4W FM Transmitter Circuit

TECHNICAL CHARACTERISTICS: 

• Stabilised tendency of catering: Vcc=12~16V
• Frequency of emission: 88~108MHz
• Consumption: 100~400mA

Circuit Diagram:

Materially:

• The resistors are 1/4W.
• R1, R2 10KOhm
• R3 47Ohm
• C1, C2 1nF
• C3 4,7uF/16V
• C4, C7, C8 0~45pF trimmer
• C5, C6 10pF
• C9 100nF
• L1 4 turns, 7mm diameter *
• L3 3 turns, 7mm diameter *
• L4 5 turns, 7mm diameter *
• L2 RFC (resistance 1MOhm with wrapped around her inductor of enough coils from fine isolated wire. Scratch of utmost inductor and you stick in utmost the resistance making thus a parallel L-r circuit.)
• T1, T2 2N2219
• ANT Simple dipole l/2.
• MIC IN Microphone dynamic or other type. (It can also connected to a cassette player unit)
• * The inductors is air from wire of coaxial 75W or other 1mm roughly.

PCB: 
 Before you print it out with microsoft paints, set the screen resolution to 1280 by 1024 in order to get the correct scale

Regulations:

• With the C4 we regulate the frequency.
• With their C7, C8 we adapt the resistance of aerial (practically to them we regulate so that it is heard our voice in the radio as long as you become cleaner).

Notes:
 The T2 wants refrigerator.

 Author: Kyriakos Kontakos, kkontak@hotmail.com
Source http://www.electronics-lab.com/

06:42 0 comments

2 Transistor FM Transmitter Circuit Diagram

Warning:

Take care with transmitter circuits. It is illegal in most countries to operate radio transmitters without a license. Although only low power this circuit may be tuned to operate over the range 87-108MHz with a range of 20 or 30 metres.

Notes
I have used a pair of BC548 transistors in this circuit. Although not strictly RF transistors, they still give good results. I have used an ECM Mic insert from Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from  Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.

One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some first hand experience of this problem. The frequency of oscillation can be found from the theory section,and an example now appears in the Circuit Analysis section. 

Source  -http://www.zen22142.zen.co.uk/Circuits/rf/2bjttx.htm

Author -Andy Collinson

08:44 0 comments

Basic Low Power AM Transmitter Circuit Schematic Diagram

Description

This transmitter is basic but allows transmission of audio to an AM radio. It consists of an RF oscillator operating in the AM broadcast band, together with a modulator stage, which mixes the incoming audio and the RF. A signal appears on the output, which has an AM component that can be picked up on a nearby AM radio receiver.

Circuit Schematic Diagram

The transmitter consists of oscillator stage Q1 and modulator/buffer stage Q2. Q1 is biased via R1, R2, and R3. L1, C3, and C4 form the tank circuit with feedback network C3-C4 providing feedback to the emitter of Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator stage Q2. Q2 is biased by base current produced by RF rectification in the base emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2 form the tank circuit for the collector of Q2. RF is taken from the junction of C9 and C10 and fed to a short-wire antenna. Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes with the RF signal in the collector circuit of Q2, producing a signal that has sum and difference frequencies if the RF carrier and AF input  along with the carrier signal.

An AM signal appears at the collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collector voltage of Q2 is needed for full modulation of the output. Because of the high level of audio needed, the modulation obtained from this circuit is somewhat limited with conventional audio sources because several volts of audio into a few hundred ohms is needed. The circuit demonstrates the principle of an AM transmitter, however, and with a suitable audio drive level, produces a well modulated AM signal.

Source - http://electroschematics.com/6821/basic-low-power-am-transmitter/

08:20 0 comments

TV Audio Video Transmitter Circuit Diagram

 Description

A very simple TV audio video transmitter circuit can be constructed using this schematic diagram . This TV audio video transmitter circuit can be used to transmit video signals from VCR ( or some other device ) to a TV without using any cable .
Video signals input at jack J1 are first terminated by resistor R6 and coupled through capacitor C1 to clamping-diode D1. Potentiometer R3 is used to set the gain of the video signal; its effect is similar to that of the contrast control on a TV set.
Bias-control R7 can be used to adjust the black level of the picture so that some level of signal is transmitted, even for a totally dark picture.
RF-transformer T1 and its internal capacitor form the tank circuit of a Hartley oscillator that's tuned to 4.5 megahertz. Audio signals input at J2 are coupled to the base of Q3 via C2 and R4: the audio signal modulates the base signal of Q3 to form an audio subcarrier that‚s 4.5-megahertz higher than the video-carrier frequency.

