27hz

27MHzField

StrengthMeter

HomeTo buy a kit: 27MHz FSM kit

$6.50 plus $4.00 post

When building 27MHz transmitters (and receivers) it is essential to have a simple piece oftest equipment called a FIELD STRENGTH METER to make sure the transmitter istransmitting and to determine the frequency of transmission. This project has both features.It is an ESSENTIAL piece of test equipment because you need to know a transmitter isworking and hen you need to know it is transmitting at the correct frequency.If the frequency of the transmitter is slightly different to the frequency of the tuned circuit ofthe receiver, the two will not communicate and it will be a very difficult job to work out if theproblem lies in the transmitter or the receiver.The reason is due to the TUNED CIRCUIT in the transmitter and receiver.This allows the transmitter to produce a signal on a very narrow band and the tuned circuit inthe receiver performs the same task.When you are trying to "matchup" the two units, you do not know if the transmitter is higheror lower than the receiver and if you don't have a STARTING POINT, you can spend hourstrying to get the two to communicate.That's why you need a Field Strength Meter.It lets you now the transmitter is working and allows you to tune it to 27MHz.Now, you only need to tune the receiver.

THE CIRCUIT

27MHz Field Strength Meter http://talkingelectronics.com/projects/27MHz Fiel...

1 de 7 09/09/15 09:35

27MHz Field Strength Meter

The project consists of two separate circuits with the current for each section going through a500uA meter. When either of these circuits detects energy in the form of electromagneticradiation, it gets rectified by the diode and passed to the transistor where it turns thetransistor ON more. This causes more current to flow through the 500uA meter and thepointer moves up-scale.The first section we use when testing a transmitter is the BROADBAND detector. This isidentified by Antenna 1. This is called an UNTUNED amplifier and simply picks up RF.This is designed to let you know the transmitter is working.But you don't know the frequency of operation.This is the function of the other circuit.It is has a TUNED front end consisting of a coil and capacitor in parallel.When the natural resonant frequency of these two components is the same as the frequencyof the transmitter, the voltage produced will be greater than 1.2v and the transistor will turnON to move the pointer up-scale.In this case, the two components are not called a TANK CIRCUIT but a TUNED CIRCUITand they are not loaded AT ALL until the voltage reaches 1.2v. This means it is very easy forthem to generate a voltage. You can see the circuit does not provide them with any voltageand so all the voltage produced comes from the RF energy absorbed from the surroundings.When the antenna is connected to the PC via a pin called a machine pin, a very small signalwill be picked up, consisting of all the "hash" in the surroundings. This will deliver a voltage tothe coil and capacitor but the signals will be coming at all different frequencies and they willbe adding and subtracting so that nothing observable will be produced.If a transmitter is brought near the antenna, the signal will be larger than all the background"hash" and it will consist of a single waveform (single frequency). It will deliver energy to thecapacitor and the capacitor will deliver energy to the coil and the coil will deliver energy backto the capacitor but at a slightly different time to that of the next wave from the transmitter.The result will be a very small signal being passed from capacitor to coil and back again.But by adjusting the air trimmer, a position will be reached where the waveform produced bythe capacitor and coil will be reinforced by the incoming signal from the transmitter and thewaveform will get larger and larger. Eventually it will increase to 1.2v. If the diode isremoved, this voltage will be even higher. But the diode drop of 0.6v and the base-emitterdrop of 0.6v limits this amplitude to 1.2v and the extra energy produced by the circuit ispassed to the transistor to turn it ON.This section is effectively your reference for a 27MHz transmitter.


2 de 7 09/09/15 09:35

That's why the coil must be exactly as supplied in the kit and shown in the photos. Adjustingthe spacing of the turns will alter the frequency at which the tuned-circuit resonates and thischange will be quite considerable.

FITTING THE ANTENNAThe antenna is firstly placed in position 1 on the board to get an indication that thetransmitter is working and then it is taken to antenna point 2.Rotate the air trimmer and the pointer on the multimeter will rise and fall as the peak ofoscillation is reached and passed.

CONSTRUCTIONA kit of components is available from Talking Electronics.All the components are included in the kit and everything is identified on the board.

