Science Buddies: "Ask an Expert"

hey,

I'm looking into doing the crystal radio but am curious how to tune it and if it can be set up to pick-up the AM transmitter

any help is greatly appreciated

Hi, I believe a crystal radio can only receive AM radio sources. There are a few ways to tune the radio to a particular frequency such as adjusting the length of the antenna wire, or adding a capacitor to the radio. Check out the first link for more information on this.

Here are some links.
http://www.midnightscience.com/howxtal.html
http://sci-toys.com/scitoys/scitoys/radio/radio.html

Hope this helps!

The traditional method of tuning a crystal radio was a sliding contact on the wound air coil that along with the capictor (called a "condenser" back then) formed the tuned "LC" (L is the symbol for "inductance" [search on term] and C is the term for "capacitance" {search on term}) circuit. The frequency of such a tuned circuit is the product (multiplication) of the inductance and capacitance, change either, and the frequency is changed. The sliding contact changed the amount of turns of the coil that was used in the circuit... this changed the inductance. Later, the sliding contactor was replaced with a fixed coil and used a moving core of soft iron called "ferrite".

Yet later radios used a variable capacitor was used. This was done by rotating a set of movable metal plates between a set of fixed metal plates. The rotation changed the amount of overlap between the plates, thus changing the capacitance between them.

A tuned LC circuit functions as a filter, either to block a frequency, or to let it pass, depending on the configuration of the circuit. Here is a useful link: http://www.hans-egebo.dk/Tutorial/tuned_circuits.htm

I built crystal radios when I was a kid. A wire antena on the roof of the house, a ground connection to the water faucet outside my window. Have fun with this project.

Good Luck!

thanks for all the info but also I want to know if the only way to tune the bottle radio is to clip the alligater clips to the different taps, if no how do i set up a capaciter to tune it... if im not understanding how it works, im sorry. can i tune the bottle radio to the frequency of the AM transmitter?

thanks in advance

Yes, you can tune the bottle.

Without specific dimensions listed, this is a guess, so just follow along. Get a selection of small capacitors in the range of 100pf to 1500pf and connect them one-at-a-time across the coil. You should hear a louder signal when in the approximately correct value. Fine tune by doing one of several things; add one of the smaller capacitors in parallel with the prior capacitor, if better then try the next larger one. Select the best combination by signal loudness. Second method is to use the biggest capacitor and move down the coil one tap at a time to tune for loudest result.

What you are doing is adjusting the resonant frequency (go lookup that term) of the coil and capacitor to match the station's frequency.

Robert Reavis