How do I make crystal radio

Thread Starter


Joined Oct 5, 2010
Hello, the diode has been added. (That's why earlier when posting, I feel like I'm missing. )

Paper towel, um what diameter???


EDIT: As for the antenna, what should it be?



Joined Feb 19, 2009
A long wire, basically, for a crystal radio, the longer the better, string it outside to a tree limb.

Just be sure the ground is earth ground, usually an 8 foot copper rod driven into moist ground, but a cold water pipe should work for you if you can find a clean copper pipe to wrap the ground wire around. NOT the hot water side, that's not always grounded.

Diameter of paper towel roll is about 1.5" (roughly 4cm), and 6-8" long (15-20 cm)


Joined Mar 24, 2008
The diode shown is a Schottky, so the only component missing is the coil.

As has been suggested, you need to make a coil, the capacitor shown is 8pf-360pf.

Get a block of wood, mount the terminal strip into it. It is your contacts. You can also mount the variable capacitor and the coil to it.

You will need to get a soldering iron eventually, with this anything will work.

I sent LF those parts. The earphone is a piezo type. I got the adaptor so he wouldn't have to cut those wires, which are a pain to work with.

You will need two antenna parts, a long peice of wire for the antenna, and something to go to a second chunk of metal, say a metal pipe going underground or bed springs. You can experiment, I did when I was in that phase of my hobby.

I'll be back and diagram some of this. I've been pretty busy lately, with one thing or another.


Joined Mar 24, 2008
They have changed since I was a kid. I suspect it is because the tuners (variable caps) are more scarce, but they want to use little loops in the coil to tune with. Ridicules!

LF, pick a coil form, be it a straw, empty toilet or paper towel core, or a smooth plastic bottle (no indents). Then tell me the diameter. I'll calculate the number of windings you need for the right coil.

You once mentioned you weren't allowed to use a soldering iron. Any chance you can talk your folks out of that stance?


Joined Mar 24, 2008
OK, I deliberately chose some pretty rugged parts. Maybe your Dad will show you how after he figures it out. I was handling a soldering iron for wood burning (which is not electronics) at age 8 with adult supervision, I was building models that required some soldering at 12, and doing electronics at 14.

I could never get my dad to understand a soldering gun is way too much. For this project it would probably work OK though, I would still recommend a small cheap iron.

For your inductor you need to be able to hit 530Khz (or 530000 Hz). The capacitor goes to 365pf (there are reasons for this, but no need to go that advanced yet).

I don't know if you've had algebra yet, but the formula is

F = 1 / 2 ∏ Square root ( L C )

Going through the math this works out to

2 π F = 1 / Square root ( L C )
( 2 * π * 530KHz)² = 1 / (L C)
L = 1 / ((2 * ∏ * 530Khz)² * C) = 1 / ((2 * ∏ * 530Khz)² * 365e-12) = 247 µH ≈ 240µH

You could buy this coil, but coils are one of the gadgets easiest to make.

Your PM said you had a cardboard tube that is 3.19" and looks something like this...

I'll get back with you on the number of windings.

You need to post this stuff publically. :) Let other people on the fun.


Joined Mar 24, 2008
OK, I went looking for a online calculator. I didn't find one I was totally happy with, but I was able to settle.

Numbers I used were

Inductance: 240µF
Coil Inner Diameter: 3.19"
Coil Length: 2.5"
Wire Gauge: 18 AWB

Take your tube, cut it down two 3.5". Punch two small holes at each end as shown, they should be around 2.5" apart.

The two holes allow you to thread and hold the wire in the tube as shown. The loose wire should be around 6" long.

Wind the wire tightly (as in each loop touching the next) until you have gone around the tube 62 times, then thread the wire through the other two holes.

If the wire is smaller than 18 gauge space the wires evenly apart.

You should now have a coil around 240µH. I'll show you how to wire it next post. Let me know how this went. Keep the other part of the tube in case I messed up.


Thread Starter


Joined Oct 5, 2010
It need saw for me to cut, I think. Bill, I am having problem lining a guide in the tube also. Sometimes it mess up the correct measurement. But the difference is only about 0.1. Is that okay?