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. Not even looking at the schematic that was on the table I notice a few problems with this.
    In the first video looking at how he solder the series. All switches would need to be on for the leds to light and if one switch is off none of the leds would light. The switches are simple spst rocker switches. in one position the prongs are in a open postion (off) in the other postion the prongs are in a closed postion (on). And there is one thing as well. the leads he used to connect everything is solder. When he solder the strand to the pieces is stayed a strand. Solder melts when touched with the high concetrated heat of a iron. the solder should not act that way.
    the biggest clue to me in the first video is at one point the entire color correction/setting of the video changes. It dulls down slightly. Also looking at how the leds look when lit. there are discrepancies of how they should look. The light spread on the table. it only goes two way. One, if the led was lighting the table the color would not be pure red since the color of the table is green. Second, the type of leds he used are front emitting diodes. all the light comes out the front. and since the leds are facing up we should not see splash light on the table.

    In the second video there is the same problem with the solder. again there is the side splash and look at the led on the right. when it lights it has the same intensity as the left. yet its angle is slightly away from the camera. so there should be a slight noticeable difference.

    conclusion. Video is faked. After Effects was used to add the light of the leds. there are too many problems to suggest that this is real. as for the AC idea. its a 9v battery. there is no AC battery. Batteries are DC and to achieve AC you would need a converter.

    1. I agree. Video trickery. Also notice that in the parallel circuit the switches are off when the leds are off. they only light when the switches are turned on. Lighting an led in post editing No Problem.

    2. “when it lights it has the same intensity as the left. yet its angle is slightly away from the camera”

      You’ll notice that the light from both is ‘over-exposed’. It’s too bright for the camera. It’s ‘off scale’, exceeding the limits of the sensor.

      Imagine you have an 8-bit ADC that works on the range 0-5V.
      Feed it 5V, it says “255”.
      Feed it 20V, it says “255”.
      Feed it 20000V, ok, it will probably explode.

      You are claiming that “20V” is the same voltage as “5V”.

      1. This isn’t fun. It’s a waste of time. It’s too bad YT is full of time-wasters. Just didn’t expect it to see on HAD.

        Now this guy is getting a lot of attention. He won’t stop bugging us now.

      2. Agreed. Anyone who knows anything about electricity knows this is just plain wrong, and if this was really done without any post editing it deserves the title of “Hack” all the more.

        Those of you who think this is a waste of time – why are you bothering to read this far down in the comments and complain? Your time could have been better spent somewhere else…

  2. Watch the light sequence in his “serial” circuit as well. At first SW1(left) turns on L1(right) but after he turns them all on, SW1 turns on L3 (left) and SW3 turns on L1. There is clearly voodoo magic smoke at work here! I better run home and kill a chicken on my doorstep to stay in the clear.

  3. I’m guessing that there are filters inside the switches. And the LEDs are tuned to low, band and high.

    Flipping the switch removes the frequency filter.

    Now, i’m assuming that the battery is actually a frequency generator.

  4. Parallel:
    If you assume that the ‘battery’ is really an AC source and both “switches” each have a diode in line with them wouldn’t each switch being closed short half of the since wave, turning off only one of the diodes?

  5. The original author claims on YT that
    1) all wires and soldering points are real
    2) no video editing has been done

    Notice that in both the serial and in the parallel circuit the leds are shining even bright, no matter how many are lit. This smells multiplexing.

    I could get the parallel circuit to work with a microcontroller in a setup similar to charlieplexing (so, no AC needed).

    In order to determine which LED should be on, first the switches need to be read. This could be done by adding a capacitor in series inside the switches and using the microcontroller as a capacitance meter.

    Here is what I mean:

    Regarding the circuit with leds in series: still thinking on this one. Help me…

    1. I thought of the same exact thing as well.

      I think it’s video editing, though.

      In the 3 LED’s video (first one in this post) look at 2:43

      Or heck, play it from 2:35 and on, and watch the right LED, his fingers and the pen.

  6. Video editing all the way.

    First video at 2:43.
    Look at the LED on the right, and then his finger.

    If you think that his finger is underneath the LED, go back a little and watch.

    The LED shines over the finger AND the pen without any interference.

    Coincidence? I THINK NOT

      1. im also 1/2 figuring if there is trickery it is with the 9v and the middle switch…i.e. the middle switch contains a button cell wired DPDT and the switch itself just changes the polarity and maybe the battery is just for show (i.e. shorted out). or maybe the resistor is actually a DIAC that has stripes drawn on it?

