To buy a kit: [27MHz Transmitter](mailto:colin@elechelp.com?Subject=Buying 27MHz transmitter 2Channel kit&Body=Please e-mail the cost of sending 27MHz Transmitter Kit by air mail to my country:****___**** and send details of how I can pay for it. My name is:____**) $6.50 plus $4.00 post**
One of the difficulties in building a 27MHz transmitter is the coil (transformer) used in the oscillator (transmitting) circuit.
Most circuits use a SLUG TUNED COIL (slug tuned transformer). This is a tapped coil wound on a former with a ferrite slug (or core) in the centre. This slug has a screw-thread so the inductance of the coil can be altered to set the frequency of transmission.
These slugs are impossible to obtain and this makes these circuits very difficult to build and impossible to produce as a kit.
The answer is to transfer the transformer (coil) to the underside of the PC board. This produces a fixed inductance so an air trimmer is needed to adjust the frequency of transmission.
This coil is quite successful and allows the project to be built on a small PC board.
We have gone further and added extra turns around the actual turns used in the transformer to gather the stray flux and pass it to the antenna. The 6 extra turns double the output and produce a benefit that no-one has done before. It’s things like this that you can design and develop through experimentation.
These projects are only designed for short-range transmission and to show how to design a transmitter / receiver and how to tune a transmitter to a particular frequency.
You will need the 27MHz FSM to show the transmitter is working and use the FSM to tune the transmitter to exactly 27MHz.
No on-off switch is needed as the circuit does not take any current when the switches are not pressed.
THE CIRCUIT
27MHz Transmitter - 2 Channel
All oscillators needs POSITIVE feedback to maintain (start, and keep) oscillating.
You cannot connect the base to the collector on a common-emitter stage to produce an oscillator because a rise in voltage on the base causes the collector voltage to fall and any direct connection between the two will be NEGATIVE feedback and the circuit will never start to oscillate.
However the coil provides POSITIVE feedback.
It works like this: The coil in the collector circuit is between the connection of the 100R and the collector. When this part of the coil has expanding flux, it passes this flux across the other turns in the coil and the centre of the coil has a voltage that is opposite to that on the collector.
In the following diagram we see the same coil.
This time it is in the form of “turns” but the same effect is produced with a printed circuit “flat” coil.
This is a VERY IMPORTANT concept to understand as it is very difficult to follow.
It is the same concept of the secondary of a centre-tapped power transformer where one end is positive and the other end is negative during one half of the waveform.
The coil in the diaphragm below has current flowing through the top half and it produces flux shown via the two red arrows. The heads of the arrows indicate the North pole but in our s case the flux is called EXPANDING FLUX.
In actual fact, the centre tap is called POSITIVE or RAIL and the top has a voltage less than rail as it is connected to the collector of the transistor and the transistor will be turn ON.
This means the bottom of the coil will produce a voltage HIGHER than rail and this voltage is an INCREASING voltage and is passed to the base to turn the transistor ON more.
The transistor gets turns ON more and more until it cannot be turned ON any more.
At this point the expanding flux stops expanding and the pulse of energy through the 47p stops. This is because a capacitor can only pass an increasing or decreasing voltage. As soon as the voltage stops increasing (or decreasing) the capacitor passes NOTHING.
The transistor suddenly stops receiving this increasing amount of energy and it turns OFF.
The magnetic flux collapses and cuts all the turns with COLLAPSING flux that produces a voltage in the opposite direction.
This puts a reverse voltage-pulse through the 47p to turn the base OFF completely and counter-acts the current coming from the 2k2 resistor. This it received a total of NOTHING.
The voltage (energy) from the coil charges the 39p and air trimmer and these items take a certain amount of time to charge and they help determine the frequency of oscillation.
We have added more turns around the outside of the coil to pick up the stray magnetic flux and send it to the antenna. This doubles the output.
The PC coil and parallel capacitors combine to form a circuit classified as a TANK CIRCUIT.
This is also known as a TUNED CIRCUIT but has the extra title of “Tank Circuit” as it stores energy during part of the cycle and delivers it to the antenna during the remainder of the cycle.
The coil also produces POSITIVE FEEDBACK to the transistor and has extra turns called an “over-wind” to deliver extra voltage to the antenna. The extra voltage produces (provides) extra amplitude and thus extra energy to the antenna.
Extra energy produces extra range.
CONSTRUCTION
A kit of components is available from [Talking Electronics](mailto:colin@elechelp.com?Subject=Buying 27MHz transmitter 2Channel kit&Body=Please e-mail the cost of sending 27MHz transmitter 2Channel kit by air mail to my country:****___**** and send details of how I can pay for it. My name is:____**).**
All the components are included in the kit and everything is identified on the board.
The layout of the components on the PC board
The complete 27MHz Transmitter
27MHz FSM PARTS LIST
$6.50 plus $4.00 post [Order a kit](mailto:colin@elechelp.com?Subject=Buying 27MHz transmitter 2Channel kit&Body=Please e-mail the cost of sending 27MHz transmitter 2Channel Kit by air mail to my country:****___**** and send details of how I can pay for it. My name is:____**)**
1 - 100R resistor
1 - 2k2 resistor
1 - 68k resistor
1 - 150k resistor
1 - 5 - 30p air trimmer
1 - 39p ceramic
1 - 47p ceramic
1 - 2n2 ceramic
1 - 10n ceramic
1 - 100n monoblock
1 - BC547 transistor
1 - BC557 transistor
2 - mini tactile switches
2 - 2032 coin cells or 2025
1 - coin cell holder
1 - 10cm 0.5mm wire for antenna
20cm very fine solder
1 - 27MHz Tx 2Ch PCB
MORE TEST EQUIPMENT
Talking Electronics has a number of pieces of TEST EQUIPMENT to help in the design and testing of projects.
Of course you can use a multimeter for most of the testing but some of the “tricky” faults need a special piece of equipment.
You may only need a LOGIC PROBE once a month, but the project you are designing will come 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 will have the whole range.
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