Making and using crystal radios

I love crystal radios. There is something magical about a radio that does not require any power source to run – at least not an obvious power source. Crystal radios do use power. Radio waves are a form of energy and it is some of the energy from the waves coming from the radio station that the listener is hearing that provides the power. Relying on this as a source of power obviously has limitations. The maximum volume is dependent on the distance to the transmitter, the power of the transmitter and the size and setup of the antenna connected to the crystal radio. Usually headphones are required to hear the sound. At my home I have only been able to successfully listen to one local station. They can be made to work on frequencies outside the usual AM radio band but I’ve not had much success with those bands.

A simple crystal radio I made. One of the switches on the front turns the headphones on and off to save wear and the other switches the transformer in and out of the circuit.

They are great fun to make and require only a few parts that can often be scavenged from old radios. Thirty five years ago I found this as exciting as a teenager today may if he connected an old picture frame, some LEDs, a few wires, a couple of batteries, an aluminium sheet and ended up with an iPad.

Early days

Crystal radios played an important role in the early days of radio in the early 1900s. They were the first type of widely used radio receiver. Even after the introduction of valve (also known as a vacuum tube) radios, it was some time before electricity was readily available to many households particularly those in remote areas. Batteries were expensive and valve radios were not very efficient. For many in remote locations a crystal radio provided an important link to the outside world. Due to their simplicity, ease of construction and relative ease of obtaining parts, these and their cousins the Foxhole radio were sometimes used in times of war in trenches and prisoner of war camps.

My first radio

My sisters had their own crystal radios and when I was old enough I got one too. We all had the same type. They were small, made in Japan and had Germanium Radio on the front, presumably because they contained a germanium diode. The brand label is long gone from mine, but I believe it is an EM-Tone.

Mum made us all little cloth bags with ties attached that we tied to the bed head. The radios hung in these bags above our pillow with the headphone wires trailing down over the pillow.

A front view of my first crystal radio. Remnants of ear wax can be seen in the earpiece proving that it was used and showing the durability of ear wax.

Back view of the radio with its back cover removed. The simplicity of the design can clearly be seen as can damage to the case near the antenna wire from an early soldering repair job.

How they work

Electromagnetic radiation is collected by the antenna and flows down the wire to one end of a coil. The other end of the coil is connected to “ground” which is usually a rod stuck in the ground. This sets up a nice pathway for the radio waves to travel.

A schematic diagram of a simple crystal radio circuit

A capacitor is usually connected in parallel across the coil. The coil and capacitor set up resonant circuit and the frequency it resonates at can be changed by changing the capacitance of the capacitor, that is turning the knob if a tuning capacitor is used or by changing the inductance of the coil. In the little radio I had there was a small fixed value ceramic capacitor and the inductance of the coil was changed by sliding a ferrite rod inside the tube that the coil was wound around. In most of the ones I have made including the one in the other photo the coil is not changed (although there may be taps in the coil to make some adjustment) and tuning is done using a tuning capacitor. A diode connected to the coil separates the audio out of the signal by only allowing electricity to flow in one direction resulting in pulses coming out the other side. They often have a few more parts, but those are the basics.

Wikipedia has a great article on crystal radios and it is a good place to learn more or just do a Google search as there is a ton of information about them.

My tips

Those little crystal earpieces are horrible. Normal headphones can be used if connected through a matching transformer e.g., 8 Ohms to 2K. 2K side connects to the radio. You may lose some volume but the overall experience I got was better.

Views of both sides of an audio transformer. One side has 2.5 and 5k taps. The other side has 2, 4, 8 and 16 ohm taps.

I have found that not all headphones are equal. Some are more sensitive than others and some have higher impedance. I have old Pioneer SE-6 headphones that are both sensitive and have a higher than usual impedance of around 150 ohms each side. Connecting them in series gives 300 ohms and although this is considerably less than the 2000 ohms recommended, these headphones work and sound good without the need of a transformer.

Headphones are the usual way to listen, but I have been able to hear a station using a small speaker connected through the matching transformer mentioned above. It wasn’t loud, but at night when there are no other sounds I was able to hear what is being said but I was only about 10km from the local radio stations antenna.

Safety: A warning if you are using one

Stringing an antenna wire up outside and connecting another to pipes in the ground and then joining them up to wires next to your skull does come with risks. It is not too different from Ben Franklin’s experiment flying a kite while storm clouds in an attempt to prove that lightning is electricity. It is extremely dangerous. He was lucky; some others that tried it were killed. You need to take care where these wires go. Make sure you do not place the antenna near electrical wires for your safety and the performance of the radio. Use insulators between the antenna and the objects it is connected to. Don’t use the earth rod for the mains ground. And finally, do not listen to a crystal radio during a thunderstorm. These are just some rules. If you do make or use one search the web to find out how to make a safe and efficient antenna and earth.

Future

The future of analogue AM radio is unclear to me. Our local AM station seems to be in decline. There will probably come a time when crystal radios will not be able to receive any stations worth listening to. That will be a little sad, but there are lots of other cool projects for the hobbyist to tinker with coming out.

The ZN414 IC is almost a complete AM radio in a single chip with just three wires. These make great projects too and only need several extra parts and will run from a small solar cell. Best of all they don’t need an external antenna or earth and can be made to run regular headphones by adding a single transistor amplifier section. I’m planning an article on this in the future.

So what about you, have you ever made a crystal radio or use them?

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Posted on June 3, 2012, in Electronics and tagged , , , . Bookmark the permalink. Comments Off on Making and using crystal radios.

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