In this article we learn how to build a professional high end walkie talkie circuit which are normally used in commercially available units.
The range of the circuit presented here is around 200 meters, which is quite a long range considering the simplicity of the design
What is a Walkie Talkie
As we all know a walkie talkie is a high frequency device which allows more than two people to communicate in a wireless mode across a specified distance. We have seen this device being extensively used by the military people, and also other defense personnel.
It basically includes a transmitter and the receiver circuit embedded in a single unit. The two modes are flipped and selected through a switch, across the two sides of communication.
In our present design also, the transmitter and the receiver circuits are incorporated within a single PCB, and the two modes are simply toggled using a selector switch. For this you will have to make two such identical circuits, such that each of the units work like a transmitter as well as a receiver across the two ends of the communication line. When one side unit is selected as the transmitter, the other side circuit is supposed to be selected as the receiver, and vice versa.
This is how real walkie talkie models also actually work.
Using Single Circuit to Work as Transmitter and Receiver
Almost all of the parts in the 4-transistor circuit are applied for both transmitting and receiving together. This helps make this an extremely cost-effective model. The frequency-generating stage merely demands the crystal to be taken away and it turns into a receiver device.
A 390R is included to the emitter of the oscillator stage to minimize disturbances and transform it into a receiver. The subsequent portion of the circuit is known as a basic foundation.
This includes 3 transistors directly connected to generate an audio amplifier having quite high gain. The very first transistor works like a pre-amplifier and the following two are connected as a super-alpha couple, typically known as Darlington pair to push the speaker transformer.
The 3rd stage comprises of the speaker. This can be a individual item as it is applied as a speaker in the receiving mode and as a dynamic microphone in the transmitting mode.
A speaker works extremely well in reverse such as this and it is known as dynamic microphone due to the coil and magnet design.
Whenever you communicate into the diaphragm, the motion of the voice coil in the magnetic field generates some millivolts output. This could be combined to a high gain amplifier to obtain very great outcomes.
Once the walkie talkie is switched in the receiver setting, the first transistor gets wired as a receiver and the audio is detected from the 4k7 load resistor through a 0.47u electrolytic.
After that it moves via a volume level control and of to the 3 transistor amplifier.
The speaker transformer couples the amplifier with the speaker and we listen to the end result. As soon as the walkie talkie circuit is toggled in the transmitter mode, the speaker gets hooked up at the input of the audio amplifier.
The audio can now be amplified and the waveform shows up as the supply voltage for the transmitter stage. The crystal is attached to the very first stage and the gain of the transistor is amplified by eliminating the 330R and solely by using a 56R for the emitter resistor.
The speaker transformer is not really employed as a transformer within this mode rather like an an INDUCTOR to pair the output of the amplifier to the power supply line and the signal established across the winding is transferred to the transmitter stage in the form of supply voltage.
As the signal goes up and down, it varies the gain of the 1st stage thereby the amplitude of the transmitted signal. In this way
In this way the transmission gets to be an Amplitude Modulated (AM) Radio Frequency (RF) signal.
How the Speaker Transformer Works
A great deal of debate could possibly be dedicated to the functioning of the speaker transformer since the form of a transformer is extremely sophisticated. You will find a couple of methods for you to model a transformer.
The first is to determine the needs from the beginning and the second is to duplicate an active design and style and create adjustments so that the desirable effect is accomplished. Duplicating and enhancing appears to be the easiest.
If you are using the assumptive strategy you are going to inevitably need to customize the style to have it functioning properly. The speaker transformer utilized in is 1k to 8 ohm.
They are the impedance valuations calculated at 1kHz. Typically the DC resistance of the primary is 42 ohms and also the secondary is 1 ohm.
The transformer DC resistance is actually distinct from the impedance number.
When the transformer is bigger, the diameter of the wire might be bigger and the DC resistance might be just 10 ohm and 0.5 ohms. The impedance will be the resistance as noticed by the transistor with 1kHz.
It "views" a 1k load at 1kHz and a higher impedance at a higher frequency. Electricity is moved through the primary to the secondary by way of magnetism.
The primary generates a magnetic flux which travels into the core encircling the windings. This magnetic flux slices the winding of the secondary and constitutes a voltage inside it.
The voltage developed is proportionate to the amount of winding implemented. For the present circuit the primary possesses 525 turns and the secondary includes 75 turns.
This is precisely a 7:1 ratio and it implies the transformer may in theory transform a 7v waveform at 10mA into a 1v waveform having a current of 70mA.
A tiny transformer such as this may have an efficiency of approximately 50 - 70% but it really is carrying out an extremely significant job, coordinating 1k to 8 ohms and the speaker may not perform if hooked up directly to the transistor.
Using a Small Transformer as an Inductor
While the transformer can be used being an inductor in the transmitting mode, the loudspeaker is unconnected and also the secondary will not sense any load.
This implies the primary is not going to experience "mirrored" load and the impedance of the transformer is actually amplified substantially.
The result is the transistor perceives an increased impedance which suggests it sees it much easier to establish a signal over the primary.
To offer a brilliant example, the transformer (using the speaker attached) is similar to an incredibly rigid spring. Once the speaker is disconnected, the transformer becomes like a really weakened spring.
The transistor sees it a piece of cake to draw the base end of the spring straight down (the upper section is attached to the positive supply line).
If a transmission is processed by the Darlington pair in transmitter mode, the emitter is retained firm through the 33uf and the exclusive factor that may occur may be the weakened spring will get drawn downwards.
Referring to the circuit diagram, the lower terminals of the transformer gets to be the supply line of the crystal oscillator.
As the voltage on the transformer goes up and down, the supply voltage for the oscillator raises and reduces. and impacts the oscillator gain.
At this point we arrive at the challenging portion of detailing how a voltage is created over the primary winding.
In the course of the quiescent (idle) mode, about 1.5v is lowered across the 42 ohm resistance of the primary. Each time a signal is generated by the Darlington transistor, the resistance between collector and emitter will be decreased and a increased current start moving.
The particular motion of current boost results in an widening magnetic flux within the transformer which slices the surrounding primary winding and induces a voltage within each one of the turns in the reverse path.
What this means is the voltage created by the transistor must be higher, so that they can force current into the inductor.
This voltage is harvested off the inductor and handed down to the 1st stage in the circuit and turns into the power supply.
The rising and falling power supply varies the gain of the stage and creates an amplitude modulated 27MHz transmission to generate sound on the carrier. By doing this an Amplitude Modulated (AM) Radio Frequency (RF) signal is transmitted.