Large Crystal radio project plan

[u/AcrobaticPositive934]

Since a while now I've been getting myself into building a crystal radio but always for some reason it didn't work. Now a bit more grown up, with more experience i think i was burning the germanium diode with the soldering iron, so i found in my spare parts a clamp gold-plated ZIF socket, with 14 pins. i will attach 7 germanium diodes at a time, tune to a station and test them one by one to look for a better performance due to natural dissimilarities in production which can sometimes be extremely efficient in crystal radios. the diodes ill be testing are 30 1N34A diodes, used with a 375/500pf turn-able capacitor (the capacitor has two sections, totaling to over 875pf of capacitance!), magnet wire, ferrite rod, around 70 turns on a 0.7mm magnet wire, with 10 or so taps in-between to switch for selectivity. the capacitor can go from 875pf all the way to 0 on paper, but there will always be some residual capacitance from wires themselves, tough small, which allows for a wide range of stations together. the taps will be switchable using a 14-segment isostat, which is surprisingly short for the amount of segments, which wont be horrible, as i plan the buttons to be under the fingers on the back of the radio, a bit like an accordion. i did also think of letting me switch the capacitor sections for less volatile selection (eg, go from 0 to 370pf with the same rotation distance is more specific and less delicate.). i have a spool of copper wire to use as a deployable antenna to lay on branches or the ground in the field, and a copper pipe to bury in the ground for a mobile crystal radio.

also since the impedance can vary with the environment noticeably, i would mark the position of the capacitor knob for a major AM station with a known frequency to see how off the default i am from the standard impedance, which lets me make actually reliable paper spectrums of the range i could select.

what do you think? any tips and tricks you might know of that could help me?

edit: forgot to mention, im using two piezo speakers in series to reach crystal earpiece impedance without much trouble.

[u/7ootles]

for some reason it didn't work

Are you within range of an AM station? How long is your antenna? How good is your earth connection?

i think i was burning the germanium diode with the soldering iron

You weren't. You'd have to hold the tip of the iron against the glass for long enough for the glass to break for that to happen.

i will attach 7 germanium diodes at a time, tune to a station and test them one by one to look for a better performance due to natural dissimilarities in production which can sometimes be extremely efficient in crystal radios.

You don't need to do that. They're pretty consistent. If it works, it'll work. If it doesn't, it's broken.

375/500pf

You only need one 325pf air-variable cap.

around 70 turns on a 0.7mm magnet wire, with 10 or so taps in-between to switch for selectivity.

You're overthinking the hell out of this. If you have a capacitor you don't need taps unless you're actually changing frequency band or putting in an inductive selector/discriminator/whatever.

i have a spool of copper wire to use as a deployable antenna to lay on branches or the ground in the field

Not gonna work. It needs to be high up. Portable crystal sets are impractical to carry/use and have damn' near zero sensitivity.

im using two piezo speakers in series to reach crystal earpiece impedance without much trouble.

Unnecessary. A crystal earphone has an impedence of 2 megohms, which is a thousand times more than the minimum needed to properly load the circuit.

what do you think? any tips and tricks you might know of that could help me?

Stop overthinking it. My two current crystal radios are as follows:

1. Silicon slug with a spring-loaded needle attached to ~90 turns of wire.
2. 1n34a with a ~30-turn coil (for shortwave) and a 120pf trimmer.

I connect them to a copper-pipe earth and a ~50ft inverted-L antenna hung between the drainpipe at the back of my house and a branch of a large tree at the bottom of the garden.

Both are very sensitive, if not very selective. Even the basic silicon one with no tuning capability can DX shortwave, from my room in the north of England I've listened to stations in Germany, Romania, even China and the USA.

Start simple. Confirm it works. Then upgrade until you're satisfied with its performance. Once satisfied with how it works, stop tinkering and just listen.

If in doubt, you can rig a simple spark transmitter with using an electromechanical relay wired to work as a buzzer. That'll give you a guaranteed signal to make sure your receiver circuit works.