Ask Hackaday: Troll Physics Edition

[Martin] sent in two videos he found while cruising the tubes. The first video is a simple circuit with a resistor, three switches, and three LEDs. All the components are soldered together right in front of the camera. When a battery is connected, turning the first switch on makes the first LED light up. Turning the second switch on makes the second LED light up, and the same thing goes for the third switch and LED. Obviously we’re dealing with powers that are incomprehensible with even several cups of coffee.

The second video features the same resistor/switches/LEDs, this time in a parallel circuit. Turning on the first switch makes the first LED light up, and the second switch makes the second LED light up. Truly we are dealing with an expert in troll physics.

This is probably something really benign and uninteresting, but it sure is enough to wake up enough brain cells on a Monday morning. We’re not going to hypothesize, so check out the comments where we expect the correct answer to be.


223 thoughts on “Ask Hackaday: Troll Physics Edition

      1. Excellent illusion. Back in the day when I used to teach electronics, this would have been great to spring on DC students right after their introduction to series circuits, and then pulll back out after they had completed AC and Semiconductor Theory to explain the solution. Nice job hiding the circuitry – I correclty assumed that you had done that, and that the solution had to do with AC, filtering, and polarity sensing/recitification capabilities oof the LEDs, but that’s as far as I got. Had to Google the rest. I would never have come up with this design by myself. Very creative!

    1. I am pretty sure that visual effects are easier than microscopic components or modified leds. Notice how careful he is about the positioning of the leds before flipping switches. I see the plausibility of the modified leds, to respond to specific voltages or frequencies. Though, if the goal is to confuse the viewer with simple science gone wrong, tricky editing seems like the way to go.

  1. Maybe the wires have multiple layers to them to create separate paths for the power to flow through. But this still doesn’t make any sense to me. Interested in finding out how it works.

      1. So, can it be that the switches and LEDs always let current pass and the switch when ON just send a RF to the LEDs that have a normal LED that turns when it receives the radio signal and a wire with the same resistance as the LED when not receiving the signal?
        Sorry for my bad English

  2. Expected behavior:

    Series: all switches must be closed in order for current to go through the LED’s and either all 3 are on or all 3 are off…

    Parallel: if either of the switches are closed, there is a short and both the LED’s will be turned off, otherwise both LED’s are on, but with a slightly brighter value ( assuming resistance is unchanged ) since we have the same voltage across both.

    How does it work in the video? Adobe After Effects of course.

    Either that or I don’t know circuit theory.

  3. It would be conceivable to make led’s that only operate at a specific voltage because of some internal circuitry.

    It’s also possible that the 9v battery has some “extras” stuffed into it as well.

    1. That’s what I was thinking. Are there frequency sensitive LEDs? Maybe there’s three inductors in the battery, a battery and some circuitry. In the switches there could be an inductor and a capacitor. There might be other components, but the basic idea seems plausible.

  4. My first instinct is to say that they’re cheating somehow; basic electronics knowledge says that shouldn’t work. However, I am not one to lightly throw around accusations. Maybe it has something to do with the diode effect of the light-emitting diodes in the parallel video, but I am at a loss for the series video. In any case, they’re certainly fun videos, and good for getting the mind going on a Monday.

      1. @raidscsi Uh, calm down dude. The video is fun. We know he’s tricking us. Its just a magic trick.

        Do you think all Magicians are just trolls and we shouldn’t be linking their videos and giving them traffic too?

        He made a fun little puzzle for us to figure out, there’s no reason to avoid giving him channel views.

  5. @mythgarr, I was thinking that perhaps the switches were current limiting, rather than on/off, and that the LEDs can only operate at a specific level of current, but I can’t explain how they can all work at the same time.

    1. Hey i was thinking that too.

      Maybe the switches have some voltage varying effects, and the LEDs thus lighten up when their corresponding switches are turned on the allow their effects

  6. I’ve watched the videos for 2 switches & Leds in series, and 2 switches & Leds in parallel…

    The battery is not just a battery – it is a low voltage AC source…

    In parallel – the switches have been modified – they have an internal diode that is shorted out when the switch is off – each switch has a the diode a different way round – the Leds are not the same way round..

    In series for just 2 switches the switches when on short out a diode – the leds have a very small surface mount diode across the legs – some surface mount diodes are very small…

    3 switches – No Idea…

    1. I also noticed, on the first video, 2:05 when he touch the wire the first led barely lights up.

      This is the phenomena of wave signals. Probably there are some inductors and filters.

