Posted in 7th Grade, Science 7

S7 L21 – Crystal Radio

For today’s work, I built a crystal radio receiver!  It doesn’t work, but I think I got pretty close.  All I needed was a scrap piece of wood about 15”x8”, a wide wooden piece about 1”x2”x2”, two longer poles, the first about 10”x2”, and the second near 12”x1 ½”, about half a roll of magnet wire, a piece of sandpaper, a toilet paper tube, a piece of a paint can, a 7×7 square of paper, two 6×6 squares of aluminum foil, a towel roll, a 1N34A diode, and a roll of tape (any will work, I used electrical tape).

Building the Base

Okay, you’re going to need a drill for this part, so if you don’t have one, you might want to go get one.  Now that you have a drill and hopefully some screws, get ready for some construction.  First take the large plate and set it on the table.  The largest side should be facing up.  Take the 10-inch post and line it up exactly 90 degrees with the plate, and predrill holes through it into the side of the flat piece.  Secure the pole and screw it on.  Next, take the other 12-inch piece and place it, parallel with the flat plate, and with 1 1/2” face up, and screw it onto the top of the first pole.  Now that that is done, take the 2×2 square and screw it into the plate from the bottom.

Wiring it up

I got all the wires, and to start, wound a piece of wire 25 times a 1/8” from the end of a  toilet paper roll, and then once I finished that,  cut the wire and started another coil.  This coil is 1/8 of an inch away from the end of the first one and had 90 turns.  Sand the ends of all the wires and the top of the 90 turn coil.  The exposed copper should be a shade lighter than the rest of the wire.  Hook the starting end of the short coil up to an antenna (about 15’ of wire; I stuck mine a cardboard box about 5”x4”x3”).  Don’t forget to sand the beginning of that too.  Then create a variable capacitor by taping the first 6×6 aluminum square onto the toilet paper roll.  Tape the second onto the square of paper, and then roll up the paper with the foil on it so it faces outwards.  Push this onto the paper towel roll, and tape it to the correct size.  The aluminum plates should not be touching.  Sand the end of another wire and tape it to the stationary piece of aluminum on the towel roll, and do the same with the other side.  Do not forget to sand the wire!  Mine doesn’t work, and I’m pretty sure that’s why; I forgot to sand the ends of all the wires I used.  Slide the capacitor onto the top, level pole on the frame, and secure it with a thumbtack.

Sand the end of the wire on the sliding piece of the capacitor and clip it to the end of the 1N34A diode, and do the same with the end of the long coil closer to the short one.  Hook the positive end of the diode to an earpiece or speaker, and hook the other terminal of the speaker to ground.    To make the tuning bar, just cut out a piece of a paint can and bend the end into a V-shape.  screw this onto the 2×2 block on the frame, with a wire under it so it has metal to metal contact.  Don’t forget to sand!


There are a mass of wires that go to ground, so here’s a list: the one on the stationary half of the capacitor, the finish end of the 25 turn coil, the end of the earpiece, and the tuning bar.  Hook the ends of these wires to a single wire (don’t forget to sand!)  and then run that to either a metal cold water pipe, a metal chain fence, or a dedicated ground rod.  Unless you have experience with this type of thing, don’t try to plug it into the wall ground!  You may blow a circuit, which could hurt you badly; I don’t want to lose any readers to a wall socket!

Really that’s all I have for today’s Super Science project, which is not only cool, it is useful and if you attach a transmitter, is insanely awesome!


I'm 10, in the Ron Paul curriculum, and working on my blog,!

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