Got absolutely sniped today
I was asked so how does this thing work?
From papanek.org via smjny.com
This is the ‘Tin Can Radio’ by Victor Papanek and George Seeger. There appears to exist exactly three images of this thing online. It’s a radio reciever from 1962 intended for ’third world’ countries. From texts referencing it, it’s generally explained that the radio is not selective (it can not be tuned), that it is somehow powered by burning candle wax or cow dung in the tin can, it features a transitor (?), a 9 cent bill of materials and is possible to assemble by hand.
Okay, but how does it work? I don’t know the first thing about RF circuitry but in short:
It’s a selector-less crystal radio utilizing a tunnel diode and a loop antenna, powered by a thermopile.
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The few online resources describing it are not particularly clear on how it actually works. A 2021 paper by Marotta et al. references a 1971 book by Victor Papanek himself, which luckily is available on the internet archive. Page 162 begins a more comprehensive story:
(Click to maximize.)
From this, we get a bit more of an idea:
The transistor element in the radio is a tunnel diode. This makes sense, as diodes are an essential element to crystal radios. Why exactly a tunnel diode, I’m not entirely sure, I’m not familiar with tech like this. From the wikipedia page, I can imagine the tunnelling itself, its low capacitance and what appears to be a low forward voltage may all be part of it…
Heat from the tin-can is converted to electricity using a thermocouple. This explainns the peculiar wire wreath at the top of the can: For the thermocouple to work, it needs a hot and a cold end. Whether the thermocouples are wired in series or parallel, I’m not entirely sure. Pure guesswork, but if we assume the thermocouples to be a type J (iron/constantan) with a sensitivity of 50 μV/°C, and a generous temperature differential of 1000 °C, we might get an output voltage of 50mV. Returning to the tunnel diode wikipedia page, this curve trace the ’negative resistance’ area of such diodes appears to begin at around 50mV… How about that. So maybe we’re looking at a series parallel setup?
Continuing - the big coil of wire in the picture may give the impression of a tuning coil, but as the text explains, this design has no tuning. More likely, this seems to just be the antenna. The 1971 text describes it as a radial (?) antenna. Whether this is an old term or a wrong term, I’m pretty sure it’s just a loop antenna. Since there’s no tuning of the device, it makes sense the antenna itself has a length matched to the broadcast frequency (Though loop antennas appear to behave differently than mono/dipoles, so I’m not sure how much the cable length matters vs coil radius). As the text mentions, these radios were expected to be used in areas with only a single active radio transmission, so they could be pre-tuned, and interference would be a limited issue.
Following the wikipedia article for crystal radios, ones without a tuned circuit do indeed exist. Also, they need to be grounded, which the 1971 text explains is what the iron nail is for. Per the wikipedia article, this radio would be recieving an AM signal.
Also, in my search it appears a modernized version of the radio was built in 1985:
From BIO 26
In which we can see the earpiece, the diode, a modern thermocouple above the can and a loop antenna..? What is going on with the extra taps on that coil? As I said, I don’t know the first thing about RF.
