Introduction: White Noise Radio
overview: We created a simple crystal radio inspired by the following website http://www.crystalradio.net/cub_scouts/index.shtm... Materials: piece of wood/cardboard
Materials:
4 in piece of one inch diameter PVC pipe
large paper clip
alligator clip
30-100 ft of antenna wire
27 feet of copper wire, coated
sandpaper/knife
drill 1 N34A signal diode
47000 Ω carbon resistor
ceramic earphone
electrical tape
Diagram
Test results - We attempted to use this radio both inside and outside, outside worked better; it allowed more white noise and almost voices to come through. The voices get louder as you silence the people around you and create a web of wire to nest yourself in. We would suggest having closer to fifty feet of antenna wire to get stations more consistently.
Physics explanation
Radio waves alter the electric and magnetic fields around the antenna, moving electrons between it and the ground. The coil (whose length is effectively adjusted by the movement of the paper clip) acts as a capacitor which charges and discharges at a resonant frequency corresponding to the frequency to which the radio is tuned, enhancing the desired signal and discarding the others. The diode converts the signal to an intermittent direct current at the frequency of the radio waves whose average intensity over short periods of time affects the movement of the speakers, thus decoding an amplitude-modulated signal.
8 Comments
8 years ago on Introduction
Hey, I always thought you had to have a capacitor parallel with the coil to make a resonant R-C circuit, didn't know it would work this way. Would a capacitor give you a better result?
Reply 8 years ago on Introduction
You are quite correct, In this case the capacitance will be the self capacitance of the coil, which is due to the coil wires lying alongside each other. Normally a coil designer tries to minimise self capacitance by careful design and winding of the coil, but in this case -- why not use it and save an extra component? Also the rest of the wiring may have enough self capacitance to be part of the L-C resonant circuit. The problem is that the exact layout of the coil and the wiring will have a big effect on the tuning, so different builders with different layouts may get quite different results.
You ask the question,"would a capacitor give a better result?". Well, you can try one if you build the circuit. Once you have got it working as per the original, then you could experiment with it. I would guess (complete guess!) at trying 100pF or less. You will need to move the coil wiper wire to a different point to bring the signal back.
Ps. It is an L-C resonant circuit not R-C. The symbol for an inductor (coil) is L. I suppose they couldn't use I for inductor because that is already used for current. Umm-m -- and C can't be used for current because it is used for capacitance!! Oh dear -- gets complicated!
Reply 8 years ago on Introduction
Very interesting, thanks for that.
I did know it should be L-C, that was just a mistake, my brain is old and tired.
I did do some crystal sets many years ago, but always with a capacitor, usually a variable one for tuning. I got an audible signal from the power of the transmission, which was something. What I was always fascinated by was to use a piece of coke as a diode, but I never managed it.....
8 years ago on Introduction
Well done. These radios can be difficult to get going so to anybody trying to make one - don't give up if you don't seem to get it working first time.
The earth connection is not mentioned or shown in the diagram. The bottom end of the coil , labelled "to alligator clip" should be connect to an "earth" such as a water pipe or metal radiator etc. It will improve the reception no end. (NB - don't use a mains earth. It is just amateurish! If you are going to be an electronic engineer then you need to learn respect for the mains early on.)
Also make sure you use a ceramic or crystal earpiece as shown here. Don't use a magnetic one which on the surface can look exactly the same as a ceramic one.
The main thing that worries me are the taped connections. The most successful versions of these radios that I have seen have been built on small bits of wood. Flat headed wood screws can be used to hold down and join the wires and components together firmly.
Reply 8 years ago on Introduction
I've just enlarged the picture of the connections to the earpiece jack socket and to me, it looks as though one of the wires has slipped out of place. My advice would be to go the whole hog with the earpoece connection and buy a matching jack socket. It will be much easier to make reliable connections to
8 years ago
Almost a 'breadboard' radio.
8 years ago
Google "oatmeal box radio".
8 years ago on Introduction
Great project, thank you for sharing this. I need to make this with my kids!