27MHz Receiver
2 Channel

 


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To buy a kit:  27MHz Receiver
$10.50 plus $4.00 post

 

This project is the receiver section of a 2-Channel 27MHz link. It has 4 outputs and these can be individually controlled by pressing button A for a short or long period of time and the same with button B.
But basically is is a 2-Channel Receiver.
See the 27MHz transmitter HERE.

Detecting the 2 Channels
The antenna picks up all the surrounding frequencies but an exact frequency of 27MHz makes the front-end produce a higher waveform that has a frequency of 27MHz.
This frequency is called the carrier frequency and the transmitter increases and decreases the amplitude at the rate of 600 times per second for one of the "tones" and 1,200 times per second for the other tone.
A small-amplitude 27MHz waveform is produced by the first transistor and it increases in amplitude when it detects a 27MHz signal from the surroundings and this amplitude increases and decreases at a rate of 600 times per second.
Only the 600 cycles per second signal is passed to the amplifying stages and any noise above this frequency is removed by the feedback components (capacitors) on two of the stages. The only signal that appears at the output of the fourth transistor is a 600 or 1,200 cycle signal.
Detecting one signal from the other is not easy. The two frequencies are very close and producing two circuits to separate them requires many components. 
The simplest solution is to add a chip to count the number of cycles in a short period of time and produce two separate outputs.
We have done this with a PIC chip and provided two other features to detect a long tone from switch A and a long tone from switch B.
This produces 4 separate outputs.
 


THE CIRCUIT


27MHz Receiver -  2 Channel

THE "FRONT-END"
The secret to getting a long-range 27MHz link is a powerful transmitter and a sensitive "front-end" on the receiver.
A 27MHz transmitter of only a few milliwatts (10 to 30 mW) will reach 100 metres providing the receiver has a very sensitive FRONT END.
The Front End is the first stage of a receiver.
In our case it is a very weak 27MHz oscillator and thus it is actually a 27MHz transmitter (or more accurately - a 27MHz radiator) as it fills the surroundings with a 27MHz signal.
When another 27MHz signal enters this field it upsets the transmission of the receiver and increases the amplitude of the oscillator. This has the effect of producing a cleaner signal and the background noise or "hash" is reduced.
This is a very clever way of making the front-end very sensitive as it takes a lot of energy to "excite" an oscillator that is sitting in a dormant condition. It is also very difficult to get an oscillator to sit in a condition that is just before the point of oscillation. So we cause it to oscillate at a very low level and an incoming signal will increase the amplitude.
The fact that the transistor in the front end is oscillating can also be referred to as a REGENERATION circuit as the output of the transistor - at the collector - is fed back into the circuit via the 39p between the collector and emitter.
The signal delivered by the 39p is prevented from being lost to the 0v rail by the 50 turn inductor. This is called "emitter injection" as the transistor is configured as a common base amplifier in which the base is held firm by the 22n and the signal fed into it via the emitter.
The operation of the circuit is kept at a low amplitude and when the antenna picks up a signal of exactly the same frequency, the amplitude increases. This is called SUPER REGENERATION or increase of the regeneration. 
It is a very simple way of getting an enormous result from very few components. Normally you would require 2 or more stages of amplification to produce the same result.
The only problem with a SUPER REGENERATIVE circuit is the background noise it produces. But since this project is deigned only to activate a load, this background "hash" is not a problem. Nearly all the background noise is removed by the feedback capacitors on each stage.
A capacitor placed between the collector and base of a transistor has an enormous effect on reducing the high frequencies.
That's why a small capacitor such as 2n2 can be used. The gain of the transistor (about 70) multiplies the effective resistance (impedance) of the capacitor by about 70 and the background noise is removed because it mostly consists of high frequencies.
But there is still a lot of skill to get the front-end to oscillate very lightly, while being sensitive to signals coming from the antenna.
A high-value resistor in the collector only allows a small current to flow and if the supply voltage is high, this produces a circuit that will
oscillate with a small waveform and can be easily "upset" or "modified" by an injection at the highest point of oscillation. If this injection is timed accurately, it will increase the amplitude of oscillation and the high voltage supply will allow this to occur.

The output of the front end is taken from the collector of the transistor - but not directly from the collector as this would load the circuit and stop it oscillating. The top of the oscillator coil will have a small percentage of the waveform and we can then amplify it via three stages of amplification. The important thing is we can only pick off a very small percentage of the energy so the front end keeps oscillating.   
The signal appearing at the "pick-off" point consists of:
1.  -  27MHz called the "carrier frequency," 
2. - a lot of noise and "hash" produced by the circuit and also from the antenna picking up background noise from the surroundings and
3.  - a tone from the transmitter.
The tone is about 1kHz and its frequency is enormously different to 27MHz so a simple PASS FILTER can be used to remove the 27MHz and only allow the tone and noise to pass to the next stage.
The component that does this is the 22n across the 4k7 in the supply-line.
This capacitor effectively passes (shunts - removes) the 27MHz to the positive rail where it is passed to the 0v rail via the 47u electrolytic.
We only want to VERY LIGHTLY load the front so that we don't stop the circuit oscillating.
We do this by using a resistor and capacitor in series.
If we just use a capacitor, the "resistance" of the capacitor will be quite small at some of the frequencies of the "hash" and it will load the circuit and reduce the amplitude when a signal is being received.
The combination of the resistor and capacitor in series reduces the LOADING effect.
We now have a very small signal that can be amplified by the following stages.
The second transistor amplifies this signal and removes a lot of high frequencies (hash) via the 2n2 feedback capacitor.
The third transistor does exactly the same and we finish up with a signal that is almost equal to rail voltage from the fourth transistor.
We need a very high amplitude signal for the PIC12F629 microcontroller. That's why there is three stages of amplification.
The microcontroller counts the number of waves in 100mS and determines if the tone is from button A or B.
It also counts in increments of 100mS and if the signal is present for 5 x 100mS, it is counted as a long pulse.


 

 

 



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


27MHz Receiver  PCB layout


The most critical part of construction is the tuned circuit for antenna point 2.
The coil supplied in the kit is 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 not produce a peak amplitude for 27MHz. 
 

 

Here is a close-up of the tinned copper wire holding the cell in place. Use fine tinned copper wire and make sure the wires do not create a short-circuit.

 

27MHz Receiver-2Ch
PARTS LIST
$10.50 plus $4.00 post
Order a kit

1 - 220R resistor
4 - 470R resistors
1 - 1k resistor
1 - 1k5 resistor
3 - 4k7 resistors
1 - 10k resistor
1 - 47k resistor
2 - 470k resistors
1 - 1M resistor
1 - 1M with 1metre 0.095mm wire for
                inductor

1 - 5-30p air trimmer
2 - 39p ceramics
3 - 2n2 ceramics
5 - 22n ceramics
1 - 100n monoblock
2 - 100n surface mount

1 - 47u electrolytic

1 - 1N4148 diode
4 - BC338 transistors
4 - BC547 transistors

1 - 13 turn coil
1 - PIC12F629 micro with 2ChRx
1 - 78L05ACM 5v regulator IC
1 - 10cm 0.5mm wire for antenna
20cm very fine solder

1 - 27MHz Rx 2Ch PCB

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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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