The FM modulated subcarrier is applied to the modulator section through C5 and R9.
Resistor R9 adjusts the level of the subcarrier with respect to the video signal.
Transistors Q1 and Q2 amplitude modulate the video and audio signals onto an RF-carrier signal. The operating frequency is set by coil L4, which is 3.5 turns of 24- gauge enameled wire on a form containing a standard ferrite slug.

Circuit Diagram

The RF output from the oscillator (L4, C7 and C9 ) section is amplified by Q5 and Q6, whose supply voltage comes from the modulator . Antenna matching and low-pass filtering is performed by C12, C13, and L1.
Resistor R12 is optional; it is added to help match the output signal to any kind of antenna.
To align this audio video transmitter you need to tune a TV receiver to an unused channel between 2 and 6. The TV must have an indoor antenna connected directly to it; an outdoor antenna or cable won't work. Make sure both potentiometers (R3, R7) are in middle position and apply power to the transmitter. Adjust L4 with a nonmetallic tool until the TV screen goes blank ,then fine-adjust L4 for the "most-blank" picture.
Connect the video and audio outputs from a VCR(AV source) to jacks J1 and J2 (respectively) of the transmitter .
After that you should see a picture on the TV screen: if you do, readjust L4 for the best picture; if you don't, check the board for any bad connections. Next, adjust R3 for the best picture brightness and R7 for the best overall picture.
Finally, adjust T1 with a nonmetallic tool for the best sound .
The TV transmitter combines line level audio and video signals, and transmits the resulting signal up to 300 feet. The circuit can be powered from a 9-12V power supply circuit .

Source -  http://www.electroniq.net/radio-frequency/tv-audio-video-transmitter.html

13:24 0 comments

Small Radio Transmitter Circuit

Circuit diagram 

DESCRIPTION 

 Contains information about building a small radio transmitter, which has a PCB 1.75" x 2.5" (45mm x 68 mm) and has a range of about 30 yards or so. The documentation with the circuit says the freq range is 100-108 MHz, but I have found it to be more like 85-100 MHz. The circuit is (of course) only mono, and accepts an audio input from either a microphone or other source. The input impedance is 1Mohm. The input sensitivity is 5mV and the max input signal is 10mV. The transmitted signal can be picked up on a FM radio. The circuit can be used for short-range transmission, eg. for wireless microphones. The actual circuit comes from a 'Kit', available from Veleman electronics (USA distributor is Tapto Corp., PO Box 1339, CLAREMONT NH-03743-US. UK distributor is High-Q Electronics, 382 Edgware Road, London, W2 1EB). The kit number is K1771. It is a very good transmitter. I bought the kit, and made the circuit, which worked very well. I wanted two transmitters, so I made my own 'copy' PCB and built the circuit, and in fact my home-made version seems to work better than the original!! So there is no need to buy the kit really, as it is quite a simple circuit, and is the best 'home-made' transmitter I have seen. CIRCUIT.GIF is the circuit diagram. PCBPLAN.GIF shows the PCB layout from above (components shown). PCBPLAN.GIF is an accurate layout, scanned from the instruction sheet. I have used * to mark one corner for reference. TRACKS.GIF shows the track layout on the soldering side of the board. This is NOT a very accurate layout. This is because I didn't actually have a plan of the track layout. To get TRACKS.GIF, I put a bit of OHP film onto the bottom of the PCB, and traced the tracks with an OHP pen. I then scanned this in. I have marked the component leg holes (approximately) with white blobs.

CONSTRUCTION

 Start off by scaling PCBPLAN.GIF and TRACKS.GIF by the same amount so that they measure approximately the correct size (1.75" x 2.5") when printed out. Then make your PCB. As mentioned earlier, PCBPLAN.GIF gives the accurate positioning of the holes, whereas TRACKS.GIF gives the positions only approximately. So use PCBPLAN when drilling the holes in your PCB board. Then draw on the tracks, using TRACKS.GIF as a guide. The important thing is to make sure you draw the 'printed coil' correctly on the PCB - those lines are there for a reason!

