LED Xmas lights.

[Trevor]

The cheap led lights xmas trees that have a flashing module I want them to remain on static as I detest flashing lights Having bypassed the control module and just connecting the 24 volt ac transformer all lamps are on but have a strobe-ing effect
has any one a schematic or scheme that can overcome this!
in anticipation Trev

[SiriusHardware]

The LEDS are DC devices and you're running them on AC, so they are 50% on, 50% off. Install a bridge rectifier and a smoothing capacitor and run them from the output of that and they should stay on, but watch out for the current through each string of leds, presumably you have red, green, blue, amber?

We can't stand Las-Vegas style Christmas tree lights either but even where they can be set to 'static' they are normally searingly bright and rather cold, not at all like the gentle glow of filament lamp sets.

On a couple of our multifunction sets I removed the original control microprocessor and grafted in a PIC12F629 which I've then programmed so that the original control button just varies the brightness of the LED strings from very dim to full brightness, using PWM control which means, of course, that the lights are actually alternating between fully on and and fully off at higher than visible speed. The apparent brightness is controlled by changing the ratio of the amount of time they spend on to the amount of time they spend off.

While we can't see that, some people are sensitive to lights which are pulsing on and off at a high rate. It works OK for us.

I also made the PIC store the last set brightness value during power down, because another thing which maddens us about those multifunction lights is that they don't always do that, some revert to full-on Blackpool seafront mode every time they are turned back on and force you to blip them all the way through to the static function, invariably the last one in a long list of effects.

[Trevor]

The leds as you say are dc devices and are wired in reverse sets so i do not even think we are getting a 50% as using the transformer and sine wave drive i suspect an on time of only 25% i was looking for a simple square wave driver with maybe a higher frequency
Trev

[evingar]

I knocked up some strings of RGB LED's a few years ago and attached them to an off the shelf PWM driver controlled by a remote.
We now have all options available from static red green or blue, to annoying flashing, but the consensual fav is a non flickering gradual transition of colour.

[SiriusHardware]

i was looking for a simple square wave driver with maybe a higher frequency

That's more or less exactly what my 'conversion' does although it utilises the existing driver transistors, etc on the control PCB.

For a non-microprocessor method of PWM dimming a lamp (or LEDs) try:

http://www.circuitstoday.com/pwm-lam...er-using-ne555

Bear in mind that the 555 will need a regulator to supply it, as I don't think you'll be able to run it directly on whatever DC supply you derive from the 24VAC incoming.

[broadgage]

Changing the supply to DC probably wont work.
Most sets of LED Christmas lights have alternate strings of LEDs wired with inverse polarity, so with DC only half of them will light.

Any attempt at rectification, smoothing, or voltage regulation will probably kill the LEDs. Most cheap LED Christmas lights have NO current limiting resistance, they rely on the stray resistance of the transformer secondary etc, to limit the current.

The peak current can be surprisingly high, but the duty cycle is low. The LEDs only light for that very small part of the mains cycle during which the transformer voltage exceeds the combined forward voltage of the LEDs.
8 or 9 LEDs in series from a 24 volt nominal transformer is common.

IMHO, the best way of getting LED light strings that don't flicker is to purchase the very cheap battery powered lights that are intended to work from 3 AA cells. These are sold everywhere at low prices.

For a single set, discard the battery holder and use a 5 volt DC PSU in series with a small silicon power diode. This will drop the voltage to about 4.2/4.3 volts, very close to the intended battery supply.

Alternatively, connect 3 sets in series from a regulated 12 V DC PSU.

IMPORTANT these battery LED lights incorporate a current limiting resistor, either in the battery box or elsewhere in the circuit. This component MUST be retained in the circuit when using an alternative power supply.

A small fuse would be a prudent addition.

[SiriusHardware]

Originally Posted by broadgage

Most sets of LED Christmas lights have alternate strings of LEDs wired with inverse polarity, so with DC only half of them will light.

I haven't actually seen that to be the case, although my experience is admittedly limited and Trevor's set obviously is as you've described. I've looked at two different kinds of four colour 'effects' sets, and one set were DC with four strings of similarly polarised interleaved LEDs, each string a different colour. Originally all four strings were having their DC power chopped up by four medium power transistors which were driven by four bits of the microcontroller.

In the other type of set there were only two strings of LEDs each with two alternating colours in the string, (one say red-yellow-red-yellow, the other green-blue-green-blue). Otherwise the arrangement was the same, both DC polarised the same way and with their supply being chopped by transistors controlled by a micro.

Unfortunately the bottom line is that you just don't know which arrangement you are buying when you buy one of these things, so your idea of buying sets which are meant to be battery powered is a good one.

I agree with your observations re: current requirements, almost all of these LED 'pretty things' require current limited, rather than constant voltage supplies but that is rarely made clear on the markings on the PSU.