      My guess that the battery is just a secondary coil inside and there are a primary coil behind the table.

    2. Definitely on the right track, both battery and switches has been re-worked the battery is just a shell which contains an atiny and the switches has been re-worked with smt diodes inside of them.

  7. I am majoring in Robotics and Embedded Systems right now. I have done tons of circuits in the past. There is definitely some trickery here. These circuits would not work the way displayed.

    My guess is very good editing. The parallel circuit video had one LED not pointing at the camera, but it was just as bright as the LED that was pointing at the camera.

  8. Line Frecuency diodes, Controlable Switches with Zener diodes. Thats it. Direct Current Source. Differente values for each pair of Switch-Line Frecuency Diode. Electrical/electronic Engineering Knowledge, in that video, sad, that it doesn’t have real life applications. :(

  9. There’s a very small jump in the first video as he takes his hands off the screen and absolutely anything could happen before he splices the video back together, including substituting a different circuit that looks the same from above.

    The second video isn’t as noticeable and he keeps his hands on the screen, but it could still be a substituted circuit, wired in the way you’d expect to achieve those results.

  10. for the first circuit –
    1) let the battery realy be a small ac supply.
    2) put a diode inside each switch in series with the contacts, miss-label the on/off sides, have the diodes be opposite in polarity orientation for the switches.
    3) have the LEDs have opposite polarities so each lights on alternatre AC half cycles.
    4) an open [realy shorted] switch prevents it’s LED from getting enough voltage to light due to it ‘shorting’ that half cycle of AC. Close [realy open] the switch, and the AC is alavible on that half cycle to light the LED.

    1. I haven’t thought it through yet, but I was leaning in this direction as well, but reverse labelling the switches makes it all sound a lot better. The LED direction and zeners helps pull it all together

  11. On the one hand, it’s sort of irrelevant _how_ it’s done, since we all know that’s not how circuits usually work. So it’s pretty obvious he’s doing some sort of trickery.

    That said, which trickery sounds easier to execute: Careful, difficult modification of components so that the end result looks completely ordinary, or an hour or so with Adobe After Effects?

  12. I vote for custom trick wire; logic dictates that the 3 LED’s are on 3 different circuits (for the series video). Why couldn’t you spin 3 (or more) threads of motor coil wire together (which is coated in insulator) with patches of exposed wire to make the circuit? Note that the wire used is silver, and quite thick (like solid core).

    I vote custom made wire containing 3 insulated cores. Very clever stuff. Bravo.

    1. Possible, but I think it’s unlikely since you see the soldering being done – that would be some incredibly deft soldering if he’s actually connecting three separate wire and avoiding shorting them.

    2. Ok, go slap together 3 TINY conductors like you say, then try to solder each one individually one handed while they wag around on a table and see if you can get a connection as easily as he did on all 3.


  13. Line Frecuency diodes, Controlable Switches with Zener diodes. Thats it. Direct Current Source. Differente values for each pair of Switch-Line Frecuency Diode. Electrical/electronic Engineering Knowledge, in that video, sad, that it doesn’t have real life applications. :(

  14. I suspect each LED secretly has a band pass filter soldered underneath it. They switches are changing the frequency of the circuit causing the correct LED to light up. It still baffles me that all three can light up, unless each switch can make a narrow band of spectrum corresponding to each LED.

      1. It does…. he briefly shorts the switches when he holds down the (assumed) positive and the LEDs… BUT only the first and the last LED glow not the middle one… if this (standard/typical) series circuit it would light the first two up brighter than the last one… UNLESS the middle one has a higher resistance (or operating voltage)…
        Anyways enough of my two sense!

  15. Notice how the movement of the circuit and the movement of the glow from the lights are slightly off… Just some post-processing here. (He’s doing this on green paper… Coincidence? I think not!)

    1. “Green Screening” and “Blue Screening” haven’t been called that in industry for at least the past 4 or 5 years. The current term is “chroma keying”, because you don’t need either a blue or a green background, you just need a solid contrasting color.

      That being said, chroma-keying is not easy to do well, especially when you have small finicky things in the scene.

  16. If someone goes out of his way to show you that there is nothing going on then you can probably be sure that something going on.

    The most likely answer would be that the LEDs never turn on and it is all post-processing.

  17. Answers:

    1st video (series led’s), its a clever video editing trick. He tracks the lets using motion tracking software (not too complex). You can tell at around 2:35 mark. The green background makes it easy to mask the hands when he passes over the leds.

    2nd Video same trick as the 1st.

