Shedding A Bit Of Light On Some Logic

When it comes to logic technologies, we like to think we’ve seen them all here at Hackaday. But our community never ceases to surprise us with its variety and ingenuity, so it should be a surprise that [Dr Cockroach] has delivered one we’ve not seen before. Light logic doesn’t use the conventional active devices you’d expect such as transistors, tubes, or even relays. Instead, it uses LEDs and CdS cells to make rudimentary switches. So far there is a NAND, a NOR, and a set-reset latch that appears in the video below the break, and it is not inconceivable that much more complex devices could be crafted.

The CdS cell switch is not too far different in operation to a transistor, with the CdS cell forming half of a potential divider as a rough equivalent of a collector-emitter circuit, and the LED feeding its light to the cell and forming a rough equivalent of a base circuit. It would probably not form a very good analog of a transistor and it seems likely that is will not be the fastest of devices, but we applaud the ingenuity in coming up with it.

CdS cells are a component that seems almost to come from another era, redolent of childhood electronic kits from days of yore. It’s no surprise we don’t see them too often, though, they pop up in the occasional automatic sunglasses.

30 thoughts on “Shedding A Bit Of Light On Some Logic

  1. Hmm, with just the right mix of (rather higher) resistor values and appropriate value and location of some old style paper capacitors one could produce a series of logic gate permutations which acted just a wee bit outside deterministic logic to act probabilistically as if a quantum (qubit like) probability engine. IOW. Adding a bit of variable fun in safe entertainment product…
    ie. Exploiting variance in some diode’s forward biasing finesse in conjunction with AND logic delays where inputs selectively ‘adjusted’ by those old style capacitors specifically selected for some temperature dependencies. Another way to craft a weird type of random number generator if you like for mere fun as if some spookiness inherent when it was far from that, religious folk welcome ;-)
    On firm basis though diode logic has an interesting and practical application space and I’ve used it a few times for convenience as well as an exercise in exploring different approaches to controls especially with relays, when some claimed ‘cant do that without a micro’ – ie. Couple of diode gates and another relay vs a pic chip two by 2 weeks to program, ugh…

    1. I’ve programmed in Q# (Microsoft’s language for quantum computing) a little bit. I have to say, I would not consider probabilistic == qubit after what I learned. Quantum programming is far more about the actual orientation of the qubits and entanglement thereof and far less about the probabilistic nature, to the point that I’m not even sure Q# can change the collapse function.

      1. Hmm, okie thanks Jim McCracken quite understand. Perchance suggest reflect on response to my comment suggest take another peek at my post though concede I could have been bit too vague, I chose elements of language/vocabulary a bit more carefully than most would expect in that tenuous paradigm with use of brackets [most] appropriate though these are purely (semantic like) mostly english like commutative unlike QM.
        IOW the pattern of crossing our collective experience of eons of history seemingly deterministically based experience is rather different to QM’s base probabilistic foundations even in terms of abstractions of intent to use of engksh descriptions to delineate the gulf ;-)

    1. I use them all the time in musical/synth applications. Since they got ROHSed, and I moved to Germany, I think I probably have half of the EU’s supply here, courtesy of US surplus stores.

      The slow response both musically useful, and it also serves as a built-in filter if you’re PWMing it. I don’t know what I will replace them with when I run out.

  2. I´m wondering if I could use something similar for an hardware, visual, analog version of a Conway Game of Life.
    With laser-cut acrylic cells forming light-pipes for the neighbor cells, embedded LED in the center of the cell, CdS sensor, surface-mounted switch to kickstart a cell and op-amp comparators.

    These are the rules of GoL::
    Any live cell with fewer than two live neighbors dies, as if by underpopulation.
    Any live cell with two or three live neighbors lives on to the next generation.
    Any live cell with more than three live neighbors dies, as if by overpopulation.
    Any dead cell with exactly three live neighbors becomes a live cell, as if by reproduction.

    (not enough or too much light on the on the sensor turns off the LED, the right amount of light turns on the LED)
    The latency of the CdS might induce spurious effects, though …

    Any thoughts ?

  3. As far as I know CdS cells are not allowed in Europe. Same for Nicad cells. Cadmium can be carcinogenic, and tests in mice suggest the body confuses it with estrogen. Can someone from Europe please confirm the availability or not of CdS cells in Europe. The US tends to follow European pollution guidelines, albeit a decade or so later, so CdS may become unobtainable in the US too some time.

    1. They are certainly obtainable from Farnell at least, however it’s most certainly a different story if you’re trying to get a commercial device RoHS-certified. I was always curious why electronics suppliers had such a wide selection of photo-transistors, but so few LDRs – now I know…

  4. I really like when hackaday features this “back to the basics” kind of post. I am an Arduino guy, but sometimes even I get tired of “Arduino-like, Arduino-based, Arduino-ed” kind of projects. Cheers to the writers and editors of this blog!

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