  7. Nice Trick, i like it.
    Maybe its the half cycle trick mentioned earlyer for two of the leds. The third led is actually a led lookalike light bulb, which needs a certain voltage to light up, and the switch corresponding Switch is, when in off position, a resistor that drops the voltage below the voltage needed by the light bulb. The leds are one with integrated voltage regulator.
    I know there are light bulbs that look like leds, but i hove no idea how to make it light only at a certain voltage.
    I hope its not a stupid idea, its way to late to be sure. No wait, its to early to be sure now.

    PS: It was mentioned that the third led to is controlled by ac trickery, i know, but would it work too?

    1. I think you’re on to something with the zener diodes!

      …I tried assuming the 3-LED series circuit started with the 2-LED circuit, and added a switch with a parallel reverse-biased zener, and a forward-biased LED in series with a reverse-biased zener both in parallel with either a resist or a forward-biased diode…

      But then switch 3 only functions when the forward-biased diode’s switch is closed. (allowing clockwise current flow)

      I even counted the different switch states – Damn, he showed all of them.

  8. Practical Electronics published a number of puzzles similar to this before LED’s, when Adobe was only mud; think diodes, zeners, and a “battery” that is rather more…

    The point here is to stretch your brain.

  9. The two LED circuits are simple diode tricks. Nothing special there other than an AC battery, these are years old.

    The three series LEDs is a little different but still easily doable without the need for video trickery. Just a battery that outputs other than what you would expect, and not just AC this time, and an extra component in each switch and across each LED.

  10. Hmm, I am thinking that battery is not real, can somebody with the actual parts put together the actual circuit and see how bright the lights would be with that given resistor?

    I just feel that those lights would be a lot brighter with 9~ volts running through them.

  11. The point is not whether video editing was used but whether it is possible to build a circuit that behaves like in the video.
    Of course the parts would be mislabled.

    the second video is clear but the first one remains a mystery.
    Of course one led could only work on positive and the other on negatife voltage, but what’s up with the third?
    The filters for different frequencies sound difficult to me, I think its not possibe to make the filters sharp enough and still get some energy through.

    Would it be possible to build a led that lights only when the average of the voltage over time is 0? This would be a capacitor and a transistor shorting the led maybe?

  12. Ok, y’all. Here’s one way to do that trick.

    Start from the LEDs. We know that on the 2-LED case, the idea is that you put a little surface-mount diode on the bottom of the LED, so that if you run AC through it, the positive half goes through the LED and the negative half goes through the diode (or vice versa). So, what else is there that’s zero-resistance to one kind of electricity, high-resitance to another, and available in tiny surface-mount packages? A capacitor. Stick a little capacitor on the bottom of your LED, and it will light up with a DC bias but ignore high-frequency AC.

    Of course, that also kills the bias that you’re using to power the other LEDs, right? Well, it only kills DC bias in one direction; the other goes through the LED and lights it up. So, put an opposite direction diode on the bottom, and it doesn’t kill DC bias in either direction.

    So, now you’ve got an LED that passes DC and AC in both directions, and lights up on DC in only one direction. Take two of those, and that’s two of your LEDs.

    The third, then, needs to light up on AC and ignore DC in either direction. You do that by putting an inductor across it — zero resistance to DC, and an open circuit for high-frequency AC.

    And then the final trick? Well, obviously you can’t actually put DC in both directions on the circuit — so, where I said DC above, read “low-frequency AC” instead. These are tiny little capacitors and inductors; 60Hz is basically DC to them. So, the generator needs to generate low-frequency AC (low enough to be equivalent to DC for the small capacitors and inductor, fast enough that the LEDs look steady to the camera) and high-frequency AC superposed on each other.

    Et voila.

    (The corresponding switch hookup is left as an exercise for the reader, but should be trivial after the LEDs are explained.)

    1. I love this explanation, I think it is truly the most comprehensive so far!

      The only problem that I could see is with providing multiple current source types over one series circuit. How would you supply DC (what you call low-freq AC) superimposed on top of High Frequency AC without experiencing the occasional full OFF current when the phases of the two match up at 0. In the video, he left all three lights on (in the series circuit) for enough time to show that no such flickers occur.

      The only other problem I could see with this, is that how could such a tiny capacitor have a long enough charge-time to block out an entire phase of even 60Hz AC. Any cap large enough to do this would be visible on the bottom of the LED’s in the first frame of the video (assuming no video editing).

      Truly perplexing…

      1. Two things:

        1) It needn’t be 60Hz – chances are that if this is correct, the two frequencies are both much higher than line voltage.

