Electronics Projects: How to Build the Coil for a Crystal Radio
When you look at a crystal radio, one of the simplest of all useful electronic circuits, the first thing you’re likely to notice is the large coil. The coil usually consists of 100 turns or more of small-gauge magnet wire wrapped around a non-conductive tube anywhere from one to five inches in diameter. The coil is an essential part of the radio’s tuning circuit.
Many different types of materials can be used to wrap the coil around. Here are a few ideas:
An empty 16 oz soda bottle. To make the coil look better, you may first want to spray-paint the bottle with black paint. (Don’t use metallic paint!)
An empty toilet-paper roll.
An empty oatmeal container.
A 6″ length of 2 or 3″ diameter PVC sprinkler pipe.
A 6″ length of a wooden closet rod.
In short, any sturdy cylindrical object that is made of an insulating material can be used as the core of your coil. As long as it’s cylindrical and not made of metal, you can use it.
The most common choice of wire to use for the coil is magnet wire, which is coated with thin enamel insulation rather than encased in plastic insulation. Wire wrapped with plastic insulation will work, but enamel insulation is thinner and thus allows the turns to be spaced closer together.
The number of turns in the coil and the diameter of the cylinder you wrap the coil around will determine how much wire you need. You’ll want to wrap at least 100 turns. Allow a foot or so of extra wire at each end of the coil to connect the coil to the radio circuit.
For the simplest type of crystal radio, the exact number of turns doesn’t affect the operation of the radio significantly. So in the example here, if you have a 50′ spool of wire, you can just wind the coil a few turns short of 100, and the radio will work just as well.
To start the coil, you must first attach one end of the wire to one end of the tube. The easiest way to do that is with a dab of hot glue. If you prefer, you can punch a hole through the tube and feed the wire through.
Be sure to leave six inches or more of wire free. This will give you plenty of wire to connect the coil to the circuit when you finish winding the coil.
Once you’ve attached one end of the coil, turn the tube slowly while feeding wire from the spool onto the tube. Each half turn or so, carefully scoot the wire you just fed up against the turns you’ve already wound. The goal is for each turn of wire to be adjacent to the previous turn, with no gaps between the turns.
You’ll find that you need to keep a bit of tension on the wire as you feed it onto the tube in order to keep the windings nice and tight. If you slip and let go of the tension, several turns may unravel, and you’ll have to untangle them to restore the coil’s tightness.
It helps if you wrap the coil in sections of about ten turns each. When you finish each section, dab a little hot glue on it to hold it in place.
When you reach the end of the tube, use a little hot glue to secure the last turn of the coil, or cut a slit in the tube and slide the wire through it. Be sure to leave about six inches of free wire after the last turn.
When you’re done, the coil should have a nice, tight appearance with no major gaps between the turns, and you should have about six inches of free wire on each end of the coil.