Circuit-Zone.com - Electronic Projects

Broadcast FM Transmitter
Posted on Tuesday, April 12, 2011   •   Category: FM Transmitters

Broadcast FM Transmitter

Here's a nice AC mains powered FM Broadcast Audio Transmitter with pre-emphasis, audio level control, and tuning control. The circuit consists of a frequency modulated oscillator, an audio preamplifier with pre emphasis to supply the frequency modulating signal, and a buffer amplifier to drive the antenna connector. Oscillator's frequency is determined by L1 resonating with the 10 pf capacitor and the total capacitance across it. The collector-base capacitance of the transistors Q3, Q4, and Q5 is a function of their revers bias. This is basically a poor man's (or lazy man's) varactor. The voltage across Q3 is set by a voltage divider and is then modulated by an Ac coupled audio signal from the pre amp, causing the reverse bias to vary with the audio signal, which changes the resonant frequency of L1's circuit, causing the frequency of the oscillator to vary with the audio signal. The capacitance of Q4 and Q5 is adjusted by DC bias from the tuning adjustment potentiometer, and this capacitance sets the center frequency of the oscillator. All of the transistors in the oscillator -Q1 through Q5, are 2N4401. The purpose of the buffer is to minimize frequency shift as loading on the antenna is changed. It was specifically designed to reduce the signal amplitude to the antenna. Transmitters should not use any more power than is necessary to achieve the task at hand, and lightly coupling the RF into the buffer's base with a gimmick capacitor did the trick. The transistor is an MPSH34.

1.5V to 5V/12V DC/DC Converter with LT1073
Posted on Monday, April 11, 2011   •   Category: AC / DC Innveters

1.5V to 5V/12V DC/DC Converter with LT1073

Small 1.5V to 5V or 12V DC/DC converter with LT1073 chip. The IC is available in three different versions, depending on output voltage. Two with fixed output voltage of 5V and 12V, and the most interesting that can be adjusted. The adjustment is done through a voltage divider with two resistors, of mass, output and Terminal 8, internally connected to the voltage comparator IC, which is responsible for stabilizing the output voltage.

RF Remote Control Light Switch
Posted on Sunday, April 10, 2011   •   Category: Remote Control

RF Remote Control Light Switch

This is a remote controllable light switch that comes with an RF remote. The only light switch is across the room from my PC and it's a pretty large room. (The building's basically a 1-room apartment) so this works out great with the remote. Of course since I'm using the remote to cut the lights when I go to bed I'm basically using the remote from two places which brings with it the unavoidable annoyance of the remote being in the wrong place all the time. Which means I have to get up and look for it which is effectively as much of an annoyance as it was meant to solve. So I wanted a second controller that would basically be a stationary switch by my bed so I could leave the portable remote around the desk.

1.5V Spy FM Transmitter Bug
Posted on Saturday, April 9, 2011   •   Category: FM Transmitters

1.5V Spy FM Transmitter Bug

Here's a tiny one transistor spy FM transmitter bug that operates from a single 1.5V AA battery. Main advantage of this circuit is that power supply is a 1.5 Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it draws 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations. Mini FM transmitters take place as one of the standard circuit types in an amateur electronics fan's beginning steps. When done right, they provide very clear wireless sound transmission through an ordinary FM radio over a remarkable distance. I've seen lots of designs through the years, some of them were so simple, some of them were powerful, some of them were hard to build etc.

Solid State Relay
Posted on Friday, April 8, 2011   •   Category: Miscellaneous

Solid State Relay

Solid state relay is a hybrid circuit, normally composed of an optocoupler which isolates the input, a trigger circuit that detects the zero crossing of the line current and a triac or similar device that acts as a circuit breaker. Its name comes from the similarity presents an electromechanical relay, this device is generally used for applications which has a continuous use of the contacts of the relay compared to a conventional relay generate a serious mechanical wear, besides being able to switch high amperage in the case of electromechanical relays in a short time would destroy the contacts. These relays allow switching speeds much higher than the electromechanical relays.

300m FM Transmitter
Posted on Thursday, April 7, 2011   •   Category: FM Transmitters

300m FM Transmitter

This FM transmitter is about the simplest and most basic FM transmitter it is possible to build and have a useful transmitting range. It is surprisingly powerful despite its small component count and 3V operating voltage. It will easily transmit over 300 meters in the open air and even more with higher voltage supply. The circuit we use is based on a proven Australian design. It may be tuned anywhere in the FM band. Or it may be tuned outside the commercial M band for greater privacy. Of course this means you must modify your FM radio to be able to receive the transmission or have a broad-band FM receiver. The output power of FM transmitter is within the legal limits of many countries. However, some countries may ban all wireless FM transmitters without a license. It is your responsibility to check the legal requirements for the operation and to obey them. FM transmitter is constructed on a single-sided printed circuit board PCB.

Power RGB LED Controller
Posted on Wednesday, April 6, 2011   •   Category: LED

Power RGB LED Controller

Power RGB LED Controller is the extension of tiny RGB to drive high power LED’s. For this project I used a 3 x 1W common Anode RGB LED. Q1 to Q3 are N-Channel HEXFet Mosfet’s with logic level drive and a RDSon at about 50mOhms. R1 to R3 are at about 2k2, R4 to R6 at about 15k and R7 to R9 depend on the LED used and VCC. If you use FET’s with higher RDSon you have to consider RDS in your calculation!

Lithium Ion Battery Charger with Microchip MCP73831
Posted on Wednesday, April 6, 2011   •   Category: Battery Chargers

Lithium Ion Battery Charger with Microchip MCP73831

Here's a simple and inexpensive compact lithium ion / polymer battery charger based on Microchip's MCP73831 IC. It features adjustable current charging from 15mA up to 500mA for single cell lipo batteries. With this lipo charger just a few external components are needed. Constant voltage power supply of 5 ~ 6V is needed. Power can also be drawn from USB port. When USB power is 150mA charge current should not be higher. Charge current can be adjusted with external resistors. External LED provides status when lipo battery is fully charged. The LED lights when battery is fully charged. The maximum charging voltage is selectable from 4.2 to 4.5. This has to be chosen at the time of purchase. Typically 4.2V is a standard charging voltage. MCP73831-2 <- 4.2V, MCP73831-3 <- 4.3V, MCP73831-4 <- 4.4V, MCP73831-5 <- 4.5V.

24V to 12V 400W DC Inverter
Posted on Wednesday, April 6, 2011   •   Category: AC / DC Innveters

24V to 12V 400W DC Inverter

24V to 12V 20A 400W DC to DC Inverter. Does little to change my PV system 12v 24v me the problem arose of what to do with investors who already had 12V. I was looking for a pattern online and found several schemes with linear regulators 20A, this solution although quite simple, due to the huge losses they have is not advisable. Ideally, a converter switched, high-performance. At the end I found nothing I liked and decided to design my own. Circuit characteristics: Output current: 20A at 12V (15A continuous and 30A Momentary), Input voltage: 18 to 30V DC, Output voltage: 5 to 20V, Operating Frequency: 70kHz, Effectiveness: 95%, 400W maximum power, Protections: Above current (30A) in the F1 circuit, D1 and F1 polarity in the circuit.

PLL FM Transmitter using LMX1601, ATtiny2313 AT90S2313
Posted on Tuesday, April 5, 2011   •   Category: FM Transmitters

PLL FM Transmitter using LMX1601, ATtiny2313 AT90S2313

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.

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