Parts

• D1 Varicap diode (eg. BB119)
• D2 1N4148
• R1 100K
• R2 220K
• R3 22R
• R4 1K trimmer
• R5 1K
• R6 56K
• R7 1M
• R8 1K2
• C1 5pF ceramic
• C2 6pF ceramic
• C3 15pF ceramic
• C4 trimmer cap
• C5 15pF ceramic
• C6 1nF ceramic
• C7 100uF electrolytic
• C8 4.7uF electrolytic
• C9 100pF ceramic
• T1 BF244A or BF245A FET
• T2 2N3819 FET
• T3 BC307/8/9 or BC557/8/9 PNP

Bear in mind that in addition to the components, there is a jumper wire which needs to be fitted (marked with a dashed line in PCBPLAN.GIF). The power supply to use is 9-14 V DC, one of the little rectangular 9V batteries is fine. Connect this to the + and - points on the PCB. The sound input goes to the points marked "MIKE". The antenna should be connected to the point marked "ANT". The emitter's output impedance is 50 ohms. You can make your own fancy antenna if you like, but I have found that a foot or so of wire is fine.

Good luck with the transmitter. If you have any improvements to the circuit, I would be glad to hear from you. --Dan Evans.

 Author: Dan Evans, dan.evans@sjc.ox.ac.uk
Source http://sable.ox.ac.uk/~sjoh0014/web/dan.html

10:24 0 comments

3W FM Transmitter Circuit Diagram

Description

This is the schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz. Although the stability isn't so bad, a PLL can be used on this circuit.

This is a circuit that I've build a few years ago for a friend, who used it in combination with the BLY88 amplifier to obtain 20 W output power. From the notes that I made at the original schematic, it worked fine with a SWR of 1 : 1.05 (quite normal at my place with my antenna).

Circuit diagram

 
Parts:
R1,R4,R14,R15 10K 1/4W Resistor
R2,R3 22K 1/4W Resistor
R5,R13 3.9K 1/4W Resistor
R6,R11 680 Ohm 1/4W Resistor
R7 150 Ohm 1/4W Resistor
R8,R12 100 Ohm 1/4W Resistor
R9 68 Ohm 1/4W Resistor
R10 6.8K 1/4W Resistor
C1 4.7pF Ceramic Disc Capacitor
C2,C3,C4,C5,C7,C11,C12 100nF Ceramic Disc Capacitor
C6,C9,C10 10nF Ceramic Disc Capacitor
C8,C14 60pF Trimmer Capacitor
C13 82pF Ceramic Disc Capacitor
C15 27pF Ceramic Disc Capacitor
C16 22pF Ceramic Disc Capacitor
C17 10uF 25V Electrolytic Capacitor
C18 33pF Ceramic Disc Capacitor
C19 18pF Ceramic Disc Capacitor
C20 12pF Ceramic Disc Capacitor
C21,C22,C23,C24 40pF Trimmer Capacitor
C25 5pF Ceramic Disc Capacitor
L1 5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm
L2,L3,L5,L7,L9 6-hole Ferroxcube Wide band HF Choke (5 WDG)
L4,L6,L8 1.5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm
L10 8 WDG, Dia 5 mm, 1 mm CuAg, Space 1 mm
D1 BB405 or BB102 or equal (most varicaps with C = 2-20 pF [approx.] will do)
Q1 2N3866
Q2,Q4 2N2219A
Q3 BF115
Q5 2N3553
U1 7810 Regulator
MIC Electret Microphone
MISC PC Board, Wire For Antenna, Heatsinks

Notes:
1. Email Rae XL Tkacik with questions, comments, etc.
2. The circuit has been tested on a normal RF-testing breadboard (with one side copper). Make some connections between the two sides. Build the transmitter in a RF-proof casing, use good connectors and cable, make a shielding between the different stages, and be aware of all the other RF rules of building.
3. Q1 and Q5 should be cooled with a heat sink. The case-pin of Q4 should be grounded.
4. C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.
5. Local laws in some states, provinces or countries may prohibit the operation of this transmitter. Check with the local authorities.

