Crystal Radio--Ferrite Coil Model #2 (4K)

I built a ferrite coil crystal radio from an internet design that worked poorly (I don't recommend building it). I recycled it into this second attempt, using a different winding scheme that worked well on an air core variable cap radio design. I recalculated the necessary turns to achieve the necessary inductance/tap and remade it. I'll show you the result.

I realized I was not clear with the wire sizes. The original ferrite was wrapped with 32 AWG (0.2mm) == 195 uH, the blue air coil was wrapped with 20 AWG (0.8 mm) == 205 uH. The second ferrite was wrapped with 24 AWG (0.55 mm) == 205 uH. Why did I change to 24 AWG? Turns out the 20 AWG wire is too wide and would not fit on the core.

MATERIALS
- Ferrite Rod 100x10mm, manganese zinc magnetic permeability 1K
- 2x hold down blocks
- 32 AWG wire 0.2032mm; Windings: 20/30/80
- Resistor 10k-40k ohms
- Variable cap: 0 to 400 pF or 0 to 500 pF
- Variable cap mount
- Brass washers
- Stainless screws
- Crystal earphone (piezoelectric)
- 1N34a diode3D

Print Ferrite bar holders https://www.thingiverse.com/thing:652...
3D Printed Mica Variable Capacitor Mount https://www.thingiverse.com/thing:652...

TOOLS
- Screw driver
- Knife
- Sandpaper (small piece, fine)
- Hand drill for screw holes
- (Optional) Soldering iron, solder

ANTENNA & GROUND
- Antenna are just a long piece of wire (I use scrap wire from old transformers). 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:
- 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.

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 and put lead outside when not in use.
- Never ground to electrical items, plugs or similar.
- If you don't know, don't do it.

OSCILLOSCOPE USED:
$25 Color Oscilloscope! The Incredible DSO138 (4K)    • $25 Color Oscilloscope! The Incredibl...  
Using The $25 DSO138 Oscilloscope - Joule Thief & Freq Generator (4K)    • Using The $25 DSO138 Oscilloscope - J...  

INTERESTING LINKS
- Knob For Variable Capacitor & Similar (D & Round Shaft Versions) https://www.thingiverse.com/thing:633...
- Knobs for capacitor tuned crystal radio. https://www.thingiverse.com/thing:619...
- Air Core: http://www.circuits.dk/single-layer-a...

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CHAPTERS
00:00 Intro
01:26 The design
03:21 New coil
04:20 Old vs New
05:09 Origin story
05:37 Parts dimensions
05:49 Set up finished & compare old & new
07:26 On the scope & conclusion