- 30 SWG enamelled copper wire
- High impedance earpiece (or a piezoelectric speaker will do), but it must be high impedance - a crystal earpiece rather than a magnetic one.
- A kitchen roll
- A scrap piece of wood to attach everything to
- Short-length self-tapping screws
- Cup washers
- Paper clip
- Safety pin
- A short length of pencil lead
- Small piece of foil
- Sand paper or emery cloth
- Wind 100-150 turns of our copper wire around the kitchen roll. Cut the spare, bare length of kitchen roll off as you don't need it and it will be in the way. Firmly attach this to the wood (Jess and I screwed it on).
- Using some sandpaper or emery cloth remove the enamel off the ends of the wire and rub the enamel off the copper wound around the roll along the length of the tube, as you can hopefully see in the diagram (click on the diagram to enlarge it)
- Rub some of the blue oxide off the end of the hacksaw blade ready for a wire to be attached to the bare metal (don't rub the oxide off all of it!)
- Attach the pencil lead to the point of the safety pin by wrapping foil tightly around them both. The lead is then "sprung loaded" against a blued bit of blade (that's why we are using a safety pin).
- Now join everything together using the schematic shown below (click on the image to see an enlargement)...
Wipe the paper clip against the exposed copper wrapped around the kitchen roll. If all is well you should be able to hear cracking from the earpiece. You should, in fact, be able to hear (probably) BBC Radio 5.
How does that work, then?
Our hacksaw blade is technically called a detector or diode. A diode allows electrons to flow one way but not the other. A coil and a capacitor together made an oscillator. Inside the oscillator electrons are actually doing the oscillating. If we adjust the position of the paper clip then we can make the electrons oscillate at different frequencies. That's how we can tune in this radio. The coil is obvious but where is the capacitor? Capacitors have two plates, and in our case the antenna is one plate and the ground beneath it is the other. The electrons are trying to oscillate back-and-forth, but the detector only allows the electrons to move one way. That means the electrons are shunted around the circuit, generating a current which is then converted to sound in the earpiece.
Now you are armed with this knowledge, if you are stuck in a dugout or a prisoner of war camp you should have a good idea of how to be able to put together a simple radio receiver for yourself!