Building an AM Radio Receiver

Building a Radio at Home

I have built a transistor radio. It’s quite a simple circuit. The parts you need are:

• An induction Coil

• An antenna

• A germanium diode

• 3 capacitors

• An LM386 amplifier

• A speaker

• A 9V battery

• Some wires and a breadboard

The Simplest Radio

I started by making the induction coil. You need a bottle with a 7.5cm diameter and a height of at least 15cm. Drill a pair of holes at the top and another at the bottom of the bottle, then trim a wire and anchor it through the top pair of holes. Now tightly wrap that same wire around the bottle but every 5 winds, strip some insulation off the wire (be careful not to cut the wire) then twist it into a loop. This is called a tap. Repeat this process 12 times to the bottom pair of holes so that you end up with 12 taps and 65 winds of wire.

The antenna collects lots of different radio frequencies, so we need the induction coil to narrow them down all the frequencies, so I only hear the ones I want. Each tap on the coil is a different frequency.

For the antenna we used a really long piece of wire (about 30 metres) and stretched it out of the window into the garden to pick up radio signals that I couldn’t find inside the house. The wire went all round the garden and if you didn’t connect a tap the speaker went crazy.

I used a breadboard to build the circuit because it makes all of the connections easy to make. The antenna and induction coil are connected to a diode. The diode only lets positive current through which means it chops the bottom half of the wave off. This is the demodulator in the circuit.

If I had a really sensitive earphone, I would be able to very quietly hear what the antenna was picking up from these components alone, but I decided to make it louder using an amplifier, some capacitors and a speaker.

The amplifier makes things louder and needs a battery to operate. The amplifier I used comes in an 8-pin chip. It has pins for 2 inputs (positive and negative), an output, a bypass, gain (positive and negative) and power (positive and negative).

Across the gain pins, I put a 10 μF (Microfarad) capacitor for maximum gain. On the out of the amplifier, there is another capacitor (220 μF), which protects the speaker from direct current (DC).

The speaker changes the electric current into the sound waves that we hear. When I connected the battery, the speaker picked up mostly static but, after moving the antenna around and adjusting the taps on the induction coil, I managed to pick up a radio station broadcasting Arabic prayer!

I could have made it clearer by using a variable capacitor to help narrow down the frequency even more, so you would hear less static.