  18. What if the battery is actually giving out an AC supply? The parallel circuit would just require a diode in the switch to ground out the unwanted polarity.

    AC might also help the series find a solution too, at work so I don’t have time to diagram it all out.

  19. The clue is in the fact there are only two LED’s
    The switches are coils (possibly capacitor tuned too), diode rectifier and switch combos.
    One LED’s and one of the “switches” are backwards :D
    The 9volt does nothing :D
    Well that’s the way I would do it :D

    Either that or frame by frame video editing as it is clearly not fixed to the table which was my first idea (through holes in the table, unlikely)

    1. I did consider button cells in the switches. But then how to power both at the same time. I suppose an SMD micro could be in there too with a tiny tactile switch but it would be pushing for space? Maybe switches have diodes and the 9volt has been cleaned out and replaced with an ac supply.

      Iv got it here is a microprocessor in the 9volt with a small powersupply (a car alarm remote battery or somethig) and mearly resistors in the switches. They are read analog and then the pins are set to output one in one polarity the other in the other polarity. And are using PWM 50% of te time in either direction (:
      That would be the easyest way I can think of (:

    2. Video Editing is definitely a reasonable way of doing it – modern software does that very easily (tracking the motion of things onscreen, etc).

      It would be much more interesting if he does it without video editing though!

  20. my guess is either adobe after affects, or the switches have tiny batteries in them, and the leds are wired so that the current must flow one direction to light one up, and the other direction to light the other. This would only work for the series one, though.

    1. Having a consistent brightness could also be consistent with the “AC battery” theory as well: Assuming a reasonably wide-band source with a reasonably good amplifier in the battery shell, notch filters in the LEDs and corresponding switches (with reasonably separated center frequencies). Switching one band on or off shouldn’t have too much effect on LEDs set to the other bands.

  21. Wireless switches and wireless LED’s?
    Both circuit configurations apply voltages to all of the components (if they aren’t normal switches). Each switch creates a wireless link to one of the LED’s

  22. An old version of the series circuit has 2 light bulbs and 2 switches, all in series. Plug it into 120vac. Like this setup, each switch independently control one light bulb!

    The trick in that case is a hidden diode across each switch, and each bulb. Turning on one switch shorts its diode, supplying halfwave AC. The corresponding bulb whose diode is reverse-biased lights. The other switch and bulb have their diodes reversed, so they operate on the other half cycle.

    So, this trick circuit might have a DC-to-AC converter hidden in the battery (555 timer etc.) so it produces 9vac, not 9vdc. That would make it easy to control 2 LEDs with 2 switches (and hidden diodes). But I’m not sure how he’d add the 3rd switch and LED.

  23. I cannot believe this got into HaD. This circuit is probably legit, but the schematic in the background not. Author of the video is a troll and deletes comments. It was already confirmed that switches are modified and there’s some kind of AC generator within one of these, nothing hard for electronics engineer.

  24. The 2nd circuit is easy as previous commenters point out:
    The supply is AC (little microcontroller inside).
    The LED’s are opposite ways round. Each switch has an inline SMT diode, such that each switch “shorts” one LED.

    The first circuit is harder. Karls solution works for 2 LED’s, but not 3. Here is my proposed answer:

    The switches are decoys. All are always closed. Each LED has a SMT inductor and capacitor behind it (in parallel). Each one has a unique “tuned frequency”. The battery has a microcontroller in it, which produces an output which is the sum of signals of frequencies which activate each LED. The user has practised the timing of the video such that flicking the switches match up with the timing in the micro controller program.

    Obviously, it could all be computer trickery, but I have a feeling that’s not the case considering the studenty laughing the background as if he is demoing it to a live audience at the same time.

    1. I think you are right in most of your theory but instead of doing the camera trick, you can have capacitor-coil-whatever inside each switch configured to have a high impedance for the appropiate frequency when not bypassed so that the current on that frequency is low and the corresponding led does not emit.

  25. Only thing I can think of is that the switches are not switches but frequency generators, and that each LED has a tiny tuned filter circuit and are wired in reverse bias with an anti-parallel surface mount diode to keep the DC flowing. Not sure how all that would be so small as to not be shown on the video.

  26. Guys there is no Adobe effects!
    TIP from author:
    “Don’t look at my schematic! Consider, how I can control this LEDs ? Tip: do you know any AC current properties ?”
    He said as well that switches are modified a bit.
    Most of guys guess well about two LED’s. Quite easy.
    Author said that it took him 10 years to find how to switch 3rd LED.
    Have ask author to reveal us his secret. ;-)

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