Crystal Radio Set - Project Ultra

Crystal Radio Antenna Coil

Project Ultra is part of The Little Whippersnappers Crystal Radio project, which is part of the Tread-Stone-Pebble program. It is a top-secret government project to get all the whippersnappers in the land to receive and decode spy radio signals in the Medium Wave (MW) and Long Wave (LW) bands.

The radio consists of 8 bands to sweep a wide range of frequencies in the AM spectrum. Your assignment, should you choose to accept, is to make this multi-band receiver.

Typically, governments would pay millions for a receiver such as this; however, The Little Whippersnappers Crystal Radio would be inexpensive. The materials list is very short and requires the rewiring of the ferrite antenna coil, and a selector switch, which costs two bucks!

Remind me to write a letter to President Obama. I just had an idea to cut the defence budget... Crystal Radios are the answer. They do not require many parts and are cheap to make. :-)

The Demands of the Whippersnapper's Part Two

After a few weeks, the whippersnappers became tired of the radio, and they realised that if you change the number of turns in the ferrite coil, you could detect other stations. Hence, they wanted to have more taps in the coil.

They tried changing the germanium diode for an LED, and even a Zener diode in the hope of increasing the signal strength, but in the end, they learnt that those components do not work.

They wanted to add an amplifier stage to their crystal radio, and then submit the work for their science project, hence, the word was given, and the work began.

Ferrite Coil Assembly - Non-NSA Approved

Band Selector Switch Schematic

In this project, you will need to make a new ferrite coil with seven taps. You will also need to find a cheap selector switch so that you can select from any one of the taps on the coil. A 1-pole to 12-way rotary switch that cost no more than two pounds was utilised for the selector circuit. There are some on eBay and one at Maplin, Code: FF73Q.

In this ferrite coil design, I made the taps after the following number of turns, 10, 30, 20, 20, 20, 20, 20, and 60.

I was thinking of using part of the Fibonacci sequence 13, 21, 34, 55, 89, 144, however, perhaps next time.

The MW band is at around 45 turns of the coil, and the LW band is at around 100 turns of the coil.

Circuit Diagram

Circuit Diagram

Please click on the image above to see the circuit diagram. Your task, should you choose to accept, is to make a new multi-band coil and test it using this crystal radio circuit diagram.

Coil - How to Make

Winding the Coil.

When you want to make a tap, cut the coil wire, tie a knot to join the two separate wires back again, and continue winding the coil. Later, you can twist the wires, scrape their ends, and solder them to another extension wire as shown below.

Coil extension wires.

For the extension wires, I am using these nice colourful wires with their ends tinned.

I am using different coloured wires for each tap, starting with black, grey, red, orange, yellow, green, blue, purple, and pink. The centre connection from the switch selector pole will be brown.

Coil Wire Soldering

Scrape the coil at the taps and solder a thin insulated wire to it. This should be done very carefully as this is a radio coil with a bundle of individually insulated wires inside. Therefore, many individual wires will require scraping.

I found that if you make a hot blob of solder and run the wire through it, pushing it along the wire, it permeates into the coil and makes a good electrical connection.

Securing the coil with insulating tape.

After soldering the wires to the coil taps, you will need to secure them with some insulating tape. You should also use a digital meter on a continuity setting to test the connection between the taps to make sure that the connection is sound and proper.

Check the connections between each tap, and then between the ground connection (the first wire of the coil) and each tap.

The other end of the wires solder directly to the selector switch. You will need to keep these wires as short as possible, and they solder to the switch in the colour sequence mentioned above.

Ferrite Coil with Taps.

If you secure all the wires very neatly at one end, then it will look like this. Each wire has two layers of insulating tape.

Ferrite Coil

After assembling the coil, you install it back into The Little Whippersnapper's Crystal Radio. If you have not made the radio yet, then you first need to consult the article called: Making a Crystal Radio

Band Selector Switch

Maplin Switch Order Code: FF73Q

I managed to get this rotary selector switch for two pounds. It is called 1 Pole 12 Way Rotary Switch. This means that I can select from a maximum of twelve different coil taps if I wanted, however, in this project there are only seven taps.

I might use the other unused switch positions for station presets in the future; hence I have left those unconnected.

Band Selector Switch Terminals

When the shaft rotates, the centre terminal makes a connection with all the other terminals on the outside.

Switch Wiring Colour Code

The colour-coded dots show the sequence of wiring to the switch. It all starts from the first terminal with a black dot next to it. You solder a black wire there. Then a grey coloured wire to the terminal with a grey dot, and then red wire to the terminal with the red dot...

In the wiring circuit diagram, and the schematic circuit diagram, I have also numbered them as well.

Switch Connections

The centre terminal has a brown coloured wire soldered to it. This wire is the selector pole and marked as S in the circuit diagram. This is how it appears after soldering all the wires.

Wiring Circuit Diagram

Crystal Radio Circuit Diagram

After soldering the band selector switch, you will need to build the crystal radio circuit to test it. You will need a tall aerial and a good earth so that the receiver picks up all the stations.

Installing the Switch

Band Selector Installation

The switch will install onto the metal bracket very well, however, there is a small plastic pillar, which gets in the way. Luckily, its use is optional, and a pair of pliers can remove it.

The other option is to drill a hole for it in the steel bracket, which I would not recommend.

Switch Mod

As you can see, a nice sharp pair of cutter pliers will cut off the plastic pillar.

8 Band Crystal Radio

Rotary Band Selector

Here is a view from the back. I have neatly tied the wires using a plastic tie and it looks very professional!

Band Installation

Here is a view from the front. It looks good to me!

Buy some radio kits today and have fun!

Assignment Completed

Project Ultra Completed

Once I connected the antenna and earth wire, I was able to test the circuit and was impressed with the reception. I managed to detect some strange sounding radio stations. I also heard some strange music, the type that would go well with a belly dancer...