Learn to Make a Walkie Talkie using FM Radio

In this post we learn how to make a simple homemade walkie talkie circuit using ordinary FM transmitters and FM radios.

Overview


In one of my earlier post we comprehensively learned the making of a compact walkie talkie design, however I found that many new hobbyists and school students found the design difficult to adjust and succeed due to its rather complex and the associated strict parameters,

In this post we try to design a walkie talkie using discrete transmitter modules and then tune them to different frequencies such that the units are able transmit and exchange the conversation across both the sides without interfering with their own receive modules.

For the FM transmitters we select the design which was earlier discussed in our wireless speaker circuit, the main reason being its ability to produce high power output, which enables the circuit to transmit the data over much longer distances compared to the other smaller FM transmitters designs.

What you will need to Make this Homemade Walkie Talkie Circuit


The Transmitters: First you will have to construct two of these identical transmitter circuits as shown below:


How the Transmitter Works.


The operating principle of this powerful little transmitter can be understood from the following points:
The MIC converts the voice signals into electrical signals which is further amplified into high amplitude low current signals by T1.

This amplified signal is fed to the base of T2 which basically forms a frequency generator stage with the help of L1, C4, C5 and C3. Together this stage forms a regenerative oscillator which resonates in the range of 50 to 200MHz depending on the setting of the relevant LC tank components settings and values.

The amplified voice signals from T1 collector gets effectively modulated over the T2 high frequency carriers waves and this modulated signal is applied to the base of T3 for enriching it with high current.

T3 basically ensures that the modulated voice signals become significantly powerful with current, and is able to be transmitted to much longer distances with the help of an appropriate antenna.

The antenna does not need to be anything special, rather an ordinary 2 feet long flexible wire will be quite enough to enable the transmission to reach over 200 meter distance.

Along with these two transmitters you will also need a couple of FM receiver units or simply FM radios, so that the transmitted signals can be received by the respective radios and a conversation over the two sides can be completed.

Thus we basically have two sets of transmitter/radio through which two individuals are able to exchange their thoughts by speaking over the respective MIC inputs.

Each of the Tx/Rx sets must have mismatched frequency while the opposite side Tx/Rx must have accurately matched frequency response, in other words the opposite side Rx/Tx must be optimally tuned with a given frequency value which must be sufficiently different from the tuned frequency value of the other opposite Tx/Rx pair.

For example if one opposite Tx/Rx pair is tuned at 90MHz, the other could be tuned at 100MHz frequency just to make sure that each of the walkie talkie unit doesn't interfere with their own set frequency value.

Using an FM Radio as the Walkie Talkie Receiver (Rx)


The radio could be any type depending on the user's choice, but preferably it must have a convenient frequency adjustment button such as a knob or a up/down keys.

Your smart phone FM radio would also work but the range would be significantly less than conventional radio having a telescopic antenna. If you intend to operate the units for a room to room conversation then probably your phone would do the job as the receiver,

How to Tune and Test The Walkie Talkie Pairs


First make sure that your transmitter is correctly built and is actually transmitting the signals. To do this keep an FM radio at around 2 meters away from the Tx circuit, switch ON the Tx and the radio, and start adjusting the frequency knob of the radio until you suddenly find a "dead" spot on the band.

Now tapping lightly on the MIC should generate a thudding sound on the radio speaker, confirming the transmission from the Tx unit.

After this, keep increasing the distance between the Tx and the radio and finally try finding the maximum possible distance that the units are able to interact optimally. This may be done by some trial and error and by some skillful fine tuning of the adjustments of the two counterparts.

Repeat the same for the other Tx/Radio pair and this will complete your homemade FM walkie talkie circuit.

Now it's just about keeping the oppositely tuned units across the two sides from where the conversation needs to be done, and then with some more adjustments you can finally get the conversation going with this simple homemade equipment.

If you have any doubts, please feel free to share them through your comments.

Parts List for the above shown transmitter


R1 =1M,

R2 = 2K2,

R3 = 470 Ohms,

R4 = 39K,

R5 = 470 Ohms,

R6 = 4k7

C1 = 0.1 uF,

C2 = 4.7 uF,

C3, C6 = 0.001uF,

C4 = 3.3pF,

C5 = 10pF,

L1 = See Text

T1, T2 = BC547B,

T3 = 2N2907B

MIC= electret MIC

Need Help? Please send your queries through Comments for quick replies!




Comments

Batuhan KOC said…
How do we have to connect c7? the connection is not shown in diagram :/
Swag said…
please ignore it, it's not there

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