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Single Transistor FM Transmitter Design

 In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency. FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. Throughout the world, the broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 - 108.0 MHz is used to transmit and receive the FM signals. Designing and assembling an FM transmitter is a difficult task. The Note given here explains how a simple FM transmitter is designed and assembled. Design Considerations The performance of an FM transmitter depends on two important aspects. 1. Tuning of the FM transmitter to the desired frequency. Even a slight change in the coil specification or slight change in the variable capacitor value can shift the harmonic frequency instead of the 88-108 MHz FM band. 2. Length of the Antenna used to transmit the frequency. The important parameters for the optimum performance of an FM transmitter are : 1. Transmitter frequency, output power and range of transmission. 2. Antenna length. 3. Coil diameter, length, number of turns and gauge of the wire used for coil winding. The circuit diagram shown below is that of a Single transistor FM transmitter with a range of 30 50 feets and 100 – 125 milli watt output.

The design details of each component are as follows. 1. Condenser MIC The condenser MIC is used to pick up the sound signals. The diaphragm inside the MIC vibrates according to the air pressure changes and generates AC signals. Variable resistor VR1 adjusts the current through the MIC and thus determines the sensitivity of MIC. The condenser MIC should be directly soldered on the PCB to get maximum sensitivity. Sleeving the MIC inside plastic tubing can increase its sensitivity enormously. 2. Decoupling Capacitors C1 is the first decoupling capacitor impedes the different frequencies of speech signals. C1 modulates the current to the base of transistor. The 4.7 uF capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current (AC) signals to pass. A large value capacitor induces bass (low frequencies) while a low value one gives treble (high frequencies). Capacitor C2 (0.01) act as the decoupling capacitor. Capacitor C3 across the transistor T1 keeps the tank circuit vibrating. As long as the current exists across the inductor coil L1 and the Trimmer capacitor, the tank circuit (Coil-Trimmer) will vibrate at the resonant frequency. When the tank circuit vibrates for long time, the frequency decays due to heating. Presence of the capacitor C3 prevents this decay. A capacitor between 4 and 10 PF is necessary. 3. Resistors Variable resistor VR1 restricts the current through the MIC. The voltage divider R1 and R2 limits the base current of T1 and R3 forms the emitter current limiter. The given values are necessary for the 2N 2222A transistor. 4. Transistor 2N 2222A is the common NPN transmitter used in general purpose amplifications. It has maximum power rating of 0.5 Watts. Over powering of 2N 2222A can generate heat and destroy the device. So maximum power output should be around 125 milli watt. Pin assignment of 2N 2222 A is 1 Emitter - 2 Base - 3 Collector (EBC) from the front side (Flat side on which the number is printed). 5. Inductor Coil The inductor used in the circuit is a hand made coil using 22 SWG (Standard Wire Gauge) enameled copper wire. The length, inner diameter, number of turns etc are the important parameters to be considered while making the inductor. Then only the inductor resonates in the 88-108 band FM frequency. For this circuit, the coil radius was selected as 0.26 inches (outer diameter) and 0.13 inner diameter. Coil can be wound around a screw driver (with same diameter) to get a 5 turn coil of 0.2 inch long. Remove the coil from the screw driver and use the 5 turn Air core coil. Remove the enamel from the tips and solder close to the transistor. The inductance of the coil can be calculated using the formula L = n2 r2 / 9r + 10 x Where r is the inner radius of the coil, x is the length of the coil and n, number of turns. The resulting value is in Micro Henry. 