The article explains a simple walkie talkie circuit that can be easily built by any hobbyist and used for communicating between rooms or floors or simply for having some fun across neighbors and friends. The range of this system is around 30 meters.
The figure shows a four stage transistorized circuit which behaves both like a transmitter and a receiver unit, making the design very economical and versatile.
An ordinary “4-pole double throw” switch serves the purpose well for transforming the unit either to a transmitter or a receiver while communicating with another identical transmitter/receiver set.
As can be seen in the diagram three transistors are directly coupled for making an audio amplifier stage set to operate at a significantly high gain.
The first transistor functions as a pre-amplifier which pulls the minute voice signals to some higher level and feeds to the next high gain Darlington stage which further amplifies the received audio frequencies and dumps it across the primary of a driver transformer.
How Driver Transformer Works
The driver transformer steps up the level of the signals such that it becomes clearly audible over the connected loudspeaker.
The speaker may be salvaged from an old small transistor radio or from a landline phone (earpice).
The speaker in the shown design is configured in an interesting manner. Depending upon the position of the walkie talkie switch, the speaker works like a sound reproducer when it’s in the receiver mode and like a super dynamic microphone when the switch is toggled in the transmitter mode.
While the speaker is being used as a sound reproducer or simply in the receiver mode, the first transistor acts like a signal receiver, picking up the audio across the 4k7 load resistor through the 0.47uF capacitor.
The signals then has to pass through a connected volume control stage to finally reach the three transistor amplifier stage discussed above.
However while the proposed walkie talkie circuit is flipped in the transmitter mode, the speaker gets rigged right at the input of the amplifier stage such that the spoken voice hits the speaker diaphragm and gets amplified by the same transistor stage.
This amplified voice signal is now applied in the form of supply voltage for the circuit in the transmitter mode. The switch also makes sure that the 27 MHz crystal gets connected with the first stage while the transistor gain is uplifted by eliminating the 390 ohm resistor and using a 59 ohm resistor at the emitter of the transistor.
In the transmitter mode the speaker transformer secondary now has no connection with the voltage step-up function rather simply acts like a series inductor for coupling the output of the audio amplifier with the supply rail and for sending the signal across the winding to the transmitter stage in the form of a fluctuating supply voltage.
As the above signal witnesses a rise and fall in response to the spoken voice, the gain of the first transistor stage is forced to change correspondingly which in turn results in a varying amplitude for the carrier waves transmitted by this stage over the attached antenna.
Thus the spoken voice now gets converted to an amplitude modulated (AM) RF 27MHz signal which may be picked by another identical unit placed in the vicinity for the same reason.
How to Wind the Antenna Coil
The coil associated with T1 is the antenna coil. It is constructed over a ready made variable inductor slug (see image below) having an approximate 3mm diameter and around 7 to 10mm height.
The wire used is a 0.3 to 0.5mm super enameled copper.
Start with the primary 9 turns first, directly on this wind the secondary 2 turns.
The coil in series with the antenna is s simple air core coil made by winding 5 turns of 0.3mm with 5mm diameter.
How to Wind the speaker coil
You may use a small audio transformer for the shown speaker transformer, or alternatively build it by winding around 70 turns for the primary (left side), and 500 turns at the secondary (speaker side).
The wire may be a 0.2mm super enameled copper wire wound over a 3 inch long iron screw.
How to Set up the Circuit
After you have built the above explained walkie talkie circuit it's time to check its response by powering it with a 9V PP3 battery.
Initially let the switch contacts be positioned for activating the transmitter stage.
For knowing whether the transmitter is generating the required 27MHz frequencies or not you will first need to make an RF sniffer circuit as explained HERE
Switch ON both the circuits, position the above RF detector circuit about 10 inches away from the walkie talkie antenna, and begin adjusting its variable inductor slug gently using an insulated screw driver which are typically used for adjusting FM radio GANG trimmers.
If every thing's done correctly you'll hopefully see the RF detector LED glowing brightly at some point of the adjustment process.
Seal and glue the variable inductor at this position, and you can assume your walkie talkie to be all set for having some great time with your friends.
However you would need to build another identical set for exchanging the conversations with the other guy, otherwise a single unit wouldn't have much of an importance.