A Radio For The Apocalypse

Penny Diode

There’s been a spate of apocalypse related articles over the last few weeks, but when I saw an AM radio made from a hand-wound coil and an oxidized British penny, I couldn’t help but be impressed. We’ve covered foxhole radios, stereotypical radios that are cobbled together from found parts during wartime.

This example uses a variable capacitor for tuning, but that’s technically optional. All that’s really needed is a coil and something to work as a diode. Surprisingly, copper oxide is a semiconductor, and the surface oxidation on a penny is enough to form a rudimentary diode. Though, note, not all pennies have that necessary coating of copper. If a penny has green oxide, it’s likely a candidate.

Need a quickly cobbled together AM radio? Have some wire and a penny? Yeah, watch the video below the break, and you’ll know how to make it happen. When the apocalypse comes, you’ll thank us.

54 thoughts on “A Radio For The Apocalypse

  1. Ok. Great. So when the apocalypse comes, I have to find a piece of wood, thumb tacks, a penny, some wire, a piece of weakiron, and lots of more wire to make the antenna.

    Then I have to find lots more wire, an ECC83 and two 6L6 radio tubes, two transformers, a bunch of capacitors and resistors, a few transistors, some more pennies to make diodes, a speaker, lots more wire, some potmeters, and a soldering iron.

    And then I’ll have my wartime trench radio!

      1. Someone flying a kite tethered with a wire instead of a string attached to a switch/telegraph key that is electrically grounded would be transmitting at the frequency determined by the length of the wire.
        QRP, of course.

        1. i dont think a wire has the Q factor to properly select a specific frequency, it is way to broadband. you’d need some capacitive/inductive filters….

          no problem….. some wire inductors and pencil-graphite-covered paper later….

      1. I have a 386 in my basement that literally came out of a barn 15 years ago and it still works (I played X-wing on it last year, got stuck on the mission where you have to disable a prisoner transport ship and then prevent everyone else from blowing it up, just like when I was a teen). So keep on looking, some old hardware still works. :)

        Maybe slightly older is actually easier to come by because they weren’t as ambitious with power supplies on the motherboard itself back then..

      2. Yes, you might have to get a bit creative. Some suitable S5/S7 sized heatsinks have turned up in the Princess Auto surplus section recently for example.

        However, I have personally noticed a sudden “die off” of pre-’98 hardware in the last 3 years, stuff I’m positive was working, suddenly isn’t. It’s not the electrolytic caps, most signs point to it being tantalum caps shorting out from growing whiskers or something. (If the hardware is run often enough it burns out the fine whiskers before they become problematic) There might be other similar issues with metals involved in connecting the silicon to the pins inside the chips. I have not had much time or ambition to get into it thoroughly.

        Post up any particular problems, and folks here might help.

    1. whoosh!

      You can’t imagine a receiver without an amplifier and loudspeaker? I bet you were listening to music off your phone thru a pair of air pods when you cobbled together your comment.

  2. Just what I was searching for, a way to listen to Rush Limbaugh from inside my radioactive cave. But how will I be able to post my selfies when I am dying of starvation and lack of toilet paper?

    1. There are quite a few things John saw that haven’t happened (seriously, go read the book, it’s wild). Talking about the coming apocalypse usually means the coming of the things that were revealed.

      1. Not sure if you’re confusing mono dynamic earphones with crystal earphones. Back in the day people made that mistake because they looked like the older crystal earphones in shape. Amplified transistor radios were highly unlikely to have used the high impedance crystal earphones and would have used the mono dynamic coil earphones as a switch-in for the 8 ohm speaker.

        However, those one hundred and eleventy in one electronics kits that were more popular back then would have had crystal earphones for the crystal radio project, so coming across them in connection with those was more likely around that time.

  3. Silicon diodes will not work. You need a germanium point contact diode, like a 1N34. Silicon’s have a threshold voltage of approximately 0.7 V or more, whilst the germanium diodes are approximately 0.3 V or less.

