In this post we are going to make simple Walkie talkie circuit project which can send and receive voice over FM frequency band with range of 250 meters. You can make this as a toy or use it for real life applications if you build it durable enough.
Note: Please follow your country’s wireless regulation strictly while using this project.
We will see:
- Explaining the idea.
- Transmitter Circuit Diagram.
- Transmitter Circuit Explanation.
- Different voice modes.
- The Receiver.
- How to use this Walkie talkie.
This project utilizes FM radio as receiver and FM transmitter to send voice. A person who wants to communicate with the other will have a set of FM radio and FM transmitter and same with the other person. Let’s consider two persons ‘A’ and ‘B’ and they want to communicate:
Here, the person A’s receiver frequency is tuned to person B’s transmitter and person B’s receiver frequency is tuned to person A’s transmitter.
You can choose any frequency available from 88 to 108 MHz but, don’t choose a frequency that has a FM station operating; this will interfere with the conversation.
The communication can be half duplex or full duplex and a switch is provided in the circuit to change between these modes. Let’s explore briefly what are half and full duplex communication.
Half-duplex: Bi-directional communication between two people but only one person can transmit and the other can receive at time. Walkie Talkie is the best example.
Full-duplex: In full duplex communication both the parties can send and receive simultaneously. A phone call is best example of full-duplex communication.
There is another communication type called simplex. Simplex is one way communication, radio and TV broadcast example of this.
In this Walkie talkie project we are providing both half and full duplex feature so that you can use according to your convenience.
Half duplex can be used where you need to make contact with the person for few seconds or you don’t want your conversation with a nearby person to reach the person on the Walkie talkie.
Full duplex can be used where you need to make conversation with a person for several minutes like a phone call.
We can see construction / crane workers where they use their phones for short distance communication because they want to communicate while instruments in their hands instead of pressing a button while speaking.
Walkie Talkie FM Transmitter circuit:
A FM transmitter cannot be simpler than this and very stable at the same time. The circuit is constructed around only one transistor and few passive components. Let’s explore the circuit now.
The microphone is connected to 10K resistor and ground; this provides the biasing for the microphone. Between microphone and 10K resistor the audio is taken out via 10uF capacitor. The capacitor allows only AC / voice signals to pass through and blocks DC signal that may enter the base terminal of the transistor. A 4.7K resistor is connected in series with the 10uF capacitor to prevent excess current reaching the base of the transistor.
Two resistors 15K and 4.7K acts like voltage divider across the base terminal which provides proper biasing to the transistor and keep the transistor in active mode where a transistor acts like an amplifier.
You can see a 0.1uF capacitor connected parallel to 4.7K resistor this is called bypass capacitor. This will bypass or simply pass the high frequency noise that may emerge from microphone to ground, preventing noise being transmitted.
You can spot a 47 ohm connected to emitter terminal, this provides stability to the transistor and prevent excess current passing from collector to emitter while transmitting. Another bypass capacitor is connected across the collector and emitter terminals, this will pass any unwanted high frequency signal generated by LC circuit to ground thus provides a clean transmission.
The variable capacitor and the inductor form the tank circuit which oscillates between 88MHz to 108MHz. We will tune this circuit to a fixed frequency between these two ranges by turning the trim capacitor.
Note: The trim capacitor is a two terminal device which comes with three terminals to provide mechanical strength; two of terminals are same.
Finally before the antenna there is a capacitor which prevents DC noise signals being transmitted.
The antenna can be up to 3 feet to get best range. This circuit can work without antenna from 50 to 100 meter depending on the obstacle between the receiver. We recommend use it with an antenna, an insulated wire with 1 feet long will do the job.
The alterations you need to do to the circuit:
In addition to the transmitter circuit you need to make the following changes in the circuit so that you can talk like Walkie talkie in half duplex mode and talk like a phone conversation in full duplex mode.
By sliding the sliding switch ON near the microphone will enable full duplex mode, sliding the switch off will enable half duplex mode. In half duplex mode you need to press the push button to transmit your voice.
You may say we can switch OFF / ON the transmitter using sliding switch for full duplex mode and use push button to turn on and off the transmitter for half duplex communication, it also saves power, right?
When you turn off the transmitter to break the communication, the FM receiver will make noise when no radio station (transmitter) is available, this will annoy the person who is listening to your conversation and also for you. So it is best to turn on and off the microphone instead of the transmitter.
You can use your phone’s build in radio as receiver with head phone plugged-in. You can also use a handheld FM radio as receiver, but there is good chance that you already got a phone with FM radio capability, so you no need to spend some bucks on a FM radio.
How to opearate this Walkie talkie:
You need to make couple of transmitter circuits and you should have couple of FM receivers to communicate between two people.
Now pick a FM receiver and tune into some frequency which don’t have a radio station. Now pick a transmitter and switch ON, use a tiny screw driver and turn the trim capacitor left and right slowly until the FM receiver gets complete silent. Now talk on the microphone, your voice should clearly hear on the receiver.
Repeat the same step with a different frequency; choose at least 0.5MHz away from the frequency you initially selected. For example: if you selected 96MHz for the first transmitter and receiver set , choose 96.5MHz or 95.5MHz for the next set of transmitter and receiver.
Now give a transmitter and receiver to another guy and try to communicate with 50 meter distance apart in full duplex mode. Gradually move apart further and test the max range.
Plug the earphones, put your phone in pocket, place the transmitter at your shoulder and walk like a pro.
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