 

Author: Rae XL Tkacik
E-mail: vocko@atlas.cz
Source: http://www.aaroncake.net/circuits/index.asp

10:45 0 comments

Transmitter FM 45W with valve

TECHNICAL CHARACTERISTICS: 

• Tendency of catering: 220V AC
• Frequency of emission at FM: 88~108MHz
• Force of expense: max 45W (without the R3) 

Circuit diagram

Materially: 

• R1 15KW/2W
• R2 1KW/10W
• R3 1KW/10W (for biggest force in the exit you replace with short-circuit).
• C1 50pF trimmer
• C2 30pF trimmer
• C3 22pF/4KV
• C4, c6, c9 10nF/1KV
• C5, c7 1nF/1KV
• C8 100mF+100mF/450V (Double electrolytic)
• C9, c10 10nF
• RFC1, rfc2, rfc3 air Inductors: 15 coils diameter 8mm, from wire 1mm.
• T1 Transformer 220V/6V-1A
• T2 Transformer of configuration with being first 4 or 8W
• T3 Inductor with core ferrite (externally it resembles with small transformer but has a turn only).
• D1 BY127 rectifier
• Lamp 807 SYLV USA or EL34 or equivalent
• ANTENNA Simple dipole L/2. (L= wave length)
• S1 Main switch of catering.
• S2 Switch of catering of rise (him we close after zestacej' the thread).

Most elements you can him find in a old back-white television with lamps.
Regulations: 

• With the C2 we regulate the frequency.
• With the C1 we adapt the resistance of aerial (practically him we regulate so that it is heard our voice in the radio as long as you become cleaner).

Notes: 

• The catering better it does not become at straight line from the network 220V but via transformer 220V/220V of isolation and safety 1A.
• When does not exist the R3, the force of expense is bigger, but respectively is increased also the hum 50Hz, because the simplicity of designing.
• The control (Audio In) can become from a kasseto'fwno or other powerful source. If it is microphone it will be supposed precedes amplifier so that it acquires a force of order of 8W roughly. 

Author: Kyriakos Kontakos
Source http://www.electronics-lab.com/

10:10 0 comments

Simple Short-Wave Transmitter

Description 

This low-cost short-wave transmitter is tunable from 10 to 15 MHz with the help of ½J gang condenser VC1, which determines the carrier frequency of the transmitter in conjunction with inductor L1. The frequency trimming can be done with VC2. The carrier is amplified by transistor T4 and coupled to RF amplifier transistor T1 (BD677) through transformer X1*. The transmitter does not use any modulator transformer.

The audio output from condenser MIC is preamplified by transistor T3 (BC548). The audio output from T3 is further amplified by transistor T2 (BD139), which modulates the RF amplifier built around transistor T1 by varying the current through it in accordance with the audio signal’s amplitude. RFC1 is used to block the carrier RF signal from transistor T2 and the power supply. The modulated RF is coupled to the antenna via capacitor C9.

Circuit diagram: 

 For antenna, one can use a 0.5m long telescopic aerial. Details of RF choke, inductor L1 and coupling RFC1 is used to block the carrier RF signal from transistor T2 and the power supply. The modulated RF is coupled to the antenna via capacitor C9. For antenna, one can use a 0.5m long telescopic aerial. Details of RF choke, inductor L1 and coupling transformer X1, we used a ready made short-wave antenna coil with tuning slug (Jawahar make), which worked satisfactorily. We tested the transmitter reception up to 75 metres and found good signal strength.

Author: EFY Mag
Source  http://www.extremecircuits.net/2010/05/simple-short-wave-transmitter.html

09:55 0 comments

Simple AM Transmitter

Circuit diagram:

Description

There are not many AM transmitters that are easier to build than this one because the inductor is not tapped and has a single winding. There is no need to wind the inductor as it is a readily available RF choke (eg, Jaycar Cat LF-1536). To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors used instead. To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency. Q1 is biased with a 1MO resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.

author: Peter Goodwin
Source http://www.extremecircuits.net/2010/05/simple-am-transmitter.html

09:41 0 comments

Long Range/Distance Infrared Transmitter

Here the long range/distance Infrared transmitter circuit, give you extra power for your Infrared transmitter. The majority of the IR remotes do the job reliably within a range of 5 metres. The circuit complexity increases in case you design the IR transmitter for good operation more than a extended distance, for example, 10 metres. To double the distance from 5 metres to 10 metres, it is advisable to enhance the transmitted power 4 times.

In case you want to realise a highly directional IR beam (very narrow beam), you could suitably make use of IR laser pointer as the IR signal source. The laser pointer is readily offered in the marketplace. Even so, using a very narrow beam from the laser pointer, you will need to take special care, lest a small jerk to the gadget may change the beam positioning and lead to loss of contact. Right here is really a simple circuit which will provide you with a fairly long distance. It utilizes three infrared transmitting LEDs (IR1 through IR3) in series to improve the radiated power. Further more, to improve the directivity and so also the power density, you may construct the IR LEDs inside the reflector of a torch.

