Here a simple method for making a walkie talkie is illustrated. The principle behind this walkie talkie is by using two pairs of fm transmitter and receivers are combined together to form a walkie talkie.
That is each device contains one receiver and one transmitter. Transmitter of one device will be set in a low fm range (88-95mhz) and the other will be tuned in high fm range (103-108mhz).
FM Transmitter
Description.
With a matching antenna, the FM transmitter circuit shown here can transmit signals up to a range of 2 kilo meters. The transistor Q1 and Q2 forms a classic high sensitive preamplifier stage. The audio signal to be transmitted is coupled to the base of Q1 through capacitor C2. R1, R3, R4, R6, R5 and R9 are the biasing resistors for the preamplifier stage comprising of Q1 and Q2. Transistor Q3 performs the collective job of oscillator, mixer and final power amplifier.C9 and L1 forms the tank circuit which is essential for creating oscillations. Inductor L2 couples the FM signal to the antenna.
Circuit diagram.
Notes.
Assemble the circuit on a good quality PCB.
The circuit can be powered from anything between 9 to 24V DC.
Inductor L3 can be a VK220J type RFC.
For L1 make 3 turns of 1mm enamelled copper wire on a 10mm diameter
plastic former. On the same core make 2 turns of 1 mm enamelled copper
wire close to L1 and that will be L2.
Frequency can be adjusted by varying C9.
R9 can be used to adjust the gain.
For optimum performance, value of C8 must be also adjusted.
Using a battery for powering the circuit will reduce noise.
FM Receiver
This is the most simple fm radio receiver with good performances that works great even if the sensitivity is not too high. The working principle of this fm receiver may seem a little unusual. It is made of an oscillator (T2 and T3) that is synchronized with the received frequency of T1. This transistor works as a broadband preamplifier in VHF range.
The oscillator is adjusted between 87 … 108 MHz with C5. Because of the synchronization, the oscillator output will have the same frequency deviation as the received signal from the fm antenna. This deviations are caused by the broadcasted audio informations. The frequency modulated signal show up on P1 + R5. Low pass filter R6/C6 extracts the audio signal and then is amplifier by T4 … T6 and transmitted at the output through C9 capacitor.
FM Receiver Circuit Diagram
The coil details are presented in the fm receiver circuit diagram. The radio receiver is adjusted on different stations with the help of C5. P1 potentiometer is adjusted untill the best reception is obtained. If we attach an audio amplifier and a speaker then this fm receiver can be made very compact as a pocket radio.
Make 2 pairs of transmitter and receiver.
Arrange boards in two cases as shown in figure
Tune both transmitters in different frequencies.
Attach antenna, speaker and mic to the respective boars.
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