        2) The frequency of where two frequencies ‘line up’ is called the resonance, and it occurs with a frequency of |f1 – f0| <– so two frequencies that are probably at least a factor of 10 apart will have a very high frequency resonance, and you wouldn't see a flicker.

        I hope this works – I want to build one of these now, just to see if I can confuse any of the EE professors around here. ^_^

      2. I agree now, it doesn’t have to be 60Hz at all, but no matter the frequency, placing single direction DC on top of AC on a single wire does not work. You will only get the sum of the two waves, be it 200kHz at 1db or 5Hz at 1db, their average together is the only thing traveling down that line.

        The resonance concept sounds very interesting, I haven’t gotten to electronics that advanced yet, hope to learn more about it on my own!

        One response I think looks interesting is the one posted with this link It seems to involve a similar sort of frequency trickery.

      3. Well, of course you have a point where the voltage from the higher-frequency oscillation and the voltage from the lower-frequency oscillation cancel out and you get zero for an instant. Depending on the frequencies, that happens thousands or millions of times a second.

        And you know what an LED that’s flickering thousands of times a second looks like on camera or in person? An LED that’s constantly lit at a slightly dimmer brightness.

  13. This should work if all the components can be hidden somehow inside the LEDs.

    The battery is AC + DC bias, and the two switches have diodes as before, but the new switch selects AC/DC couple using a capacitor. Two LEDs have very small diodes paired in reverse bias, with extra inductors wired in parallel. The third LED has a bypass capacitor. The inductors allow DC current path, while the capacitors allow AC path.

    Here’s a quick sketch

    1. Assuming this is really how it’s done, reactance of capacitor goes as 1/2pfc, and inductors as 2pfL, you need fairly high frequency for the components to be small enough to fit into the LEDs.

      The highest SMD inductor I could find that fits in the LED is 33uH, so you need the ac to be in the MHz range for 1kohms reactance needed to block the AC.

      At that frequency it’s easy to get low conductance path through skin. Notice when the LED lit up at 2:03 one of his hands touched wire to the battery, and the other to the wire going to the LEDs, by passing the switches.

  14. Obviously that’s an edited video. That LED doesn’t give out that kind of light when pointed at the camera without the camera flaring, or is getting over-saturated, I tried it. And yes, the LED’s accidentally lit at 2:04 on the video.

  15. lol i was reading about halfway down and so far noone can get it. the parallel circuit i dont care about cuz u we prolly ALL know andor said how that is done, but the series… i know how to do that and i have done the same.

    normal battery BUT WITH ONE LEAD CONNECTED TO THE CASE! so the case is either – OR +
    3 different LEDs, specificly the CAPACITENCE of the reverse junction all different and IN ORDER of value. this is why one of em always dimly lit and they different viewing angles,,, completely diff LEDs but similar appearence and ect
    three LIGHTED switches with LEDs inside (very important that the switch SHORTS the LED). non-moded,,, from car parts store
    a MEDIUM voltage rf high freq. supply UNDER THE TABLE AND ONLY UNDER PART OF IT. very important
    metal shield under table to shield part of circuit (switches) and ground battery case. the fr tx antenna is far away from battery and near LEDs. insulating AC RF + table + circuit = capacitor = CURRENT = LED light.


    i know, im not good at explaining things, maybe someone will explain better. PS YOU CAN GET RF BURNS FROM THIS EXPIEREMENT!!!!!!!! SCHOCK/PACEMAKER HAZARD!!!!!!!!!!!!!!!!!!! DONT TRY AT HOME! (AS WITH ANY CB RADIO :P NOT!)

    1. … PS the LED thats always on is maybe an RF LED; the others dont work until forced to, or maybe its the other way around… its all confusing with RF, im not an RF kinda guy. PPS he’s probably ruinning his leds in short order(reverse bias breakdown), but cool expierement either way, and yes the RF current going through an LED and then into an antenna (nowhere to go but air) can and does burn you and your LED. youve been warned!!!

      1. … in the still photo on this article;

        the middle LED is a darker plastic to my eyes;

        …confirming that maybe they all have different (and calculated and carefully chosen) reverse breakdown capacitences!

  16. I would think if anything maybe he’s using some of the circuit elements of the astable multivibrators that are hardwired into blinking LED’s. I do not think it’s video editing, and that “glow” is probably just the ambient light reflecting/refracting through the LED back toward the camera (I’ve seen this in real life too, it’s not a big deal). But I’m not entirely sure exactly how the astable multivibrators are wired in blinking LED’s, nor what would happen if you were to put 3 of them in series. But I’d be interested to find out :)

  17. Wow, too many comments here to read them all … Anyone already suggested that the LEDs might be specified to light up at different currents – and the switches simply switch from high-Ohms to low?