    Schottky’s are also usable. Their forward voltage is also around 0.3 .

    1. “Crystal” earphones are high impedance. I suspect that transistor radios of the Carter era did not use them.

      The iconic version of this radio uses an old razor blade instead of the penny. Note that modern U.S. pennies are mostly zinc.

      As for the poster worried about a power supply, when life hands you lemons, make batteries.

    2. Not strictly correct. Crystal radios rarely see even 300mv, and if they did and 1n34s didn’t conduct till then, there’d be nearly 100% even harmonic distortion. Actually, diodes start conducting very close to zero – but the current is an exponential function of the forward voltage (which is how log/exponential converters work in analog circuits). That forward drop you mention is “the knee of the curve” where the current gets “high”, and yes, point contact and germanium diodes generally hit usable current in the microvolts, where silicon takes more … It’s more a question of where does the current become enough for you to notice the effect on the audio transducer, and that does happen at a lower voltage in Ge or point contact diodes – far below your 300mv. This is something you can try at home with a big variable R and battery for a current source, and a high impedance voltmeter.

  4. For crystal radios, good substitutes for the 1n34a germanium diodes (which are hard to get) are the Bat15 and Bat63 Schottky diodes, which last i checked were made only in surface mount packages. As a bonus, they work up to the microwave range.

    1. There’s nothing magic about the 1N34. I’ve long considered it code for “germanium diode”, the germanium being the key point. It was a semiconductor diode available early to hobbyists, Rufus P. Turner was involved with it. And then there was incentive for hobby suppliers to carry it, though I’m not sure if you’d always get a 1N34 when ordering.

      About five years ago I was strioping some old radio boards and was surprised to find germanium diodes, I thought they’d mostly disappeared from consumer electronics earlier. But they were glass, and larger in suze, and had a lower voltage drop on the DMM. I think they were mostly from FM detectors, but maybe also the AM detectors.

      Certainly with a DMM set for diode testing, it’s easy to differentiate between silicon, germanium and schottky diodes, the readings are distinctive enough.

      But I thought the point of all this was to make do, create a detector when you don’t have a handy germanium diode.

  5. Homemade semiconductors are more difficult to get working but also more satisfying to actually get a signal out of them. The galena is supposed to be quite comparable in sensitivity to the germaniums, but sensitivity is highly dependent on the rock and sometimes the uglier specimens work better as rectifiers than the large crystal ones.

    The 1n270 germaniums were a little more sensitive than the 1n34as. The fabled Radio Shack loud “1n34 type” diodes from the 90s were probably 1n270s or a close cousin.

    Interestingly, most schottky diodes don’t fair well in crystal radios and it’s not easy to tell from the specification sheets. I found some if them had significant backwards conductance and some had a very pronounced knee at low voltage yielding no current for any but the strongest signals. Even low or zero bias radio detector schottky diodes.

    Fortunately, years ago, diodes could be bought from the major suppliers in onesies for pennies each and I sent out quite a few orders for a variety of them to find the best ones for crystal radios. Now single diodes are fairly expensive in eaches except a few part numbers (or taking a gamble and buying from China).

    The bat15 and moreso the bat63 are louder than the germaniums, but they have a much lower reverse breakdown voltage so if the signal is loud enough to turn the crystal headphone into a speaker, it will distort with the Schottkys.

    1. There was a book , “The Invention that Changed the Workd” by Robert Buderi, which was about the development of RADAR during WWII, and how it advanced electronics afterwards.

      A significant problem was they needed to push up the frequency limits. Transmitting was relatively easy, but receptionn was a problem.

      So they went back in time to the cat’s whisker, to make diode mixers that worked at UHF. They did start with something with a pin to find a sensitive point, but eventually they became fixed semiconductor diodes. Things like the 1N21 microwave diode.

      That led to the 1N34 after the war.

      Interestingly, the development of the transistor didn’t seem to use tgat as a foundatiin, instead going back further by those involved and starting another branch.

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