Flickering LED Strip lights - Challenge question

Discussion in 'General Electronics Chat' started by Non Amuss, Aug 12, 2012.

  1. Non Amuss

    Thread Starter New Member

    Oct 23, 2003
    Hi there,

    I have set up some LED strip lights connected to a transformer, and I have a motion detector which switches the transformer on at the mains when someone enters the room. After a time delay of no motion being detected, the transformer switches off at the wall and so do the LEDs. Works great! Except for the fact that on turning off, the LEDs produce an epilepcy-inducing flickering for a few seconds before turning off. Guessing that the transformer has some residual charge?

    Here is more detail about the set-up:

    • I have two separate transformers, each with two output wires
    • I have one motion sensor that controls both transformers and therefore all the strip lights
    • All LED strip lights turn on when motion is detected and they turn off (or rather produce epileptic inducing flashes before switching off) 5 minutes after no motion is detected

    The way I'm achieving this is as follows:

    Here is a diagram of my set-up to hopefully make things clearer!

    (1) Someone moves about in the range of the PIR motion detector (2) The motion detector sends a signal to the plug sockets (3) Both plug sockets (and hence both transformers) switch on, and so do the LED strip lights. This works nicely. The problem is that after a few minutes of no motion, the PIR is set to turn the plug sockets off. When the sockets turn off, the transformers turn off and the LEDs flicker rapidly before turning off.

    I don't know if the transformers are faulty or whether there is a way of curing this flickering whilst still being able to switch the transformers on and off at the wall. I've wondered whether it's possible to put something at the transformer outputs, in between the transformer and the LED strip lights, so that instead of flashing they fade off or turn off in a non-flickery way.

    Technical specs

    The transformer I'm using is 150W for powering 12V LED lighting. All the specs are given here:

    The strip lights specs are on this page:

    I have 5m of strip lights per transformer output, so with 2 transformers and 4 outputs I have 20m. That equates to 288W in total (72W per output).

    Tests done
    • The flickering happens whether I have 20m of strip lights or 1m.
    • The flickering happens without the PIR controller as well (i.e. if I plug the transformer directly into the wall and switch off manually, the flickering still happens).

    Any help to remove the flickering using this setup would be much appreciated!

    Many thanks!
  2. BMorse

    Senior Member

    Sep 26, 2009
    One thing, if you plug the transformer into a mains socket without using the remote controlled dimmer, do the LED's still flicker? Just wondering, since I have some of those similar sockets (by X10) and I noticed when used with certain bulbs (CFL mainly) that the light will flicker for a few minutes after turning it off with the remote... but does not happen without the socket...
  3. Non Amuss

    Thread Starter New Member

    Oct 23, 2003
    Hi BMorse, yes - the lights flicker even when I plug the transformer directly into the wall and switch off at the mains.

    I should also mention that although the remote has a dimmer functionality, I am only using it in the on/off mode.
  4. BMorse

    Senior Member

    Sep 26, 2009
    From what I can tell, the "transformer" you are using is using PWM to control the LED's, and it seems as if it just has some capacitors holding a charge as it powers down so the circuitry is still trying to do the PWM as the power goes off, which causes it to appear as "flickering"..... too bad you do not have access to the actual schematic for the transformer to see exactly what it has on the outputs, it seems something like a resistor would need to be added somewhere to bleed the caps quickly when power goes off.....
  5. domnic0723

    New Member

    Aug 13, 2012
    Hiya,Im new here,thx for your sharing.
    Is it something like intelligent control?what gonna happen if a person is sleeping in the room?Sry, Im newbie in this field.
    You guys real cool!
  6. Non Amuss

    Thread Starter New Member

    Oct 23, 2003
    Many thanks BMorse - I did suspect the transformer. Would you suggest I get a different one? If so, how will I be able to tell that another one won't have the same problem before I buy? Are you able to recommend one? Thanks :)

    Hi domnic0723 - the room is a dressing room. Nobody will be sleeping in it ;)
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Just use a relay with an ac coil and switch the DC output of the driver with the contacts. That way when you turn off the mains the relay immediately disconnects the LED's from the driver. flickering gone