Four Stage FM Transmitter

This FM transmitter schema uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A condenser microphone is connected at the input of the oscillator. 

Four-Stage FM Transmitter Circuits diagram :

Four-Stage FM Transmitter Circuits Diagram

Working of the schema is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna. Use a heat-sink with transistor 2N3866 for heat dissipation. Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88-108 MHz. Also adjust trimmers VC2 through VC7 to get maximum output at maximum range. 

Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the frequency generated. You can also use a 12V battery to power the schema.

Assemble the schema on a general-purpose PCB. Install the antenna properly for maximum range. Coils L1 through L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively.  

EFY note. This transmitter is meant only for educational purposes. use of this transmitter with outdoor antenna is illegal in most parts of the world. The author and EFY will not be responsible for any misuse of this transmitter.

Author : Pradee G.  - Copyright : EFY

Cheap 12V to 220V Inverter

Cheap 12V to 220V Inverter Circuit Diagram. Even though today’s electrical appliances are increasingly often self-powered, especially the portable ones you carry around when camping or holidaying in summer, you do still sometimes need a source of 230 V AC - and while we’re about it, why not at a frequency close to that of the mains? As long as the power required from such a source remains relatively low - here we’ve chosen 30 VA - it’s very easy to build an inverter with simple, cheap components that many electronics hobbyists may even already have.

Though it is possible to build a more powerful schema, the complexity caused by the very heavy currents to be handled on the low-voltage side leads to diagram that would be out of place in this summer issue. Let’s not forget, for example, that just to get a meager 1 amp at 230 VAC, the battery primary side would have to handle more than 20 ADC!. The schema diagram of our project is easy to follow. A classic 555 timer chip, identified as IC1, is configured as an astable multivibrator at a frequency close to 100 Hz, which can be adjusted accurately by means of potentiometer P1.

Cheap 12V to 220V Inverter Circuit Diagram

As the mark/space ratio (duty factor) of the 555 output is a long way from being 1:1 (50%), it is used to drive a D-type flip-flop produced using a CMOS type 4013 IC. This produces perfect complementary square-wave signals (i.e. in antiphase) on its Q and Q outputs suitable for driving the output power transistors. As the output current available from the CMOS 4013 is very small, Darlington power transistors are used to arrive at the necessary output current. We have chosen MJ3001s from the now defunct Motorola (only as a semi-conductor manufacturer, of course!) which are cheap and readily available, but any equivalent power Darlington could be used.

These drive a 230 V to 2 × 9 V center-tapped transformer used ‘backwards’ to produce the 230 V output. The presence of the 230 VAC voltage is indicated by a neon light, while a VDR (voltage dependent resistor) type S10K250 or S07K250 clips off the spikes and surges that may appear at the transistor switching points. The output signal this schema produces is approximately a square wave; only approximately, since it is somewhat distorted by passing through the transformer. Fortunately, it is suitable for the majority of electrical devices it is capable of supplying, whether they be light bulbs, small motors, or power supplies for electronic devices.

PCB layout:

PCB Layout For Cheap 12V to 220V Inverter Circuit Diagram

COMPONENTS LIST
Resistors
R1 = 18k?
R2 = 3k3
R3 = 1k
R4,R5 = 1k?5
R6 = VDR S10K250 (or S07K250)
P1 = 100 k potentiometer
Capacitors
C1 = 330nF
C2 = 1000 µF 25V
Semiconductor
T1,T2 = MJ3001
IC1 = 555
IC2 = 4013
Miscellaneous
LA1 = neon light 230 V
F1 = fuse, 5A
TR1 = mains transformer, 2x9V 40VA (see text)
4 solder pins

Note that, even though the schema is intended and designed for powering by a car battery, i.e. from 12 V, the transformer is specified with a 9 V primary. But at full power you need to allow for a voltage drop of around 3 V between the collector and emitter of the power transistors. This relatively high saturation voltage is in fact a ‘shortcoming’ common to all devices in Darlington configuration, which actually consists of two transistors in one case. We’re suggesting a PCB design to make it easy to construct this project; as the component overlay shows, the PCB only carries the low-power, low-voltage components.

The Darlington transistors should be fitted onto a finned anodized aluminum heat-sink using the standard insulating accessories of mica washers and shouldered washers, as their collectors are connected to the metal cans and would otherwise be short-schemaed. An output power of 30 VA implies a current consumption of the order of 3 A from the 12 V battery at the ‘primary side’. So the wires connecting the collectors of the MJ3001s [1] T1 and T2 to the transformer primary, the emitters of T1 and T2 to the battery negative terminal, and the battery positive terminal to the transformer primary will need to have a minimum cross-sectional area of 2 mm2 so as to minimize voltage drop.

The transformer can be any 230 V to 2 × 9 V type, with an E/I iron core or toroidal, rated at around 40 VA. Properly constructed on the board shown here, the schema should work at once, the only adjustment being to set the output to a frequency of 50 Hz with P1. You should keep in minds that the frequency stability of the 555 is fairly poor by today’s standards, so you shouldn’t rely on it to drive your radio-alarm correctly – but is such a device very useful or indeed desirable to have on holiday anyway? Watch out too for the fact that the output voltage of this inverter is just as dangerous as the mains from your domestic power sockets.

So you need to apply just the same safety rules! Also, the project should be enclosed in a sturdy ABS or diecast so no parts can be touched while in operation. The schema should not be too difficult to adapt to other mains voltages or frequencies, for example 110 V, 115 V or 127 V, 60 Hz. The AC voltage requires a transformer with a different primary voltage (which here becomes the secondary), and the frequency, some adjusting of P1 and possibly minor changes to the values of timing components R1 and C1 on the 555.

Author : B. Broussas Copyright  Elektor Elecronics 2008