DIY Radio - No power needed

 

Introduction

Imagine being able to listen to the radio without needing a power source, just a few household materials and a bit of ingenuity? This recipe will show you on creating an AM radio by using an oranges fruit box, a pencil, a shaving blade, and a copper coil. This fascinating project not only showcases the beauty of DIY electronics but also connects us to a remarkable chapter in history.

The foxhole radio has its roots in World War II, when soldiers in the trenches sought ways to stay informed and entertained amidst the turmoil of battle. With limited resources and the need for stealth, these resourceful individuals crafted radios from whatever they could find—often using items like tin cans, wire, and even personal belongings. The result was a simple yet effective device that could capture AM radio signals without requiring any external power, allowing them to tune in to news and music while remaining hidden from enemy forces.

In this recipe, I will explain the process of building your own power-free foxhole radio. By using just a pencil, an orange box, a blade, and a coil, you’ll not only learn about the principles of radio technology but also experience the thrill of creating something functional from everyday items.

Ingredients

• Oranges box or a piece of cardboard for the base. A wooden board are also valid.
• Safety pin.
• Pencil with graphite lead (make sure that the pencil is 8B, lighter pencils lead contains plastic and will affect the conductivity).
• Shaving blade. 
• 15 or more letters of copper cable (can be the electricity or telephony ones)
• Approx 0.1mm coated copper cable (enough to make 100-150 turns). 
• Small cylinder (can be a toilet paper can). 
• Low impedance crystal earphone or an amplified speaker. In this project I connected my radio to an amplifier via minijack cable.
• AM radio stations around. The closer the better.
• Lot of patience.

Preparation

Heat up the blade on a fire source (can be on the kitchen cooker) and leave it for a few seconds. Magically, the colour of the metal will change to a blueish colour which is the ideal for our experiment. Heating the blade creates a blue oxide layer, allowing the current to flow in only one direction. In the picture below you can see the difference between a new blade (on the left) and a blue blade (o the right):

Cut the pencil one inch from the tip and nail the safety pin to the back of the pencil, making sure to have contact between the internal graphite of the pencil and the safety pin. 

Attach the blade to the cardboard and attach the pencil and safety pin next to it. The pencil should be able to slide around the surface of the blade. What we have created here is a homemade diode: The blade allows the electrons to flow in only one direction, and the interaction between the graphite of the pencil and the metal blade helps to improve the rectification of the radio signals. At the same time, the movement of the pencil on the surface of the bread will increase or decrease the capacitance of the circuit. Definitely, a germanium diode gives way better results than this homemade experiment, but it is awesome to build this kind of stuff from the scratch.

Now wind the copper wire around the cylinder for around 100-150 turns making sure that the wires turns are very close to each other. Also ensure that you have around 2-5cm of wire ending on the left and on the right of the can. This will be the connections points of our coil. After this, use a scissors or a sandpaper to scratch those endings to remove the coating, otherwise they will not make connection with the rest of the circuit.

The antenna will be our 15 meters of more of copper cable. We need to wrap it around the house, specially on surfaces located close to the ceiling (like wardrobes or top of the doors). Also placing it next to the windows or balcony will improve the overall reception. Since our radio does not have any batteries, the energy will be obtained from the inductance from the signals that are coming from the radio station. Also by wrapping the cable around the house will create a capacitance that will help to our project. Due to its structure and interaction with its environment it can functions similarly to a capacitor. This capacitance plays a crucial role in tuning and optimising radio frequency reception and transmission.

The ground cable can be connected to either the ground of any power socket, but I personally don’t recommend this because it is too close to the AC power lines, if your cable go to the wrong whole there is a huge risk of fire or making you being electrocuted. It it is safer to just attach a copper cable to any copper pipes that are coming from your building (for example, for heating). Those pipes are connected the ground already.

Now attach your crystal earphone to the circuit (one end to the ground, another end to the safety pin with the pencil). On this project, I installed a minijack port to connect later a cable to an  amplifier that will increase the sound volume. 

Now connect all the elements of the circuit as shown below:

Serving

If everything was done properly, you will start to hear some background noise. If you are using an amplifier, it will help you to hear more loudly, as the little amount of inductance from the air signals is very low.

Move the pencil slowly and gently on the surface of the blade, paying attention to the spots that the noise intensifies, these can be potential radio stations. By doing this, we are modifying the capacitance of the blade and the pencil, having as a result that we are modifying the tuning frequency!. The formula is:

Where:

L = Inductance (henries) -> This is our copper coil.

C = Capacitance (farads) -> This is what we are modifying by moving the pencil on the surface.

The quality of the sound will be determined of how good are the blade, the pencil, the antenna and the signal strength of the radio station. So many factors that make this kind of radios a nightmare to catch some decent radio station with a minimum of quality around, so if you caught some muffled voices like I did, you can make yourself proud of your project.

Have in mind tat when these radios were implemented in the 40’s, there were a lot of AM stations and little interferences, specially outside the cities. Nowadays, many countries does not longer offer AM radio in favour of FM or DAB radio, which brings much more quality and stability. However, it is fun to create this projects. Have fun!