Christmas lights technology revisited

It is the time of year that people start to install light decoration to their house. So it is a good idea to check again this post from one year ago;
Christmas lights can be a beautiful sight to behold. Stringing lights around your home can be a real safety hazard if you’re not careful. For outdoors use the lights you need to have lights that are designed to be used outdoors


  1. Tomi Engdahl says:

    DIY LED Light Show Props (PEX Wreath, Mega Star, Tune To Sign, Flood Lights)

  2. Tomi Engdahl says:

    This Is The Most Beautiful Christmas Market In Europe

  3. Tomi Engdahl says:

    Who invented electric Christmas lights?
    Thomas Edison and Edward Johnson (1880 &1882) and Albert Sadacca (1917).

  4. Tomi Engdahl says:

    Magical OpenThread Mesh Christmas Lights!
    Build an awesome Christmas light show with the timing coordinated over the OpenThread mesh network!

  5. Tomi Engdahl says:

    Impress your guests and neighbors with Arduino IoT Cloud-controlled, color-changing holiday lights.

    Colour Change Outdoor Christmas Lights © CC BY-NC
    Ten-year old nephews are particularly impressed when they can change the colour of the outdoor Christmas lights from a tablet.

  6. Tomi Engdahl says:

    Why you can’t mix LED and tungsten in series circuits.

    This video doesn’t just apply to the classic candle arch, it applies to most situations where lamps are wired in series multiples. On fairgrounds that often use series multiples of four 60V lamps you must change over from tungsten to LED in all four series holders.

    The construction of this LED lamp was most impressive because they had clearly designed the electronic module to fit in with all the existing production machinery.

  7. Tomi Engdahl says:

    Christmas Lights – 2020 – Carol of the Bells

  8. Tomi Engdahl says:

    Very minimalist Xmas lights control PCB

    I thought I’d take apart a set of new Xmas lights to see how they are evolving.
    The answer is that I doubt they could evolve more without going to a blob chip.

    The pinout of the chip is similar to a PIC12 microcontroller, but I’m not sure what it actually is.
    The LED driving comes directly from two IO pins, so the chip has either got a built in H-bridge driver, two high current outputs, or they may just be abusing standard outputs and relying on the internal FET resistance to limit the current.

    After the video I reassembled the module and tested it on a bench power supply. It drew 84mA while alternating and 64mA while static (alternating quickly) on a 4.5V supply.

    I did a voltage/current test on an alternating mode, and the results were:-
    4.5V 80mA
    4V 60mA
    3.5V 39mA
    3V 15mA
    2.7V 4mA

    Below 2.7V the unit shut off, which is good for NiMh cells as it will reduce the risk of reverse charging of the first one to discharge completely. The intensity at 2.7V was pretty good for just 4mA.

    The use of alternating polarity LEDs saves a lot of wire for a flashing effect, and may also reduce the risk of static DC corrosion of the wires when water inevitably wicks up the LED heat shrink. Unfortunately it makes it a bit harder to run the string directly from a simple USB power cable adaptor.

    The 32.768kHz crystal frequency would need divided by 15 binary stages to get to 1Hz. A PIC microcontroller has the option of an RTCC (Real Time Clock Counter) which can be set to divide the clock frequency. It could be used to pre-divide and allow a more relaxed incrementing of the second stage counter for daily cycle timing in software while also maintaining the flashing patterns.

  9. Tomi Engdahl says:

    USB to LED string PCB prototype build

    This was partly a way to test my first panelised and V-cut PCB order from an online supplier. It turns out that the $2 for five panels soon adds up as you deviate from the norm.

  10. Tomi Engdahl says:

    eBay is STILL selling dangerous Christmas lights

    Grey import Christmas lights have been sold on eBay for a very long time. These are products that are shipped directly from countries with lower electrical safety standards than the country they’re being shipped to. In doing so they bypass a wall of testing and approval to make sure they pose a low shock and fire risk.
    Amazon seem to be getting better in this regard, but do still occasionally show products from rogue vendors.

    In the case of these eBay lights they just made a mockery of safety.

    The plug should be fitted with a 3A fuse that is actually in circuit. A typical UK socket is 240V protected by a 32A circuit breaker, and requires a suitable fuse to protect the load and its cable. In this case the cable would burn and possibly ignite.

    The flex between the plug and controller has two white cores that are very thin and appear to be copper coated aluminium.

    The controller’s cable connection point can be popped open effortlessly, exposing live connections.

    The wiring of the string is very thin and would classify as single insulated, only suited for low voltage use.

    There’s no strain relief to stop the wires being pulled out of the lights – exposing live connections.

    The heatshrink sleeve is thin and easily pierced by a wire end or solder point.

    The resistors are very small and often run at well above their rated power, risking melting of the sleeving and subsequent exposure of live connections.

    It’s not uncommon to find bare live wire or unsleeved LEDs along the cable run as nothing is properly tested.

    These lights are often sold as suitable for indoor or outdoor use. They are NOT waterproof in any way and can pose a shock risk if handled while wet, or leak current onto touchable metalwork like railings. If that happens it may be pulsing DC leakage which may defeat some protective devices.

    The cost of these lights has risen to the point they are close, if not more than a locally sourced country-compliant set that has proper insulation or an SELV (Separated Extra Low Voltage) power supply for safety.