6. Trimmer capacitor A small button type variable capacitor with a value of 22 pF can be used to adjust the resonant frequency of the tank circuit. The variable capacitor and the inductor coil form the Tank circuit (LC circuit) that resonates in the 88-108 MHz. In the tank circuit, the capacitor stores electrical energy between its plates while the inductor stores magnetic energy induced by the windings of the coil. The resonant frequency can be calculated using the formula f = 1 / 2 x √LC = Hz Where f is the frequency in hertz, x is the coil length, C is the capacitance of trimmer in Farads, and L is the inductance of coil in Hendry. Tank Circuit Every FM transmitter needs an oscillator to generate the radio Frequency (RF) carrier waves. The name 'Tank' circuit comes from the ability of the LC circuit to store energy for oscillations. The purely reactive elements, the C and the L simply store energy to be returned to the system. In the tank (LC) circuit, the 2N 2222 A transistor and the feedback 4.7 pF capacitor are the oscillating components. The feedback signal makes the base-emitter current of the transistor vary at the resonant frequency. This causes the emitter-collector current to vary at the same frequency. This signal fed to the aerial and radiated as radio waves. 7. Antenna A plastic wire or Telescopic aerial can be used as antenna. The length of the antenna is very important to transmit the signals in the suitable range. As a rule, the length of the antenna should be ¼ of the FM wave length. To determine the length of antenna, use the following equation. By multiplying the Wave frequency and wave length will give the speed of light. Speed of Light = Frequency of Oscillation x Wavelength = in Kms/ Sec Wave length = Speed of light / Frequency = in meters Antenna length = 0.25 x wavelength = in meters By using this formula it is easy to select the antennal length. For the circuit mentioned above, a 25-27 inches long antenna is sufficient. Assembling and Testing The circuit can be assembled on a Dot type common PCB or Perf board. The following tips should be considered while assembling the circuit 1. Assemble the components as close as possible, especially the transistor, trimmer and coil to prevent unwanted oscillation. 2. Lead length of capacitors, resistors, transistor should be as small as possible. 3. Solder the MIC directly on the PCB ( use the trimmed leads of the resistors to connect MIC) 4. Observe the polarity of MIC. 5. Check the pins of 2N 2222 A. The pin assignment is E-B-C (Emitter – Base – Collector) from the front side (Flat side on which the number is printed). 6. Coil should stand horizontally above the Emitter of transistor. 7. Coil should be closely wound. How to test After assembling the circuit, connect 9 volt battery. A battery operated FM pocket radio is necessary for testing. AC powered FM players will give lesser performance than the battery powered FM receivers due to noise. Tune the FM receiver to a “Dead Air space” (around 108 MHz where there is no station). Place the FM radio 2 feet away from the transmitter. Gently tap on the MIC. If the tank circuit is properly tuned, tapping sound will be heard in the radio. If no sound is heard, slightly pull the coil to separate the windings. Adjust the shaft of the Trimmer slowly with a preset screwdriver. Check again. If the sound is clear, move the FM radio and assess the range. Try again by adjusting the trimmer and position of aerial of both transmitter and FM radio. If the sound clarity is good and there is sufficient range, stick the coil with nail polish or glue to avoid frequency change. The FM transmitter is ready to use.