For improving the circuit performance, a MOSFET (BS170) has been applied, which works as being a switch and thus minimizes the electrical power loss that would happen if a transistor were being applied. To prevent any drop while in its ‘on’/‘off’ operations, a 100µF reservoir capacitor C2 is used across the battery source. Its benefit is going to be more obvious if the IR transmitter is operated by common batteries. Capacitor C2 provides additional charge while in ‘switching on’ processes.

Since the MOSFET exhibits big capacitance across gate-source terminals, a particular drive arrangement has been built implementing NPN-PNP Darlington pair of BC547 and BC557 (as emitter followers), to prevent distortion of the gate drive input. Data (CMOS-compatible) for being transmitted is utilized for modulating the 38 kHz frequency produced by CD4047 (IC1). Even so, in the circuit shown here, tactile switch S1 has been utilized for modulating and transmitting the IR signal.

Assemble the circuit on a standard PCB. Use switch S2 for power ‘on’/‘off’ control. Commercially available IR receiver modules (e.g., TSOP1738) can be utilized for effective reception of the transmitted IR signals.

Source  http://circuitdiagram.net

12:50 0 comments

500mW PLL FM transmitter 88-108MHz

Circuit diagram 500mW FM PLL transmitter 88-108MHz

This PLL transmitter is controlled and the frequency is very stable and can be programmed digitally.
Transmitter will work 88-108 MHz and output power up to 500mW.
With a small change can set the frequency of 50-150 MHz.

The output power is often set to several watts with transistors.
So therefore I decided to build a simple transmitter with great performances.
The frequency of this transmitter can easily be changed by software and space / compress air coil.
This transmitter is the oscillator colpitts. Oscillator is a VCO (voltage controlled oscillator) which is set by the PLL circuit and PIC micro controller.
This oscillator is called the Colpitts oscillator and voltage controlled to achieve the FM (frequency modulation) and PLL control. T1 must be HF transistors to work well, but in this case I use a cheap and common BC817 transistor. LC tank oscillator needs to oscillate properly.
In this case the LC tank consists of L1 with the C1, C2, C3, and varicap BB139.
Coil parallel to the C1 and C2 in series. The same with the varicap and C3.
You may think that L is parallel to the [(C1 / / C2) + (Varicap / / C3)]
C3 will determine the value range VCO. Large value of C3 will be broader in the range VCO can be.
PLL and Microcontroller
Oscillator is made to work as a “Voltage Controlled Oscillator” VCO.
To control the frequency synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil (L2) is connected to pin 6.
This coil should be placed close to the coil L1 to take some of the energy oscillates.
The LMX2306 PLL in to use this frequency to adjust and lock the VCO to the desired frequency.
Systems also need to set the external reference crystal. In this case I use 12.8 MHz.
Pin 2 of MX2306 you will find the PLL filter to form a VM that is set voltage of the VCO.
The PLL tries to arrange so that the oscillator frequency Fout kept locked to the desired frequency.
The desired frequency programmed into the PIC EEPROM and clocked into the synthesizer (LMX2306) at power up.
I will below explain how to program the EEPROM to different frequencies.
In the pin14 of your synthesizer control output. In this output you will find a reference frequency for testing.
(I must warn you that the signal is not symmetrical in form. Pulsa positive only a few microseconds, so you will be hard to see on the oscilloscope.) I solved by connecting it to 74HC4020 (14-stage Binary Counter) to input pin 10 Hours. In Q0 (pin 9) you will have a symmetrical square wave with a frequency half since the circuit is a table. In Q1 pin 7 will be divided by 4, see data sheet for more information.
LF input
You want to send audio must be connected to the audio input (left schematic).
Will affect the signal and thus modulate the FM varicap RF carrier frequency.

A potentiometer P1 was added to adjust the depth of modulation (FM Wide or Narrow FM). You may have to play a bit with a value of P1 because it tends to modulate the lot. You may need to add the 500k – 1M potentiometer only. You test and find out for himself.