    These lights pose a hazard to kids and pets who are attracted to the flashing lights and will handle them. In the right circumstances they could deliver a dangerous shock.

    Only buy your lights from a prominent local retailer or known brands who have to comply with local standards.

  11. Tomi Engdahl says:

    Most Christmas lights divided by sections. Each section has several bulbs connected in series. On traditional construction when one bulb burned, whole section is out. Some newer lights have special bulbs that are designed to go short circuit when they burn so keep rest on chain on (maybe somewhat brighter). When too many lights are out in a section are out the rest burn out more quickly. Those “go short when burned bulbs” do not always go to short circuit when burns and there can be intentionally some traditional bulbs (without short when burned feature) for fail-safe reasons.

  12. Tomi Engdahl says:

    How to fix your low voltage LED Christmas lights

    A brief exploration of three styles of wiring used in common low voltage light strings.

    These circuits and repairs are only for the low voltage sets of lights that run from battery, solar, USB or plug in power supplies.

    Note that for the two/three wire flashing/effects strings the circuitry is similar to the 3-wire strings, but with the LEDs alternating polarity along the string. If rejoining a damaged set then the incorrect polarity of the two LED wires will cause a colour change at that point in the string during flashing effects.

  13. Tomi Engdahl says:

    Custom Christmas Light Controller Blocks Blinks

    Finding that his recently purchased LED Christmas lights defaulted to an annoying blinking pattern that took a ridiculous seven button presses to disable each time they were powered up, [Matthew Millman] decided to build a new power supply that keeps things nice and simple. In his words, the goal was to enable “all lights on, no blinking or patterns of any sort”.

    Connecting the existing power supply to his oscilloscope, [Matthew] found the stock “steady on” setting was a 72 VAC peak-to-peak square wave at about 500 Hz. To recreate this, he essentially needed to find a 32 VDC power supply and swap the polarity back and forth at the same frequency. In the end the closest thing he could find in the parts bin was a HP printer power supply that put out 30 volts, so the lights aren’t quite as bright as they were before, but at least they aren’t blinking.

    To turn that into a pair of AC square waves, the power supply is connected to a common L298 H-Bridge module.

    Is it really this difficult to get modern LED Christmas lights to default to “on” ?

    Wrapped your house in LED Christmas lights, put it all on timer (or some other automated control mechanism), only to discover that the default setting for your LED Christmas lights is an annoying comically blinky pattern, certain to invoke questions about your standing in society?

    That happens to be just the position I recently found myself in and apparently this is a common annoyance of 2-wire LED Christmas lights, presently the most common type. In this YouTube video Big Clive reveals that some power supplies have an unpopulated option for a memory chip, which preserves the selected pattern after power-off.

    Unfortunately you probably didn’t get the option to have that installed in your lights, and also like me have discovered that even if such an option exists inside the power supply, opening it non destructively is near impossible. Some of these units have an 8 hours on, 16 hours off timer which sort of half solves this problem, but not in my, and many others opinion.

    Rather than messing around modifying an existing LED Christmas light power supply, I was keen to see how an entirely new LED Christmas light supply could be built, in particular one which defaults to “all lights on, no blinking or patterns of any sort”.

    From the above we can see that it outputs a 72 volt peak-to-peak (36 VAC) square wave alternating current voltage, at around 500 hertz. In theory this is simple to generate, just find a 36 volt DC source and swap the polarity 500 times a second, but in practice it is annoyingly difficult. The voltage will vary depending on the manufacturer, but will always be printed on the side of the unit.

  14. Tomi Engdahl says:

    LED Christmas light balls – analysis

    It’s Christmas, so let’s analyze some Christmas lights with 10 balls :).

  15. Tomi Engdahl says:

    Chinese 120V to 220V LED lensed-globe string (with schematic)

    This set of lights would fail most countries electrical safety tests, but is very typical of the grey imports from China.

    With its ultra thin copper coated aluminium flex, very simple power supply and mains referenced LED string with resistors being run well above their normal rating, it makes me wonder if it’s just naivety of the designer or if they want to hasten the failure of the product for future sales.

    These WILL work on 120V, running at lower current and with cooler resistors. I’ve tested that and they still give a good effect.

    Note that these lights are not safe for use within reach of pets or children. They should also be used with a fused adaptor with a low current fuse.

    Other than that’s they’re quite nice lights and probably worth getting for the caps alone, which emit a nice pattern of dots. The two colour alternating LEDs are very common in these products.

  16. Tomi Engdahl says:

    Run LED strings on USB chargers and power banks.

    This is a very simple project to make an adaptor that lets you run parallel strings of LEDs from USB power supplies and power banks. The types of string it can run are the parallel wired strings often found with solar powered strings or the battery powered strings. If you use the newer “effect” strings where alternate LEDs are wired in inverse parallel then only half will light depending on which way you plug it in.
    The circuit is designed to run the LEDs at a nice low level for efficiency and to maximise the lifespan of the LEDs. This also allows even a basic USB power bank to run a set for a very long time. Note that the cheaper power banks do not have proper low battery cut-off, so if the LEDs suddenly go dim then that’s the time to unplug them and recharge the power bank. With better quality power banks the LEDs will cut off suddenly when the battery is too low.


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