The 27MHz FSM PC board

27MHz Field Strength Meter fully assembled

The most critical part of construction is the tuned circuit for antenna point 2.The coil supplied in the kit has the correct dimensions for a 27MHz transmitter.This is very critical and the coil must not be stretched or squashed as the tuned circuit will


3 de 7 09/09/15 09:35

not produce a peak amplitude for 27MHz. Just the last two turns need to be extended toproduce 27MHz as shown in the photo above.Use fine tinned copper wire to hold the cell in place and make sure the wires do not create ashort-circuit.

CONNECTING THE METERThe PC board is connected to a multimeter via two paper clips.They are connected to the board at the holes identified by 500uA. The paper clips are madefrom spring steel and are very hard. They must not be cut with delicate side cutters. Simplyuse a file to score a mark on the side of the wire and bend it back and forth a few times andit will break.Squeeze the paper clip together so it fits down the holes in the multimeter and makes a goodSLIDING connection.Almost any meter can be used on the 1mA or 0.5mA range.When you turn the project ON, the meter will show about half-scale deflection to representthe quiescent current taken by the circuit.When the antenna detects RF, one of the circuits will turn ON more and take more current.This will show on the meter.That's about all you need to know.

This 27MHz link is designed to get you into understanding the concepts of transmission - it'sthe place to start.The 27MHz link consists of a 27MHz Transmitter - 2-Channel27MHz receiver - 2-Channel

27MHz FSMPARTS LIST$6.50 plus $4.00 post

Order a kit

2 - 2k2 resistors2 - 1M resistors

1 - 5-30p air trimmer1 - 100p ceramic2 - 1N4148 diode2 - BC547 transistors

1 - 16 turn coil2 - paper clips for meter3 - machine pins for antenna1 - 10cm 0.5mm wire for antenna1 - AG1 cell fine tinned copper wire for cell1 - mini slide switch20cm very fine solder

1 - 27MHz FSM PCB


4 de 7 09/09/15 09:35

LED TESTERTests LEDs.$1.50 plus $4.00 post

(buy a number of kits and payonly one postage)

CONTINUITY TESTEROnly responds to resistance lessthan 50 ohms.Ideal for digital projects as it testsconnections very quickly.$12.50 plus $6.50 post


LOGIC PROBE withPULSER - slimlineDetects HIGH and LOW signals onboth TTL and CMOS circuits.The piezo allows you to hear lowfrequency signals and the signalinjector (Pulser) will over-ride clocksignals to make a circuit operate ata reduced frequency.$8.00 plus $6.50 post

MORE TEST EQUIPMENTTalking Electronics has a number of pieces of TEST EQUIPMENT to help in the design andtesting of projects.Of course you can use a multimeter for most of the testing but some of the "tricky" faultsneed a special piece of equipment.You may only need a LOGIC PROBE once a month, but the project you are designing willcome to a stand-still if you can't locate a problem.We designed all these projects because we needed them ourselves.Add one of them to each order you place with Talking Electronics and eventually you willhave the whole range.


5 de 7 09/09/15 09:35

SUPER PROBE20 different functions.See article for the complete list offunctions.$18.00 plus $6.50 post

COMBO-2 TRANSISTORTESTERTests transistors and shows thegain of the transistor.Also has Signal Injector probe.$21.50 plus $6.50 post

Simple Transistor and LEDTester - 3Tests PNP and NPN transistorsand LEDs.Also teaches the amazing propertyof an air-cored coil in producing ahigh fly-back voltage.$4.00 plus $3.00 postage.


MAINS TRACERDetects 240v AC mains hidden inwalls etc.Will also pick up RF signals from akeyboard to show you whereElectromagnetic Radiation iscoming from and giving you aheadache.$10.00 plus $4.50 post


6 de 7 09/09/15 09:35

CABLE TRACER - 100MHzTraces cables when the power isOFF.Uses an FM radio to pickup beeps.$10.00 plus $4.50 postage.


OP-AMP TRAINER andTESTERTeaches how an op-amp works byusing pots to control the voltageson the two inputs.$24.50 plus $6.50 post(comes with instructions)

PIC Fx-1 MICRO (8 pin)PROGRAM DEVELOPERand TESTERLearn to program PIC chips.Comes with a pre-programmedPIC12F629 chip with 3 routines.$12.00 plus $6.50 post

Model Railway POINT MOTORCONTROLLER andTESTER CDU-InlineThe cheapest CDU project you canget.$8.50 plus $6.50 post


7 de 7 09/09/15 09:35

27hz

Documents

Transcript of 27hz