    1. dammit! i have a feeling you sound more right then i do. why didnt i think of that? it is simpler then my theory… but requires LEDs with INTERNAL resistors in PARALLEL. my theory uses off the shelf parts. your theory is eaiser and simpler to pull off,,, and uses no “RF-magic” and causes no burns! lol

  18. About 27 years ! ago our physiks-teacher showed us a serial circuit like the first one, only with 2 light-bulbs. He used a 9 V – batterie like in the video. We really needed a long time to find the answer.
    The 9 V batterie is a hoax! He put two 1,5 V batteries in it and a discreet circuit (astable multivibrator) to create an “AC”-current. Look at the small black strip at the end of the batterie in the video. The light bulbs had diods solderd in their sockets and the switches to. So, no video-manipulation, thats an old joke.
    The paralell works the same.

  19. This is Hack so it fine for it to be here.
    I go with the theory that the battery is AC and diodes are used in the switches and LEDs as well. The 3rd series LED is the biggest puzzle, perhaps a little diode pump might do it.

  20. My 3 possible ideas (assuming no video trickery):

    1. PWM or other frequency control to match a switch to a light, then have the sum of the frequencies turn on all lights. I find this unlikely, however because bandpass filters are not visible in the first frame of the first video: the bottom of the LED’s appear unaltered.

    2. Multi-Phase AC? Maybe individual phases of 3 or higher phase AC can be controlled separately so that at high frequency each light corresponding to a phase appears solidly on… (please correct me if I’m wrong here)

    3. Very, very, very well timed charging capacitors in the switches, oriented in different polarities, producing different amounts of current and polarity for each switch, which could correspond to LED’s which have cutoff voltages (possible above and below, such that they only illuminate when above 4v and below 5v), this would explain why it took him 10 years to figure out, as the 10 minutes spent at my workbench have only resulted in 3 rapidly flickering LED’s with only controlled by 2 switches.

    All that being said, it’s an incredible video with a great potential for provoking thought and ideas in viewers, I hope we solve this HaD!

    (I’m 15; any errors made are inexperience, not ignorance nikisweeting at gmail dot com)

    1. In the comments he says that the battery is the only unmodified element, so those are not ordinary LEDs.

      Also he didn’t say it took him 10 years to figure out the 3rd diode but he was thinking about adding the 3rd diode for a couple of years.

      1. I stand corrected.
        The battery being unmodified also makes this a much harder problem, unless he somehow managed to stuff a multivibrator into one of the switches. I have no doubt that the LED’s are unmodified, but I’m still baffled by how he’s managed to fit such complex circuitry inside the base of a 2mm or 3mm LED. Then again, maybe we are all over-thinking this and it’s in fact an incredibly simple solution.

  21. BUT STILL! A series circuit is a series circuit! even if 1 switch out of 3 is turned on, that would still be considered an open circuit. It doesn’t matter if you use PWM, or junctions, or whatever; a switch-in-series will only be considered CLOSED when all switches are closed. That’s an equivalent to a Logical AND Gate people.

    1. Unless there are invisible wires or other means of bypassing the switches, the switches are obviously tampered with, either remaining closed at all times or only allowing portions of current to pass through. This has been assumed from the beginning of the discussion. Electronics 101.

      1. As I said earlier. If the switches have resistors in them of diferent values. And the battery has been removed from the case and replaced with a smaller one and a microcontroler. The controller reads resistance from the terminals and so knows which one of the four combinations the switches are in. Then sends the appropriate signal out -/+,+/- or ac depending on one the other or both. And one LED is reversed :D
        Simple. That’s the way I would do it anyway without much fiddling around and without editing the video…
        Personaly I prefer the hack to the video edit approach myself, shows a bit of creativity (:

      2. Even if the battery was producing AC, and the LED’s were flipped, wouldn’t it turn on 1 LED on 1 switch, and 2 LED’s on the other? Or even if there’s a micro controller behind this, you cannot address individual LED that are in series (assuming there are no hidden wires). But srsly, did you see the guy solder “extra” connections or “hidden wires?”.

  22. What’s confusing us here, like in any good magic trick, is our *assumption* of what we’re seeing.

    These switches are likely not traditional switches at all, and the LEDs are likely not of the same properties.

    The “trick” is that we see a “simply switch” and a “simply LED” in a “simple circuit” and we immediately think “that’s not how circuits work!”

