What is a Crystal Earphone
Crystal earphone also called Ceramic earphone/earpiece is the soul of crystal radios. They are one of a few types of earphones that are sensitive enough to produce sound from a Crystal Radio. Regular HIFI headphones will not work with a crystal radio. The Crystal radio runs without battery. It runs purely by the radio wave captured by the antenna and convert that into tiny electric current (< 1uA) that is yet sufficient to produce sound through the highly sensitive crystal earphone.
Wikipedia definition of Crystal Earphone
The original crystal earphone (shown above) cannot be purchased in Hong Kong since 10 years ago. You can still buy it from UK through the online shopping sites, e.g. Amazon, or e-Bay, the cost is not too expansive, but the shipping cost alone will kill this idea. The total cost to buy one oversea may exceed HKD 120, for something valued around HKD 30 max.
SPECIFICATIONS of a Crystal Earphone:
- Impedance: 20K ohm
- Capacity: 15,000 pf
- Sensitivity: 57 dB at 1KHz
- Frequency Range: 200 ~ 8000 Hz
However, with some readily available components you can buy from HK Shamshuipo, Ip Liu Street, you make one yourselves at HKD 10 following the instructions here.
Remember to leave me some comments (at the comment box) at the bottom of this if you have read this blog.
Parts you'll need.
- Piezo-Electric Buzzer (HK$ 7).
- Cut out 20mm of the tip from a used plastic ball pen (HK$0).
- A 2.5mm head Jack & wire from a used/broken headphone (HK$0. If you don't have that, you can also buy some spare wires and a 2.5mm mono headphone Jack (the one in black below) (HK$ 3).
- A rubber earbud cousin from existing earbuds (top right on the picture below).
How to make it.
Watch this video or follow the instructions below:
- Using a screwdriver and brutal force, remove the lid of the buzzer
- Find the Piezo-electric plate (the circle plate in gold and white and the circular circuit board with some other electronic components.
- Remove all other electronic components using a solder iron and a clamp; as these components will reduce the impedance and also lower the audio output of the crystal earphone we are going to make.This is how it looks after all the unnecessary electronic components have been removed from the circuit board.
- Using a solder iron, connect two wires from the earphone jack directly onto the two points on the plate. One wire touching the circumference (gold), a second wire touching the inner plate (white). There may be a third white plate which is smaller, connect the wire from there to the other bigger white plate to get maximum audio output. Polarity is not important. So, you can choose which wire to go where.
- If you are using a separate earphone jack, connect the other end of the wire to an 2.5mm mono headphone jack.
- Before you close the lid, test the connections it by shorting the terminals of the jack together using a screw driver or touching the tip and the base of the earphone jack with a multimeter set to Ohm-meter. You should hear some "click" sounds each time you shorten the terminals of the jack.
- Close the lid and secure it by applying glue. Do not use the strong instant super glue. Use some regular glue so can still open up the lid later for maintenance in case the wire inside is broken.
- Line up the hole from the tip of the ball pen to onto the hole in front of the buzzer. Stick them together using instant super glue. The function of the ball pen tip is to focus the sound to the ear.
- Attach the earbud cousin to the tip of the pen.
- Connect it to your Crystal Radio to test.