turns. Pin configuration is shown in the diagram.

Wednesday, July 9, 2008

AM DSB Transmitter for HAMS

This circuit of AM transmitter is designed to transmit AM(amplitude modulated) DSB (double side Band) signals. A modulated AM signal consists of a carrier and two symmetrically spaced side bands. The two side bands have the same amplitude and the carry same information. In fact, the carrier itself conveys or carries no information. In a 100% modulated AM signal 2/3rd of the power is wasted in the carrier and only 1/6th of the power in each side band
In this transmitter we remove the carrier and transmits only the two side bands. The effective output of the circuit is three times that of an equivalent AM transmitter.

Opamp IC741 is added here as a microphone amplifier to amplify the audio signals from the condenser mike. The output of the Opamp is fed to the double balanced modulator build around In4148 diodes. The modulation level can be adjusted with the help of preset VR1.
The carrier using cristal wired around BC548 transistor T2. The carrier is further amplified bt transistor T1, which also acts as a buffer between modulator. The working frequencey of the transmitter can be changed by using the cristals of the diffreent frequencies. For multi-frequency operation, selection of different cristals can be made using a selector switch. The level of the carrier coupled to the DEM(double Balanced Modulator) cna be adjusted with the help of preset VR2.

The output of the DBM contains only the product (of audio and carrier) frequencise. The DBM supress both the input signals anf produce the double side band supressed carrier(DSBSC) at its output. However, since the diodes used in the balanced modulator are not fully matched , the output of the DBM does contain some residual carrier. This is known as carrier leckage. By adjusting the 100-ohm preset(VR2) and the trimmer (c7) you can nullify the carrier leckage.

To receive DSb signals you need a Beat Frequency Oscillator(BFO) to reinsert the missing carrier. If you dodn't have a BFO, or want to transmit only AM signal, adjust preset VR2 to leak some carrier so thar you can receive the signals on any ordinary radio receiver. In AM mode 100% modulation can be attained by adjusting presets VR1 and VR2.
The DSBSC signal available at the output of the balanced modulator is amplified by two stages of RF libnear amplifiers. Transisor 2n2222A(t3) is used as an RF amplifier., which provides enough signal amplification to drive the final power amplifier around transistor SL100B. The output of the final power amplifier is connected to the antienna.

All coils are to be wound on ferrite balun cores(same ast used in TV balun transformer of size 1.4cm x 0.6 cm) using 24 SWG enameled copper wire. proper heat-sink should be provided for SL100B transistor used as final power amplifier.

Range of the order of a few kilometers can be easily achieved by proper choice of the site, type of antenna (such as a resonant half-wave dipole of lenght 10 meters for 7.08 Mhz frequency) and proper matching of transmitter to the antanna. Use good quality shielded wire of short length to connect the crystals.