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I think the best lesson that can be learned from this cheap USB disco light is the importance of supply rail decoupling in the vicinity of audio circuitry.
There's an audio amplitude detector circuit that triggers continually when used on anything other than a perfectly noise free USB power supply.
That said, it's a good demonstration of a simple electret microphone based audio detector with a minimalist single transistor amplifier.
The lens projects fairly good beams of light from the hard-driven (50mA) LEDs, so it might find use in other projects.
The light came from a UK pound-shop called One Below.
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A usb powered disco light that i found in a pound shop in the uk pound shops are the equivalent of a dollar store in america. So this thing it says usbc, but it actually has a standard, usb connector and a usb adapter on it. Let's get it open and we'll take a look at what it looks like so there's a little adapter, which i wonder if that's a standardized one that could be useful for other things, and here is the disco light. Let's plug it in and see what happens.

I'll. Warn you advance, it does say flashing, so just in case it is flashing, we shall go with a warning that is just a swallow, went for a warning that is hideous. Oh, that is annoying and it's supposedly sound active. It doesn't seem to be responding to impacts, but that is very, very annoying hey hold on i'm just going to turn the light off, so you can actually see this yeah.

It's actually pretty bright um, but that is super annoying yeah. That is annoying. I'm not sure what the tapping does it almost. Maybe it just increments it through patterns, that's horrible watch, your eyes.

The light is coming back. The light is back, let's take it apart. That is very, very annoying. Is this thing glued together, where's? The spudger here is the spudger.

Let's see if it's just clipped together, oh it's kind of it's kind of moving. Is it glued and clipped? No, it's just clipped revealing a circuit board hold on this thing should pop out should pop out isn't popping out there. It goes so revealing the circuit board, with a i'll just zoom, down it, a microcontroller, some resistors, presumably for the leds and a very, very basic audio circuit, based electronic microphone and some support components right. One moment: please, i'm just going to take a picture of this.

So we can take a look at it right. It turns out that this thing is slightly over sensitive if you plug it into a power supply the electrical noise, the power supply triggers it and the of the power bank, if you put it into one of these power supplies it's better. But if you add an extra capacitor, it works a lot better because it's triggered into that strobing mode when it detects a disturbance, usually the audio in the microphone. But in this case it was triggering all the time other times when there's no audio, it will just go into sort of a more passive sort of pattern effect, which is less disruptive, because it's quite ugly at other times.

Okay, now we've ascertained it can be fixed by soldering a capacitor across these terminals. Here that's between this sort of pad and this sort of pad that improves it, but it's still when you plug it into the power bank. It still kind of just goes into it's. Uh, oh it's not it's not bad! Oh no! Nope there! It goes yeah.

It's not great! It's very fussy about what it's plugged into okay. Let's take a look at this circuitry, so we have the connector coming on and a microcontroller. So this is bringing the five volts in we've got three leds coming to the positive rail and each led has its own resistor. The green and blue leds have a 10 ohm resistor.

The red led has a 12 ohm resistor, just because it's got a slightly lower voltage, there's a decoupling capacitor here and then there's the audio detection circuitry, which is just a very simple microphone with a capacitive coupling to a lightly biased transistor. And it's just a generic transistor, it's quite a high gains of low current sort of thing. Let me bring in the schematic. I shall zoom down in this for maximum resolution.

Here's a usb command! There is a randomly placed coupling capacitor. That's something worth mentioning: the decoupling capacitor could have been put directly across the microcontroller here, but they didn't they've actually placed it here. Wonder if that's an attempt to try and make the uh the audio circuitry more stable, it didn't work. The audio detection circuitry is based on a electric microphone.

Electric microphone is a microphone. That's got a charged film and when it detects audio that film moves backwards and forwards, and it's an enough for charge, it can actually turn on an internal field effect transistor, but just basically undulate its resistance. So to speak, so it actually acts as a audio, responsive. Resistor is the best way to describe it.

There's a 10k resistor forming a resistor network with a divider so that, as a result, that audio will cause the voltage here to fluctuate that gets covered across the base of the transistor, which is a one mega. Ohm resistor. Just gently biasing it to the positive rail just to keep it in a semi on state and, and that means this transistor will affect the lip pulse. When there's audio, the microcontroller is probably pulling that in high internally with a resistor just like a button input, and so it would see the audio just a series of pulses in that pin.