How to make audio level VU meter using transistors

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TRAIN DETECTORS

 TRAIN DETECTORS In response to a reader who wanted to parallel TRAIN DETECTORS, here is a diode OR-circuit. The resistor values on each detector will need to be adjusted (changed) according to the voltage of the supply and the types of detector being used. Any number of detectors can be added. See Talking Electronics website for train circuits and kits including Air Horn, Capacitor Discharge Unit for operating point motors without overheating the windings, Signals, Pedestrian Crossing Lights and many more

LED DETECTS LIGHT

 

LED DETECTS LIGHT The LED in this circuit will detect light to turn on the oscillator. Ordinary red LEDs do not work. But green LEDs, yellow LEDs and high-bright white LEDs and high-bright red LEDs work very well. The output voltage of the LED is up to 600mV when detecting very bright illumination. When light is detected by the LED, its resistance decreases and a very small current flows into the base of the first transistor. The transistor amplifies this current about 200 times and the resistance between collector and emitter decreases. The 330k resistor on the collector is a current limiting resistor as the middle transistor only needs a very small current for the circuit to oscillate. If the current is too high, the circuit will "freeze." The piezo diaphragm does not contain any active components and relies on the circuit to drive it to produce the tone.

INCREASING THE OUTPUT CURRENT

 INCREASING THE OUTPUT CURRENT The output current of all 3-terminal regulators can be increased by including a pass transistor. This transistor simply allows the current to flow through the collector-emitter leads. The output voltage is maintained by the 3-terminal regulator but the current flows through the "pass transistor." This transistor is a power transistor and must be adequately heatsinked. Normally a 2N3055 or TIP3055 is used for this application as it will handle up to 10 amps and creates a 10 amp power supply. The regulator can be 78L05 as all the current is delivered by the pass transistor.

CAMERA ACTIVATOR

 CAMERA ACTIVATOR This circuit was designed for a customer who wanted to trigger a camera after a short delay. The output goes HIGH about 2 seconds after the switch is pressed. The LED turns on for about 0.25 seconds. The circuit will accept either active HIGH or LOW input and the switch can remain pressed and it will not upset the operation of the circuit. The timing c

BOOK LIGHT

 This circuit keeps the globe illuminated for a few seconds after the switch is pressed. There is one minor fault in the circuit. The 10k should be increased to 100k to increase the "ON" time. The photo shows the circuit built with surface-mount components:
 

DARK DETECTOR with beep-beep-beep Alarm

 This circuit detects darkness and produces a beep-beep-beep alarm. The first two transistors form a high-gain amplifier with feedback via the 4u7 to produce a low-frequency oscillator. This provides voltage for the second oscillator (across the 1k resistor) to drive a speaker.

150 WATT AMPLIFIER CIRCUIT

 Description This is the cheapest 150 Watt amplifier circuit you can make,I think.Based on two Darlington power transistors TIP 142 and TIP 147 ,this circuit can deliver a blasting 150 W Rms to a 4 Ohm speaker.Enough for you to get rocked?;then try out this. TIP 147 and 142 are complementary Darlington pair transistors which can handle 5 A current and 100V ,famous for their ruggedness. Here two BC 558 transistors Q5 and Q4 are wired as pre amplifier and TIP 142 ,TIP 147 together with TIP41 (Q1,Q2,Q3) is used for driving the speaker.This circuit is designed so rugged that this can be assembled even on a perf board or even by pin to pin soldering.The circuit can be powered from a +/-45V, 5A dual power supply.You must try this circuit.Its working great! The preamplifier section of this circuit is based around Q4 and Q5 which forms a differential amplifier. The use of a differential amplifier in the input stage reduces noise and also provides a means for applying negative feedback. Thus overall performance of the amplifier is improved. Input signal is applied to the base of Q5 through the DC decoupling capacitor C2. Feedback voltage is applied to the base of Q4 from the junction of 0.33 ohm resistors through the 22K resistor. A complementary Class AB push-pull stage is built around the transistors Q1 and Q2 for driving the loud speaker. Diodes D1 and D2 biases the complementary pair and ensures Class AB operation. Transistor Q3 drives the push-pull pair and its base is directly coupled to the collector of Q5.

Notes.  Remember TIP 142 and 147 are Darlington pairs .They are shown as conventional transistors in figure for ease.So don’t get confused.Even though each of them have 2 transistors ,2 resistors and 1 diode inside ,only three pins ,base emitter and collector are coming out.Rest are connected internally.So its quite OK to assume each of them as transistor for ease.  Use a well regulated and filtered power supply.  Connect a 10K POT in series with the input as volume control if you need.Not shown in circuit diagram.  All electrolytic capacitors must be rated at least 50volts.

27MHz RECEIVER

 27MHz RECEIVER The 27MHz receiver is really a transmitter. It's a very weak transmitter and delivers a low level signal to the surroundings via the antenna. When another signal (from the transmitter) comes in contact with the transmission from the receiver it creates an interference pattern that reflects down the antenna and into the first stage of the receiver. The receiver is a super-regenerative design. It is self-oscillating (or already oscillating) and makes it very sensitive to nearby signals. See full description in 27MHz Links article

5-TRANSISTOR RADIO

 5-TRANSISTOR RADIO If you are not able to get the ZN414 IC, this circuit uses two transistors to take the place of the chip.