Electronic – only use a crystal earpiece for the crystal radio


I am building a crystal radio for my science fair project, and I am having some issues. For my earpiece, I am using a telephone receiver which may be the issue. For my ground, I am using a water spigot in my backyard which is completely metal. My diode is a germanium diode, which works. My coil is a bottle wrapped in enamel coated magnet wire that is between 18 and 22 gauge. Instead of sanding a section of the wire and using a wiper blade, I have taps with the enamel sanded off. I have tested all of my parts with a voltmeter, so the circuit isn't the issue. Any advice? What might I be doing wrong?

Best Answer

The problem involves the "impedance" concept, so it's usually not discussed in beginners' crystal radio projects.

Crystal radios need a high-resistance earphone. This is the type that produces tiny sounds when driven by several volts, while only drawing a few hundred microamps. For AC signals it appears to be a large resistor, 5K or higher.

A standard 8-ohm earphone appears as a much smaller resistor for AC; roughly eight ohms, and it expects a signal of hundreds of millivolts, while drawing a few tens of milliamps.

Yet the RF signal from a crystal radio must be high voltage at low current, so it can well exceed the 0.3V detector voltage. It's designed to produce few-volts DC output, not few-tenths. A standard earphone will just short out your receiver, and won't convert very much of the DC output into sound.

There of course is a simple cure. Connect a small audio transformer to your earphone. You want to step down the few-volts output by a factor of 20 to 50, converting 8ohms into many Kohms. Small transformers like this are available, called audio matching transformers, or audio output transformers for old-style transistor radios. Connect one of these to your tiny earphone, so the high-volt, high-ohms side connects to your crystal radio output.

The above will work, but unfortunately this transformer uses up some energy (as wire heating,) as does the coil in your earphone. A piezo-crystal earphone is far more efficient than a coil/magnet earphone.

So, for the same milliwatts of EM energy being received by your radio, a crystal earphone will sound distinctly louder than a coil earphone, even if exactly the right audio transformer is being used. The missing audio ends up as coil-heating.