The output is via the resistors, 12, 10 and 10 for red, green and blue red, green and blue, and that is fundamentally it it's just going through a bit of software inside to sequence, these leds and when it detects disturbance it such does a wee strobing effect On them, it's very simple: it's hideous, but you know what it's uh, the circuit board itself or the base you could flip the connector out. You could make your own circuit board it's kind of a good starting point for another novel light, because these lights will. Let me shine a flashlight through it, it's quite nice because it creates that's a nice dotty pattern uh when you actually shine the lights through it. It's that a bug's eye type pattern, but there we go it's interesting.

It was worth uh exploring just to see this of their simple audio circuitry and just basically get the components for a personal project, perhaps but quite a neat little device.

18 thoughts on “Pound shop usb disco light teardown with schematic”
  1. Avataaar/Circle Created with python_avatars morpheox says:

    Weren't many early microphones basically some sort of carbon (grain/pellet) based audio responsibe resistors? Many phones in Sweden used the type.

  2. Avataaar/Circle Created with python_avatars Jim's videos says:

    Handy marker light for RC cars or model rockets possibly.

  3. Avataaar/Circle Created with python_avatars Little Jackalo says:

    When can we get back to real electronics content instead of dollar store junk and fairy lights.

  4. Avataaar/Circle Created with python_avatars The Internet Helpdesk says:

    You're like Ashens but with over intense detail. I like it.

  5. Avataaar/Circle Created with python_avatars FunkyJunky says:

    Haha, I would buy this thing just for the USB-C adapter. 😛

  6. Avataaar/Circle Created with python_avatars Mijc Osis says:

    The dulcet tones of Clive, just what the doctor ordered before bed

  7. Avataaar/Circle Created with python_avatars Pringle is cool says:

    i never see things like this in my dollar stores 🙁 i want stupid stuff like this

  8. Avataaar/Circle Created with python_avatars Kevin Hardisty says:

    Audio responsive resistor, great description. Now I understand how they work

  9. Avataaar/Circle Created with python_avatars Ellis The DJ says:

    as a DJ that light is annoying if it had a speed & sensitivity dial on it you probably vould have toned down the flashing rapidly when you powered it via a power bank but as it cost £1 it does not leave much room for improvement

  10. Avataaar/Circle Created with python_avatars George Dorn says:

    I bet if you replaced the resistors setting the bias to the transistor with an appropriately-sized audio-taper potentiometer, you could adjust the sensitivity on the fly.

  11. Avataaar/Circle Created with python_avatars Gregory Bell says:

    Still wonder about the C to A adapter. Just power rails in there?

  12. Avataaar/Circle Created with python_avatars TinPlate Geek Too says:

    Yep that flashing light is bad.
    As I've become older, I've noticed that these fast flashing lights are now causing mild nausea even just the small amount shown here. The joys of being over the hill and heading fast down the other side. 😃

  13. Avataaar/Circle Created with python_avatars RobotWashingMachine says:

    I miss the vise of knowledge, and the Stanley X-ray unit! They haven’t made an appearance in a while :^]

  14. Avataaar/Circle Created with python_avatars Tapio Peltonen says:

    Hm, that'd be a neat project, maybe just throw away the circuit board and replace it with a slow colour changing LED and a resistor. Or even better, three colour changing LEDs so they'd vary the patterns when they naturally drift out of sync.

  15. Avataaar/Circle Created with python_avatars Lee Adama says:

    if you disable the mic input it might actually be a neat effect.

  16. Avataaar/Circle Created with python_avatars Steve Durbin says:

    Interesting piece of tat… Was the USB-C adaptor fully wired or just +/- connections?

  17. Avataaar/Circle Created with python_avatars Peter Kay says:

    Another in the series of Clive's "e-waste" tat. It's a shame we don't have a set of border controls that look at products coming in, that rejects the most pointless and terrible at point of entry. Perhaps they'd stop making this straight-to-landfill guff…?

  18. Avataaar/Circle Created with python_avatars GMManBZFlag says:

    I have one of those and was always wondering what's in it. Surprisingly simple

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