DIY Crystal Radio

How to build a crystal radio. They can be made for fun or as a real piece of art. This one is easy and robust. DIY step by step. No special tools or skills needed. Most parts can be had from recycled things. The purchased parts run about $3. Good during power failures, when batteries are not available. This one is loosely based on the Cub Scout / Wolf Scout / Boy Scout crystal radio design.
Depending on your design (coil mostly) it can receive in the AM broadcast band, approximately 535 to 1700 kHz.

https://www.youtube.com/watch?v=mt-s7pf-U5Q

See Warnings below.

OPERATION TIPS & TRICKS
- A good antenna is important. Longer is usually better (see antenna warning below), a good ground is next most important. More below
- With some stations, you can put a finger or 2 or 3 inside the tuner coil to improve reception. Move them around for best reception.
- A finger near the coil can also improve reception. Touching the coil directly usually lowers sound volume, BUT a piece of paper or thin plastic between finger and coil often improves sound / tuning. Move finger around.
- Touching different parts of the radio or getting near them can affect performance.


TOOLS
- Screw driver
- Knife
- Sandpaper (small piece, fine)
- (Optional) Saw, if you do not have the wood & tube already cut
- (Optional) Hand drill for screw holes
- (Optional) Soldering iron, solder

MATERIALS
- 1 x Germanium Crystal Diode (1N34A)(bought: Amazon, Radio Shack 276-1123, Digikey, Jameco, Mouser...)
- 1 x Resistor (10k ohm)(bought: ditto)(see note below)
- 1 x Crystal earphone (can NOT be other type)(bought: Amazon, Jaycar, Jameco?) MUST be like the type in the video. Regular iPod or similar will NOT work.
- 1 x Base 21 x 17.5 cms x 2 cms thick or more (wood scrap)
- 1 x PVC pipe 15 cms long or stiff cardboard tube x 2.5 inch diameter
- 1 x motorcycle / bicycle spoke, 2mm diameter or brass rod or stainless rod
- Enameled bell wire (can be from old transformer, solenoid, ...) 0.3 to 1.5mm (note: I used 0.5mm or roughly 19-20 AWG, the number of turns and length of pipe change with wire size.) Bigger wire has lower resistance (better sound), BUT you get fewer turns per centimeter (less tuning).
- Screws, stainless or brass (6 for wire hold downs, 2+ for coil mounting),
- Fahnestock clips (6) (bought: Amazon & electrical shops). OR brass washers (13) (bought, hardware store)
- 2 bigger washers for wiper.
- 1 bigger, large head screw for wiper.


ANTENNA & GROUND
- The antenna is just a long piece of wire (I use scrap wire from old transformers, fluorescent light ballasts and such). The longer the better.
- Try different lengths and orientations (vertical, horizontal)
- A good ground helps. A wire to a ground rod is good. Metal water pipes also work well.


NOTES:
- I ran the calculations on the coil and with 2.25 inch tube and 170 turns it's about 425 micro Henries (but it varies up to 650 depending on whose calculator I use). That's a bit more than the typical design which runs about 350 - 400, but still very workable. It means you can get by with more like 150 turns of wire.
- The resistor is necessary to get the best sound from these "new" crystal earphones. The new ones are piezoelectric and have lots of capacitance. Without the resistor the charge builds up then discharges so you get constant "static" sounds. The resistor bleeds off charge so you get louder, and much clearer sound.
- Also called a foxhole radio.


WARNINGS:
- Never connect radio to any source of power. Not house current, nor batteries, nothing!
- Never use the radio during lightning storms.
- Never run an antenna over or near power lines.
- Disconnect antenna when not in use.
- Never ground to electrical items, plugs or similar.
- If you don't know, don't do it.



Re-edited for annotations 20190105