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Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Back when I was younger I got a crystal radio set kit which included a 1N34A Ge diode as far as I can remember, I don't have it anymore.
I wanted to build one again recently and all I had a was a Schottky diode with a Vf of around 0. I heard no sound from it through a piezo buzzer pressed against my ear and figured it was maybe because of the diode, so I looked around for the datasheet for the 1N34A to read its characteristics.
It lists Vf as 1V at 5mA. It is my understanding that Ge diodes were the choice for crystal sets because their low Vf does not load the tuned circuit. Perhaps I am misunderstanding the datasheet? Thank you. The trick lies in the combination. With a lower If, Vf is also lower. Since a crystal earphone draws very little current, Vf is also lower.
As you can see from this diagram from here,. The 1N34A will conduct 0. You need the crystal earphone to avoid loading the tuning circuit and to keep the Vf low enough for the diode to demodulate the signal.
In reponse to comment: It may well be the diode that is causing problems. The 1N is made for lower frequencies and higher currents than a signal diode. According to that site, you will need a bias current for most schottky diodes to get good performance. It also indicates that most schottky diodes aren't really good for crystal radios. From my own experience in building crystal radios, you would be well advised to build the first one using exactly the parts recommended and exactly the schematic of someone who has built one successfully.
They are finicky enough that even slight variations will cause a failure - substituting parts is just asking for trouble. Another thing to keep in mind is that they don't have fanstatic range.
My biggest problem when building one as a kid was that we lived so far away from any transmitter that you pretty much had to get everything perfect and have a good antenna in order to pickup anything at all. Being a kid, of course, meant that I never got things really good enough. Please, please, please! Use a proper crystal radio earphone! A piezo buzzer is meant to BUZZ at a particular frequency. How well it will do anything else is anybody's guess.
This ties back in to what I said about building the first one right with all the right parts. Once you have a working receiver, you can try using the piezo buzzer and see what it does and how it changes things - but that is an experiment for AFTER you have a working receiver. Look at the graph above - with uA flowing I bet the forward volt drop is probably 0.
Look at this one: -. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Ask Question. Asked 4 years, 2 months ago. Active 4 years, 2 months ago. Viewed 6k times. MichaelK MichaelK 6 6 silver badges 14 14 bronze badges. Also worth noting is that Vf is dependent on temperature. I guess it didn't help that I tested this radio outside in -5 degrees Celsius. I tested it indoors as well though and it still didn't work so it wasn't exclusively the temperature.
Active Oldest Votes. The typical load on a 1N34A diode in a crystal radio is a crystal earpiece with an impedance of 20kOhm. The 1N34A conducts much smaller currents with a lower Vf. As you can see from this diagram from here, The 1N34A will conduct 0. You need a low Vf because of the modulation scheme, not to avoid loading the tuning circuit. But at mA it looks like it conducts 0.
Would this still be a suitable replacement for a Ge diode? It is completely possible that another part of my circuit is to blame, I just want to rule out the diode. Try locating an actual crystal earpiece and give it a go. How did I miss that? If not, I hath been lead astray by this link and videos. Thanks again for your replies. A buzzer is made to work specifically at a particular frequency.
Whether and how well it works at other audio frequencies is anybody's guess. It may work well, it may not. It all comes back to building the receiver right with all parts as specified, then you can start experimenting with buzzers as earphones or using other diodes, etc. Get it working right, first, though.
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Datasheet 1N34A - Microsemi DIODE, GERMANIUM
1N34A Diode. Datasheet pdf. Equivalent
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1N34A - 1N34A Germanium Diode Datasheet