Buffer stage
Here you find other HF transistors and work in the class C.
Resistor R1 and resistor Re2 regulate the flow of DC. In this case I find that 9.1k will give a good output power and thus equal to 150. If you want to increase the power should be lower Re2. You can add another 150 ohm resistor in parallel.
In the table below I’ll show the output power with different voltages and resistor values of Re2.
I advice you to not run the transmitter with a high output power. Transistor I use is small and tends to be hot.
I advice you to run the unit from the 0 – to 200mW. At the transistor will 500mW pain …* smiles *
At the output you will find a network T. This “filter” will match the transmitter to the antenna impedance output stage.
You have two variables 60pF capacitors to tune the transmitter for best performance.
The antenna I use I a 1 / 4 wave whip antenna (wire) about 75cm long.
Smaller antenna types, but not so good performance as a dipole.
With a dipole you will be more long distance transmitter.
How long can I pass?
It is a very difficult question because the environment affects the transmission distance is very much.
In a city environment with concrete buildings transmitter will send maybe 200m.
I will send a proposed open 2000m.
I did the test and filed with 70mW output power into a “bad” whip antenna is placed in the room I can send 200-300m to a park without a problem.

Source - http://www.afiata.com

12:35 0 comments

Simple FM Transmitter

Description

This FM transmitter (FM Tx) is about the simplest and most basic FM Tx 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 penetrate over three floors of an apartment building and go over 300 meters in the open air. 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 this FM Tx is below
the legal limits of many countries (eg, USA and Australia). However, some countries may ban ALL wireless transmissions without a license. It is the responsibility of the builder to check the legal requirements for the operation of this circuit and to obey them.

CIRCUIT DESCRIPTION:

The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio picked up and
amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output
from the collector is fed into the base of the second transistor where it modulates the resonant frequency of
the tank circuit (the 5 turn coil and the trimcap) by varying the junction capacitance of the transistor. Junction
capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is
connected in a Colpitts oscillator circuit.

CIRCUIT CALIBRATION:

Place the transmitter about 10 feet from a FM radio. Set the radio to somewhere about 89 - 90 MHz. Walk back to
the Fm Tx and turn it on. Spread the winding of the coil apart by approximately 1mm from each other. No coil winding  should be touching another winding. Use a small screw driver to tune the trim cap. Remove the screwdriver from the trim screw after every adjustment so the LC circuit is not affected by stray capacitance. Or use a plastic screwdriver. If you have difficulty finding the transmitting frequency then have a second person tune up and down the FM dial after every adjustment. One full turn of the trim cap will cover its full range of capacitance from 6pF to 45pF. The normal FM band tunes in over about one tenth of the full range of the tuning cap. So it is best to adjust it in steps of 5 to 10 degrees at each turn. So tuning takes a little patience but is not difficult. The reason that there must be at least 10 ft. separation between the radio and the Tx is that the Tx emits harmonics; it does not only emit on one frequency but on several different frequencies close to each other.

NOTE:
You may experiment with using 6V or 9V with the circuit to see how this increases the range of the transmitter. The sensitivity may be increased by lowering the 22K resistor to 10K. Try it and see.
 

10:00 1 comments

FM Transmitter Circuit

This circuit is a simple two transistor (2N2222) FM transmitter. No license is required for this transmitter according to FCC regulations regarding wireless microphones. If powered by a 9 volt battery and used with an antenna no longer than 12 inches, the transmitter will be within the FCC limits. The microphone is amplified by Q1. Q2, C5, and L1 form an oscillator that operates in the 80 to 130 MHz range. The oscillator is voltage controlled, so it is modulated by the audio signal that is applied to the base of Q2. R6 limits the input to the RF section, and it's value can be adjusted as necessary to limit the volume of the input. L1 and C6 can be made with wire and a pencil. The inductor (L1) is made by winding two pieces of 24 gauge insulated wire, laid side by side, around a pencil six times. Remove the coil you have formed and unscrew the two coils apart from each other. One of these coils (the better looking of the two) will be used in the tank circuit, and the other can be used in the next one you build. The antenna (24 gauge wire) should be soldered to the coil you made, about 2 turns up from the bottom, on the transistor side, and should be 8-12 inches long. To make C6, take a 4 inch piece of 24 gauge insulated wire, bend it over double and, beginning 1/2" from the open end, twist the wire as if you were forming a rope. When you have about 1" of twisted wire, stop and cut the looped end off, leaving about 1/2" of twisted wire (this forms the capacitor) and 1/2" of untwisted wire for leads.

09:19 0 comments

88-108MHz FM Transmitter Circuit Diagram

Description: The following circuit shows about 88-108MHz FM Transmitter Circuit Diagram. Features: 1W transmitter Tetsuo style. Component:  Capacitor, Antenna, transistor, IC, diode.[thing.net]

11:52 0 comments