    Consider, instead, that they are not on/off switches and that each of those LEDs are not identical… now you should be able to understand what’s going on….

  23. The switches are definitely bogus. Any SPST rocker or toggle switch usually has the terminals on the _opposite_ side of the on position. That’s because when you flip the level, it pivots, and slides the contacts in the opposite direction.

    You can see in the videos that when the switch is in the off position, the “sliding under carriage” is actually closing the contacts.

    1. Definitely a smart observation. Its almost a given that the switches are modified in some way, but what mysterious magic is going on between the blue rocker and the metal prongs coming out the bottom.

      I can only think how disappointed this entire forum will be if its proven to be fake, on the other hand, the collective intelligence of everyone here should be able to find a valid solution.

  24. MY THEORY:
    if the “LED”s are standard then i STILL say RF-magic! (read my above post, WAY above)

    agtrier’s THEORY:
    but its still possible the “LED”s are actually something else, maybe in paralell with an INTERNAL resistor for current (mA) indication, and switches actually switchs+resistors

    PS: whats the resistence of brush on liquid drying conductive paint??? or a thin strand of conductive thread??? you could make very small and impossible to see resistors on the base of LED without it being seen, but thats too easy… i still think its more complex, “just for fun”.

    1. As Brooks and Tanvach both addressed earlier, all three LEDs have likely been modified. I imagine the reason that it’s visible on the first and third is that he had to solder a cap and a diode to each in order to pass the AC and DC, while he only needed an inductor on the center LED.

  25. VS = 9V
    VR1 = 3V

    SW1 parallel zener (Vsw1) = 1V
    SW2 parallel zener (Vsw2) = 2V
    SW3 parallel zener (Vsw3) = 3V

    LED1 Forward V (Vf1) = 1V
    LED2 Forward V (Vf2) = 2V
    LED3 Forward V (Vf3) = 3V

    SW1 Closed (Vf) = (VS-VR1)-(Vsw2+Vsw3)
    = 6V-5V = 1V = Vf1

    SW2 Closed (Vf) = (VS-VR1)-(Vsw1+Vsw3)
    = 6V-4V = 2V = Vf2

    SW3 Closed (Vf) = (VS-VR1)-(Vsw1+Vsw2)
    = 6V-3V = 3V = Vf3

    For the first approximation that the leds
    emit only at their Vf.

    1. This looks like a great solution, the breakdown voltage math certainly looks right, but do Zener diodes allow enough current to flow to the next diode above AND below their breakdown voltage? I’m not that well-versed on how they work, so correct me if I’m wrong, but wouldn’t this also be foiled by his ability to turn on 2 at the same time, but not the third…?

      I’m losing sleep over this problem…

      1. By experiment, I can light one of two leds in series with the one emitting having the highest forward voltage. True the other doesn’t light but still passes enough current for the other to emit.

  26. notice that the question mark he draws at the end is actually an AC symbol sideways? he draws it next to different components in both videos, maybe the AC generator is not in the battery? The tape on the battery could be a decoy. Its a common magician’s trick: divert your attention so you miss something.

  27. Each LED has a tiny inductor (L) and capacitor (C) across it. Each LC pair is tuned to a different frequency. The LC is a short at DC, but at f1, f2, f3, go high impedance and allow current through the corresponding LED. The switches have a resistor (R) and capacitor (C) in parallel across them. The C large enough to allow f1, f2, f3 through, and 3 different values for R. The battery has 3 oscillators, each of which is enabled depending on the DC current value – set by switches.

  28. OK, here is (the simplest?) solution for the parallel puzzle:
    – add diodes in the switch
    – put microcontroller in battery holder
    – reverse 2 output pins hi/lo alternating as in charlieplexing
    I simulated, it works
    The voltage drop of diodes (0.2-0.7V) is smaller than voltage drop of leds (2-4V), so a diode with a led in parallel will switch of the led.

  29. The “parallel” circuit seems to fairly well explained by others as simply using an ac source and diodes to bypass the switches when they are open.

    For the series I think I’ve come up with a solution that only requires a standard ac source, and only uses diodes:

    My solution has one led powered on the negative cycle of the supply. The other two have zener diodes places in front of them to block the current if the voltage is below some threshold. The zener diode on the first led has a higher breakdown voltage than the one on the second.
    Likewise the voltage drop across the first two bybass diodes (on the switches) are different.
    I think this setup could work if the appropriate values were selected for each of the diodes.
    Not sure if all of the components could be hidden inside the switches and LEDs though.

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