A diode is used to demodulate an amplitude modulated (AM) signal. It is a semiconductor material, which behaves as a rectifier.
The earliest diodes used germanium as the semiconductor material because a germanium junction can conduct even when the voltage falls as low as 0.2 V. This meant that they were very sensitive to low voltage signals of the type coming from a ferrite coil.
In contrast, silicon junctions will not conduct if the voltage is lower than 0.6 V. This means that silicon diodes will not work for crystal radios, because they are not sensitive enough.
Modern germanium diodes are point contact diodes consisting of a germanium wafer with a wire contact. Their current rating tends to be in the milliamp region with a low inverse voltage and high reverse voltage. Due to this limitation, they are utilised as small-signal diodes for detection.
Although there are very few modern electronic applications for germanium, as silicon is now a more widely accepted semiconductor, it is still required for crystal radio projects.
For crystal a radio, you need a good quality germanium diode, which will provide a nice strong signal.
The best diodes that provide the strongest and clearest signal were vintage ones that had been recycled out of old radios. The large 8 mm long, glass diode shown above, has two red bands, similar to a resistor colour code, signifying 22. It produced the best and strongest signal, and the crystal radio detected eight stations!
If your crystal radio is not working, then getting the wrong type of diode is the number one reason for this.
If you are able to receive just one station that is faint and broken up, then chances are that the diode is the wrong type. If you have a digital meter with the diode symbol, then you can measure its forward voltage characteristics. This will provide a good indication if the diode is silicon or germanium. Many low-cost meters have this capability. Have a look at the Germanium Diode Test article to see how to measure this.
Generally, the best crystal radio diodes tend to be germanium ones in large 8 mm long glass tubes like this one in the photograph.
The Mystery of the 1N60P...
I did a search on eBay for germanium diodes and found a number of sellers selling the 1N60P as a "Germanium Detector Diode". These are tiny ones in a 4 mm long glass body. This one had me puzzled for a while...
As you can see from the marking, this is definitely a 1N60P. You can see the N and the 6. It is in a 4 mm long glass body. What was puzzling was that the datasheet indicated the body to be 7.2 mm long, and states it is excellent for AM detection; however, it will not work with a crystal radio! So what is the deal here?
After further investigation, I managed to figure out that there are in fact two different types of diodes with the same name! I managed to download their datasheets and it appears that both components are different but with the same name!
There is a 1N 60 P manufactured by Semtech Electronics Ltd. This is a point-contact germanium diode in a 7.2 mm long glass body. Notice the spaces between the letters.
Then, there is also a 1N60P manufactured by LGE (Luguang) Corporation. This is a silicon junction small signal Schottky diode. This diode has a 3.8 mm long glass body.
The seller on eBay, for whatever reasons, lists the silicon diode as a "Germanium Detector Diode". However you can tell straight away, because if the glass body is around 4 mm long then it is not the correct diode. That is why your Crystal Radio does not work. Mystery Solved!
Ding Dong Diodes Don't Demodulate...
I managed to buy 10 germanium diodes for £1.30 including postage from a Chinese seller called Ding Dong, :-) obviously, not related to Leslie Philips the actor who says ding-dong as his catchphrase.
I tested the diodes and they appeared to be genuine diodes! Ding Dong! However they did not work with the crystal radio, furthermore, the diodes are not marked.
I also bought germanium diodes from other sellers because they were only a pound for ten. However, one UK seller sent ordinary 1N4148 silicon diodes! Hence, be very careful, do not simply take someone's word that the component is what he or she says it is.
One simple way to tell is, if you have a crystal radio set that is working, swap the diode and see if it still works. If they have sent you something dodgy and not up to spec, then you will hear only one faint signal that is broken up. The other way is the check the diode with a meter.
I tested my diodes more scientifically by comparing the characteristic curves, and found that many of these modern diodes are off spec compared to earlier designs. Comparing my OA80 and OA90 from 20 years ago, with the modern versions of the same model reveals that they must have made changes in the manufacturing process, because the modern versions do not perform as well.
Crystal Radio Not Working
You can get wrong diodes from local shops as well. Sometimes the seller thinks, hey I will just give him this, he will not know the difference. Then of course, you end up wasting hours trying to figure out what might be wrong, and if you are just starting out, you might be disheartened.
This component tends to look the same to non-technical persons, sometimes the stock is incorrect, and the shop assistant brings the wrong component from the bin. It happens all the time with resistors.
There is also some dodgy stock from China, where sellers will try to sell an ordinary silicon diode as a germanium one simply because they can fetch more money. Sometimes it can be a genuine mistake as well, where the seller does not know the difference.
If it looks like a IN4148 with a tiny 4 mm long body, then I treat it with suspicion.
Pete's Crystal Diode Test
Once I got the crystal radios working, I decided to see if there were any diodes in my collection that might work. I have a huge assortment of Zener, Schottky, LEDs in my recycle box. I even tried the junctions of germanium transistors, however, none of the ones I had worked.
The only ones that worked were these 8 mm long glass tube diodes. These vintage germanium diodes provided the best signal. In fact, as soon as I held one terminal between my fingers, the crystal earpiece was buzzing with a loud station!
This Article Continues...Making a Crystal Radio
Making a Crystal Radio - The Little Whippersnapper's Parts List
Crystal Radio Baseboard 4 inch by 12 inch by 1/2 inch
No. 6 Brass Screw Cups & No. 6 Screws 1/2-inch
Marking Out the Baseboard and Installing the Screws
Making the coil
Crystal Radio Coil
Crystal Radio Coil Winding
Crystal Radio Coil Terminals
Connecting the Crystal Radio Coil
Crystal Radio Diode
Germanium Diode Test
Crystal Radio Earphone
Crystal Radio Circuit
Crystal Radio Specification
Potentiometer Mounting Bracket
RFC - Radio Frequency Choke
Project Power Supply Wiring
10k Potentiometer Wiring
BC549C Lead Extensions for Future Amplifier Projects
Wiring the Speaker to the LT700 Transformer
The Little Whippersnapper's Radio