|Image Courtesy: Elektor Electronics|
It's basically based on a superregenerative audion receiver circuit where the use of minimal components becomes the main feature of the unit.
However fewer components also means a few compromises involved, here the receiver requires a large metal base for grounding the unwanted signals, and for keeping the noise factor to the lowest, and also this system would work only in places where the reception is rather strong and thus may not be suitable in areas where the signal strength is thinner.
How the One Transistor FM Radio Receiver Works
As mentioned above, the circuit is basically a single transistor superregenerative RF oscillator with a constant amplitude.
Here we have tried to enhance the design such that the amplitude becomes considerably magnified in order to turn OFF the transistor completely during the oscillations.
This called for an increase in the feedback capacitor and also to use a transistor specifically designed for handling extreme high frequency ranges such as a BF494.
Further modifications include an inductor with the emitter of the transistor, and a capacitor across the emitter resistor of the transistor.
Due to this the transistor is switched ON as soon as the base emitter voltage of the transistor falls significantly, resulting in an abrupt cut off in the oscillations.
However this prompts the emitter capacitor to discharge, allowing the collector current to yet again resume its flow, initiating a fresh cycle of oscillation.
The above happening forces the circuit to flip flop between two situations, oscillator OFF and oscillator ON, resulting a sawtooth frequency of about 50kHz at the output.
Each time the circuit flips across the above ON/OFF states, results in a significant stepping up of the amplitude which in turn constitutes greater amplification of the received signals. The procedure also gives rise to noise but only as long as a station is not being detected.
The above design has one drawback, though. The output received from the above circuit would have greater content of sawtooth noise compared to the actual FM reception.
A smart technique can be seen employed in the following single transistor FM radio circuit to attribute better efficiency to this simple design.
Here we pull out the emitter capacitor C5 ground link and connect it with the output. This results in a fall in the collector voltage as the collector current rises, which in turn forces the emitter voltage to rise, prompting the emitter capacitor to negate the situation at the output.
This enforcement results in making the sawtooth effect on the received signal practically to zero, thus presenting an FM audio with much reduced background noise.
Single Transistor Radio with Audio Amplifier
To make the above circuit self-contained, an additional transistor stage may be introduced for enabling the radio to play the music loudly over a small loudspeaker. The circuit is self explanatory, just the inclusion of
a general purpose BC559 transistor along with a few inexpensive passive components can be witnessed in the design.
How to Make the Inductors
The involved coils or the inductors are very simply to wind.
L1 which is the oscillator coil is an air cored inductor, meaning no core is required, wire is super enamelled type, 0.8mm in thickness, diameter of 8mm, with five turns.
L2 is wound over R6 itself using 0.2mm super enameled copper wire with 20 turns.
How to Set Up the Circuit
Initially when the circuit is switched ON, the output will be accompanied with substantial background noise which will gradually tend to disappear on detection of am FM station. This may be done by carefully tuning C2 with the help of an insulated screwdriver. Try to keep the tuning at the edge of the band of the particular FM station, with some practice and patience this would get easier with time. Once tuned, the circuit would respond to that reception every time its switched without the need for further alignment.
As indicated at the beginning of the article, the circuit should be installed over a wide circular meta plate, preferably a solderable material, and all the ground of the circuit soldered on this plate. This is important to keep the circuit stable and avoid drifting away of the received stations and also for cancelling unwanted noise.
The antenna in the proposed single transistor FM radio receiver circuit is not crucial and in fact should be kept as small as possible, a 10cm wire would be just enough.
Remember, the circuit also acts a like an effective transmitter circuit, therefore keeping the antenna size bigger would mean transmitting noise across the ether and disturbing your neighbors radio reception.
The upside being that the design can also used as a walkie talkie within a small radial distance....more on this next time.