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Credit card crystal radio Answered

What do you think about the feasibility of a credit card sized crystal radio? (Fits inside a wallet) Some possible i though so far are... A simple pcb with a diode, and a capacitor and a coil wrapped(protected by duct tape) around it. The antenna has to be tied to a hole on a corner. Or it is coiled to the card, ready to be unwrapped. The ear piece can either be external... Or can be glued to the card itself, but you must put the card near you ear... Instead of a winded coil, maybe it can be part of the pcb design itself... and to tune it, you unhook a beady wire and place it in the holes placed in a interval. What do you think? Any more ideas? In fact. a competition can be made for this idea.



7 years ago


Here is one that looks pretty good. The coil is a spiral trace on a PCB. Antenna, ground and earphone connect using alligator clips.

73 Dave NM0S

The size of the coil necessary for a simple unpowered crystal radio would be too large to fit on a credit card. What I'd suggest is that you search Google for a simple transistor radio circuit. With modern components you may be able to squeeze something like that on a credit card sized board.

I dunno about this one, LV. Like a lotta kids, I built a xtal set w/ a germanium diode. The coil was wrapped around a toilet paper tube. But I don't think it had so many courses that it couldn't work on a credit card size form (especially if very fine wire were used.) I think this one might be worth a go... But if I recall correctly, the antenna length was pretty hefty...

The coil is only half the problem, the antenna has to be substantial too if I remember correctly. I had to stretch over 50 ft of wire across my back yard to get anything decent (unless you like local AM stations :-) .


Might be a problem, however you can make the antenna optional using this extremely simple design.

By the looks of the coil, i'm am quite confident that there is a way to compact the Coil/Antenna.

To keep the coil folded for storage in wallet, we can use fridge magnets or a simple binding string.

Also another useful idea, is that you can get rid of the capacitor itself and it may still work. This is because you can use the coil spacing as the dielectric itself. according to the plans i seen, the coil can still be tuned without a cap, by inserting a tap into the coil at various intervals.

So in essence, a credit card crystal radio would only need:
- A flat and foldable ribbon cable(as a coil/antenna), connected to a preferably thin pcb circuit.
- A crystal diode
- A resistor
- And a crystal radio headset
- A thin short wire for tap selection

- (Optional) a capacitor
- (Optional) an antenna and ground hole

That looks worth exploring. You think this might work without the variable capacitor, eh? Don't see why not, as normally a coil 'slider' is used for tuning. And those vari cap things are usually at least 3/8 in thick (I destroyed many small radios as a kid.)

Actually, you could make a variable capacitor from two thin sheets of PCB material. Last night I designed one that was just one plate on the 'credit card' and another plate on a rotating disk of PCB material.
The simplest would be a vari cap that had capacity proportional to its angle of rotation.
Just to be different, I wanted one that would when paired with a coil would have a frequency proportional to its angle of rotation (or a capacity with a square root proportional to its angle of rotation).

I got the idea from the radio in the book "Cache Lake Country" by John J. Rowlands. He slid squares of foil over each other along their diagonal axies.
The design is perfectly linear.

Oh, by the way, some commercially made PCBs have multiple layers of copper foils on each side. So we could print quite a coil on part of the 'credit card'.

Yep, I made one out of a book and some aluminum foil sheets once :-)

If you were really careful at laying out a circuit board you could print a multiple turn pancake coil on one or both sides of the card (just keep the electrons going clockwise on one side of the card and counterclockwise on the other side so their magnetic fields aid each other).
Tuning might be by coil taps or by sliding or rotating second card that formed a variable capacitor using a thin insulator (such as split mica).
Brass eyelets could form tip jacks for old style headphones.
Holes might have to be made in the card for mounting wire lead diodes such as the 1N34A.

Novel approach! Yes it can be done using prewound RF Chokes or Inductors. Possibly wound torroids would work here. For AM you would need roughly 33 microHenries and 220 microHenries. You would have to build it off the card. The easiest way may be by using a cc sized piece of either pefr board, poster board, a 3x5 card or thin coardboard using brass thumb tacks for solder points. Before assembly or gluing to the credit card carefully remove the magnetic strip 'cause this will interfere with your radio working. Use only a glue that is not metal based. Hot clue will work. Let me know how it goes, and if you need help I would be glad to share what little I know since I am new to crystal set building myself. Your idead gives me an idead. GOOD JOB AND GOOD LUCK!

Have you ever seen surface mounted Inductors? They can be as small as surface mounted resistors, diodes, capacitors or transistors.

They make PCB material that is as thin as a credit card- some of it is even thinner- take a good look at the IC chip embeded in a FED-EX/ KINKOS smart card for using their computers or copy machines.

After WWI a vetran made a pancake coil crystal set that was a part of a wallet.

In the early 1950's there was a chalange to make a tube radio that could be mailed in an envelope. It was done using a 5"x7" envelope, sheets of corigated cardboard, alligator clips, a pancake coil, and wire lead hearing aid tubes. It was about half an inch thick. Try the GOOGLE BOOKS archives and look for: 'radio in an envelope'.

In the late 1960's or early 1970's someone did an article on a crystal radio made from a greeting card.

I remember that it's possible to make one with flat bar ferrite rod antennas, and if you are good with surface mount devices, you can build a one transistor radio using SMD resistors, capacitors, and the diode is small enough to fit in a small enclosure. Looking for a variable tuning capacitor? Use a varactor diode and flat potentiometer a SMD type, use it with the amplifer circuit. If you're good with etch circuit boards then you can make a coil antenna using the loop antenna configuration. Problem is here are you going to find a credit card sized enclosure?

Any chance you could double-purpose the wire for the ear piece as the antenna?