Radios have always piqued my interest and it has been my dream to build one since my high school. Finally, I was able to make it and that too with a very few components. The radio I made is quite minimalistic and can be reproduced very easily. Also, the working is quite easy to understand. It basically consists of four parts:
1) Tuner part - A variable capacitor and an inductor in parallel which pick up radio signals.
2) Amplifier part - A voltage amplifier circuit built using LM324.
3) Detector or demodulator - I used a simple germanium diode for this process.
4) Speaker - I used normal earphones for this purpose.
So, for building this radio, you will be needing:
a) 2 Chocolate wrappers/Aluminium foil along with a thick book (for building the capacitor)
b) A wire of length 4-10 meters (for building the inductive coil)
c) Some electronic components: 2 x 100 ohm resistors, 1 x 10 K variable resistor (potentiometer), 1 x LM324 IC, 1 x 9V battery (for building the voltage amplifier part)
d) A germanium diode (OA-79, 1N34A or any other)
e) Earphones or less power requiring speakers.
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Step 1: Building the Tuner Circuit
A capacitor can be made by placing two metal plates at a distance from each other. I thought of using chocolate wrappers in stead of the metal plates, although one can also use aluminium foil pieces for this purpose. Chocolate wrappers have aluminium coating on one side which makes them ideal for acting as capacitor plates. Since the capacitor can be built only if the plates are separated by a distance from each other, one can place the two wrappers within a book with a separation of one page between them. In order to vary the capacitance, one can increase the number of pages present between the two plates or pull one of them out of the book slowly (in order to vary the area of overlap between the capacitor plates). Finally, I used tape to connect two wires to the two foils. This makes up the variable capacitor.
The inductor can be created by coiling a wire around a cylindrical object and its inductance can be approximately calculated by using the formula for inductance of air cored coils if the cylindrical object is non-magnetic. I built a coil with a radius of approximately 0.65 inches, length of approximately 1.5 inches and totally 35 number of turns (which indicates an inductance of approximately 24 microhenrys). After you build the coil connect it in parallel with your variable capacitor. This combination is the tuned circuit which picks up the radio signals from the air.
Step 2: Building the Amplifier and the Detector Stage
The voltage amplifier part of the radio consists of an opamp with resistors connected to form a non-inverting amplifier configuration (full circuit of the radio built is shown above). This stage amplifies the signals from the tuned circuit about 200 times if the potentiometer is set to its maximum value. I used a LM324 IC for the opamp because it can work on quite low power supply voltages.
After the amplifier, comes the detector which is a single germanium diode. The diode acts as a rectifier and increases the average voltage transferred to the earphones. Germanium diode is used as it has a very low forward drop voltage, nearly around 0.3 Volts.
Step 3: Connecting the Earphone
The earphone is the final unit which is used to hear the sound. In order to connect it to the circuit, we need two leads out from the earphone. Cut out the main wire and you will find several twisted bundles colored differently. This color is from the insulation provided to the individual wires. You can scratch it out or burn the insulation in fire to get the naked wires but keep the bundles separate. Two of these will be used to connect the earphones to the circuit. After the copper wires are exposed, connect the bundles to proper wires by either soldering or just twisting together at the ends. Finally, check which pair of wires respond to voltage provided by the battery. You will hear a click in the case when the right pair is obtained. After that, connect the whole circuit together as shown previously. If you hear static, the hiss kind of sound which comes when a radio is tuned, you have built the radio successfully.
Step 4: Experimenting and Further Reading
In order to listen to different channels, tune the capacitor by varying the number of pages between the foils or by varying the area of overlap between them. This radio also works if the capacitor is removed. In that case, however, you need to move your finger near the inductive coil in order to create the capacitive effect. For people who are making this circuit, in case if any problem arises, keep trying because believe me it is a thing worth making!! The time when you hear someone's voice for the first time on this thing, it is really awesome. I would suggest readers to go further and build something more minimalistic as well as efficient.
As far as sources are concerned, while making this radio, I took help from two sources:
1) A book named "Building and designing transistor radios" by R.H. Warring
2) A website on homemade radio building by Simon Quellen Field