DIY crystal radio that does not require power

bqgup

DIY crystal radio that does not require power [Copy link]

 

 

A crystal radio is a passive radio without an amplification circuit, which is composed of an antenna, a ground wire , a basic tuning circuit and a crystal as a detector . It is the simplest radio receiving device and is mainly used for receiving medium-wave public radio broadcasts.

People are used to calling those radios that do not use power and have only one semiconductor component in the circuit "crystal radios". The name comes from the fact that crystal was used as the detector in the beginning.

Let’s take a look at the actual picture:

Simple circuit diagram of crystal radio:

Upload the video effect of the demonstration: Crystal radio video demonstration effect

Analysis of the principle of crystal radio:

1. AM signal. We all know that the frequency range of voice signals is 20Hz-20KHz, and the frequency range of sound signals is 300Hz-3.4KHz. These signals cannot be transmitted over long distances because of their low frequencies. We need to graft these low-frequency signals onto high-frequency signals so that our voices and speech can be transmitted far away. This grafting process is called modulation, the high-frequency signal is called the carrier wave, and the low-frequency signal is called the modulating wave. The waveform after modulation is as follows:

Carrier wave 1KHz Modulation wave 100Hz

Carrier wave 100KHz Modulation wave 1000Hz

This type of signal broadcast is still used in our country. Here is a schematic diagram for easier understanding:

2. Resonant circuit. The LC resonant circuit has the function of frequency selection. It will allow signals that meet the frequency requirements to pass according to the values of inductance and capacitance, and other signals will be filtered out.

3. Demodulation. The demodulation here uses diode detection. In the past, the crystal of the crystal radio was equivalent to the detection diode, and the unidirectional conductivity of the diode was used for detection. The detection diode and capacitor form a modulator to demodulate the low-frequency signal.

4. Play. This signal can be played through a high impedance speaker. You must use a high impedance speaker, 350 ohms. The higher the impedance, the better. You can listen to it after installation. Because it does not require external energy, the sound is a little small, but it does not consume electricity and can be played all the time.

lcofjp

Awesome! But there are few radio stations in the AM band now, and the interference is too serious

chunyang

AM radio stations are gone now.

When I was 10 years old, I tried to make a crystal radio, but I failed after a long time because I used a 2CP rectifier tube, and the junction capacitance was too large to detect. Then I got a few 2AP9 and 3AG1 tubes, and made a rifle regenerative single-tube radio, and I became an electronics enthusiast from then on...

maychang

"A crystal radio that does not require power"

Seeing the words “crystal radio”, it felt like a lifetime ago.

maychang

At that time, detection was indeed done with ore. The most common "ore" was "natural copper" bought from a Chinese medicine shop for 10 cents, and 10 cents could buy a few small pieces.

File a thin copper wire to a point and gently touch it to the surface of the ore to form a unidirectional conductive element for detection. Sometimes the sound is small at one point, so gently move the copper wire and touch another point, and the sound may become louder.

maychang

From the first picture, it can be seen that the OP used a flat magnetic bar coil. This coil, especially when winding with multi-strand wire (Ritz wire), requires some manual skill.

bigbat

It is not easy for the original poster to buy these components. Are there any AM radio stations now? Is the original poster in Shanghai or Beijing? There is definitely no signal for our crystal radio here.

bqgup

bigbat posted on 2019-12-31 17:58 It is not easy for the OP to buy these components. Are there any AM radio stations now? Is the OP in Shanghai or Beijing? We have crystal radios here...

Hello, senior, the AM station is still available. I am from Huangshi, Hubei.

bqgup

maychang posted on 2019-12-31 16:15 From the first post, it can be seen that the OP used a flat magnetic bar coil. This coil, especially when winding with multi-strand wire (Ritz wire), requires some skill.

I took this coil out of a radio and used it directly.

bqgup

lcofjp posted on 2019-12-31 12:30 Awesome! But there are no radio stations in the AM band now, and the interference is too serious

Yes, the electromagnetic environment has become complicated

gmchen

When I made the crystal radio, there was no magnetic rod yet. The coil was wound on a paper tube filled with wax paper. The variable capacitor was an air single-connected capacitor (this name may not be heard by many people now). The antenna was hung on the roof and the ground wire was connected to the water pipe.

nmg

gmchen posted on 2020-1-1 12:49 When I made the crystal radio, there was no magnetic bar, and the coil was wound on a paper tube filled with wax paper. The variable capacitor is an air single-connected (this name may be used by many people now...

A huge project

gmchen

nmg posted on 2020-1-2 09:31 A vast project

Haha, it was indeed a big project. In fact, it was not only the assembly and production, but also the search for materials was a big project at that time. I was still a middle school student at that time, and I never got tired of it. I didn't expect that my interest at that time would become my profession in the future.

maychang

At that time, the antenna was set up on two trees, and a pulley was added to prevent the trees from shaking and breaking the antenna. The next morning, I found that the antenna was gone (it was a multi-strand copper wire). It might have been stolen at night and sold for copper. I was sad for several days. Later, I only dared to set up the antenna under the eaves, and I didn't dare to set it up high.

btty038

There seems to be another channel that broadcasts CCTV's

bqgup

gmchen posted on 2020-1-1 12:49 When I made the crystal radio, there was no magnetic bar, and the coil was wound on a paper tube filled with wax paper. The variable capacitor is an air single-connected (this name may be used by many people now...

You are all seniors, I admire you.

gmchen

maychang posted on 2020-1-2 14:33 The antenna was mounted on two trees, and a pulley was added to prevent the trees from shaking and breaking the antenna. The next morning, the antenna was found to be missing (it was a multi-strand copper wire), but...

Haha. I was fine back then. I lived on the third floor. There were bungalows in front of the building. The eaves under the window were already a commanding height. I didn't have the experience of being stolen. It was just a little dangerous when setting it up.

symic

There is a feeling of uncertainty